Basement conversion and basement extension guide — in depth information on how to successfully tackle basements

What do you do if you want to extend your home, but your garden is too small to allow it, or your loft is unsuitable for conversion? You may think the only way to get extra space is to move house. However, there is another possibility. Consider a basement conversion, or basement extension, excavating below ground to gain space from a basement.

Basements are slowly becoming more popular in the UK as people look for more space and don’t have the ability to extend wider, longer or higher.

Sometimes, the only way to get more room without moving house is to go down into a basement. This is you ultimate guide on basement conversions and basement extensions to help you achieve a basement that works for you. 

Basement conversion and basement extension advantages

In some parts of the UK, especially in parts of London, and more and more in Cambridge and St Albans, where property values are at a premium and stamp duty is high, many people are turning to basement conversions and extensions as a viable option.

Another significant advantage of a basement conversion or extension is that you can create a whole new floor that can be used in a variety of different ways. In comparison, if you were converting an attic, for example, space is so far from the living areas that it can only really be used for bedrooms, studies or bathrooms.

practical advantages that basements can provide


Borrowed light

External light wells in basement conversions and extensions can be used as a means of escape. If there is no clay in the vicinity and the water table is not high, these can be constructed from reinforced plastic, helping to keep costs down. However, if there is clay and a high water table, light wells will need to be constructed from concrete or other substantial material will be needed. 

Courtyards can create a garden space, with glass bi-folds or sliding doors opening into it. 

Borrowed light can also come from double height spaces or glazed floors.

Direct light

In a basement extension or new build, depending on the site constraints, the basement can be designed to get light from either front or back, or possibly the side. This could allow the positioning of living spaces adjacent to light with service spaces to the rear. 

Sun tunnels

These use mirrors to reflect sky and light from above.

Tricks to enhance light once within the space

Keep the space as open as possible with minimal structures like walls
Consider a glass staircase with glass balusters and treads, and open risers. 

Rooms with no light

Rooms that work well without light are wine cellars, cinema and entertainment rooms, utility rooms, storage rooms and perhaps bathrooms and WC.

basement conversion and basement extension disadvantages

On the flip side, the major disadvantage of going down into a basement can be the cost. However, in areas where building land is scarce and property values are high, basements can work financially. If the property has an existing basement in your property, even if it is in a bad state of repair or with a low ceiling, it will still be cheaper than a traditional new build extension.

Another disadvantage is that the construction process of a basement conversion or basement extension can be complex. Like any building work it will be disruptive, noisy and messy especially if underpinning is required. Even though excavation is usually accessed externally the breakthrough isn’t until it’s time to install a connecting staircase and the neighbours won’t like it.

What basements can be used for

Basement extensions and basement conversions can offer interesting spaces quite different in character to the rest of the house. Basements are close enough to the ground floor that they can easily be designated as playrooms, living rooms, kitchens or separate bedrooms. 

With the growth of multi-generational homes, basements are also a viable option for granny flats or living spaces for teenagers or grown up children.

With the demands of modern lifestyles, basement conversions and extensions are increasingly being turned into leisure areas with pool tables, home gyms or even swimming pools. If you have luxurious tastes and a love of entertaining, basement rooms make excellent wine cellars. The relative quiet of a basement means it could make an ideal cinema room, or for the increasing numbers of people working from home, a quiet office. You could even consider a basement garage and storage with external access via a driveway.

Ideas and inspiration for basement conversions and basement extensions

View our Pinterest board on basements for inspiration. Click the image for access.


Definition and history of basements

A basement is a storey whose floor level is more than 2 metres below ground level. A semi‐basement is where the basement floor level is at least 1 metre below the adjoining ground level. Dwellings on sloping sites where one side of the lowest floor level relates similarly to the ground on that side of the building are also considered as basements since they share many of the physical characteristics with other basement types.

Basements were once commonplace in many domestic buildings in the UK. The use of basements in many larger houses was due to a social factor. Basements were the so‐called ‘working areas’ of houses, such as kitchens, storerooms and offices.

Basements in smaller properties were often of low quality, used for basic storage, such as coal or raw foods. They tended to be dark, damp and poorly ventilated. Generally the land used was good‐quality greenfield sites and was relatively cheap. Poor‐quality or difficult land was often left unused. After the First World War, land was significantly cheaper. Large numbers of houses were built at relatively low cost and basements were not considered necessary, and were generally omitted. 

Although land costs have been steadily increasing over the years, basements have generally been ignored until recently. This is because they were considered to be expensive to construct and it was felt that potential buyers were not interested in basements. As a result the norm has been to focus on alternative methods of fitting additional space within the building shell, such as, developing the roof space with a loft conversion. However, roof spaces can be more restrictive both in usable area and inaccessibility.

By comparison, in Europe and North America, basements have continued to be incorporated into new dwellings. The reasons for this vary. Traditionally they were used as storage space in the colder parts of Northern Europe where households stored produce during long winters. As the need for such storage diminished the use of basements has been replaced by different needs such as additional living space, hobby rooms and utility rooms.In North America, basements are commonplace to provide additional rooms at ground level, even when land is available. Since there has been a continuing demand in these locations, industry has developed improved methods of constructing basements to make them cheaper to build, warmer and drier. 

The scene is now changing in the UK with house buyers, self‐build groups and established builders showing more interest in houses with basements. But there remains certain issues to be resolved to enable basements to become more of a norm in the UK.

Basement types

  1. Basement refurbishment or conversion of an existing cellar. This is the most straightforward option that uses old cellar spaces which often have bare brick walls and lack waterproofing. They tend to exist beneath Victorian and Edwardian houses. If there is enough headroom such a basement conversion would involve damp proofing, improving the staircase and access, and finishing works. If there isn't enough headroom the build gets more complex as the basement conversion will need the floor to be lowered and the ceiling to be raised.
  2. A retrofit basement is when a basement is added to an existing home. Adding a new basement to an existing building requires extensive and pricey excavations and possibly underpinning to party walls. However, such basement conversions are still viable in high value properties and neighbourhoods or when there is no other way to achieve more required space. Such basement extensions can be underneath your property or extend further out.
  3. Building a new basement from scratch. This is when basements are constructed as part of a new build. This is a straight forward option and can be fully or semi underground. Including a basement in a new build design can be a great way to get more space from your land.

Establishing if your property is suitable for a basement conversion or basement extension

Property buildability and contractor access

One of the first things to consider is how constrained the site is? What level of access would builders have for such things as the removal of spoil, and delivery and storage of materials? How close are the neighbours and what effect will any works have on them?

Property reparability

What is the structural state of the existing property? How much extra will it cost just to bring the property up to par and is it possible to do so?

Surrounding geology

What are the local ground conditions and soil types in the immediate area? A good place to start is to talk to your local building control officer and pick his knowledge of his patch. Obviously, carrying out a couple of trial holes will also establish this information quickly. These tend to be carried out during a site investigation, soil survey or geology survey. They should be carried out before construction begins as the results should inform the design and build of the basement and property.

Water table

Establishing the drainage characteristics of the local soil and water table level is important because it has a direct effect on the choice of structure and waterproofing of the property. If there is an existing basement, consider if it has a history of flooding, or, is it currently damp? Can anything be done to control this? It is important to seek expert advice to ensure costs do not spiral out of control if the issues are not dealt with early enough. 


Are there level differences that can be exploited for the provision of external access, light and ventilation? 


Is there any ventilation to any existing basement? If not how easily could it be provided?

Access from the existing property

Consider the constraints of the existing context and ground floor layout. For example, how will internal access and fire escape be provided and comply with building regulations?

Existing foul drainage

What are the existing foul drainage provisions? What will the impact of the basement conversion or basement extension be?

consider your basement conversion and basement extension requirements

Careful planning of the basement space is crucial to its success. All too often basement rooms are lifeless areas, starved of natural light or key connection to the outside because the design hasn’t been properly considered. 

Some people might assume basements to be dark and dingy zones, but in reality, the right balance of illumination and good ceiling height (ideally at least 2.6 metres) will create bright, spacious, rooms that feel connected with the rest of the house.

The design and layout of your proposal should revolve around how you’re going to use the space. Are you creating extra room for general living areas or will the basement play a specific role, such as a cinema, gym or spa zone?

Your basement should feel like a natural continuation of your home in the same way as a good horizontal ground level extension would create a seamless flow between the stories by things like matching flooring and style. The staircase position between the basement and the rest of the building has an important role to play. Tucked away staircases can make a new storey feel like a secretive area, whilst a more visible design will add a sense of connection.

Setting the brief for your basement conversion or basement extension

Your basement brief is an important stage that outlines what you want your basement conversion or basement extension to achieve. Having established that your property is likely to be suitable for a basement, the next step is to define the scope of your project. What follows are some basic questions you might ask yourself:

  1. How would you like to use the new basement space?
  2. How big a footprint do you want from your basement? It does not need to be as big as the house above, or it could be bigger and extend under the garden to the rear.
  3. For circulation, where do you want a staircase?
  4. What is the desired headroom for the basement?
  5. Is external access direct from the basement level required?
  6. How much natural light is required?

Having clear ideas on these points will help ensure professionals like architects can help ensure your basement conversion or basement extension provides you with what you want. They can offer priceless design advice and if some aspects of your brief are not possible, they can explain why and offer alternatives.

Basement conversion and basement extension design considerations

How to bring natural light into a basement

Lighting is essential to the success of a basement conversion project. The only rooms which could work without natural light are cinema rooms, utility rooms, storage rooms and wine cellars.

A popular way to get daylight into a basement is with light wells. These are pits dug around the outside walls of a basement to allow light to penetrate into the rooms below ground. Basements do not always have to be fully underground. Parts of a basement could be partially above ground, especially on a sloping site. And this area of the basement could be designed to allow natural light into the basement. Even a small amount of daylight can relieve the feeling of claustrophobia that may mar an otherwise successful basement extension.

If introducing natural light is not possible then you’ll have to depend on borrowed light and/or a carefully planned artificial lighting schemes. Borrowed light comes from open plan spaces, or for example, a staircase with glass railings that draws light down. Artificial lighting can be softened by concealing the source, and reflecting it off surfaces. This trick works well in basements and is an effective way of illuminating the ceiling. The lighter the ceiling, the higher it will seem. 

Coelux-Generation-2, for when there is no natural light

If there are certain places that you simply can't get natural light, the artificial light product, the “Coelux-Generation-2” has just been launched. Watch the video to see this great artificial skylight product. 

Circulation - how the basement conversion or basement extension connects to the rest of the house

Along with the lack of daylight, another problem a basement designer faces is connecting the new zone to the rest of the house. Ideally, the new stairwell should link seamlessly into the hallway. A basement should always feel like it is part of the home, so it is important to give some thought to how it will connect to upper levels. 

The staircase must provide a safe and direct route outside from the new lower level, otherwise, there may be problems with Building Regulations. (Note, this may not apply if you are upgrading an existing basement). The best location for the staircase is immediately below the ground-to-first-floor flight. Locating it elsewhere just eats into existing rooms, therefore reduces the project’s cost-effectiveness and use of existing space.

Another way to seamlessly connect zones is by creating an internal balcony or a galleried landing, accessed via an internal staircase. This would not only create a dramatic double height area but would allow natural light into the basement conversion or basement extension.


How to bring ventilation into a basement conversion or basement extension

Any new habitable rooms will need to be ventilated and this can sometimes present a problem. Ventilation should be considered early in the design stage because if mechanical ventilation is required, the ductwork will need to be accommodated for.

Where the basement contains a light well, it can sometimes be adapted to include an opening window equivalent to 1/20th of the floor area of the room with a trickle vent at high level. In any bath or shower rooms and extractor fan should be fitted. If there are no opening windows in these rooms, extractor fans which are triggered by the light switch with overrun timers should be fitted. This allows the fan to remain on for a period of time after the light is turned off. Where natural ventilation through windows is not practical, a mechanical ventilation system will need to be installed. Various package systems are available on the market. 

What about extending out under the garden?

The simplest way to construct a basement is to follow the line of the existing structure above. However, it is also possible to extend horizontally beyond the footprint of the house. This can be particularly effective at the rear of a house, where it is usually capped with a raised terrace or patio. This allows roof lights to be set into the ground, which floods the rooms below with natural sunlight.

Technical considerations for basement conversions and basement extensions

If basement conversions or new extensions go wrong, they really go wrong. The worst-case scenario is that you end up with an underground swimming pool. So, it may be essential to bring in some professional and specialist help depending on your level of expertise. However, do not be discouraged. The technology to excavate, construct and waterproof basements is improving all the time, and there are many viable options for a headache free basement conversion or extension.

The right construction method for your basement mainly depends on your budget, soil and conditions.

A soil report is considered essential. Soil can have a low or high water content, can be crumbly or can gather water if you are on a hill. Different soil types will need different types of waterproofing and various types of construction methods.

Requirements of walls below ground

  1. Walls below ground are required to act in a hostile environment and must be highly durable
  2. They are required to cope with lateral and axial loads 
  3. The requirement to exclude moisture is highly significant due to positive hydrostatic pressure in the ground.

Methods of construction for basements

There are many different approaches to the construction of basements. Your choices will depend on location, geological conditions, adjacent structures and access limitations. What follows is a description of the most common methods used in domestic construction for existing buildings and new builds.

Construction methods for existing basements

Underpinning and piling are by far the most common techniques used for basements built underneath or close to existing structures. There are several underpinning methods:

  1. Mass concrete underpinning
  2. Reinforced concrete underpinning
  3. Pile underpinning (used in repair)

In practice mass concrete underpinning is the most used method. This is because of access and the need to work from the inside of the property of most domestic properties, particularly turn of the century terraced townhouses with shared party walls, where neighbours also need to be considered.

Pier and beam underpinning and pile underpinning are more likely to be used for remedial works when buildings have failed, and access from the outside is good.


Underpinning is the technique by which an existing foundation is provided with increased depth. This method has been used historically for foundation repair and strengthening.

Underpinning relies on the ability of a wall to span unsupported for a short length (usually 1.2 meters’ maximum) and a new structure to be built directly underneath. In this way, new sections of lower level basement foundation and wall can be built in a sequence until the existing wall is supported to its full length.

Historically for repair the underpins were transferring vertical load from the existing foundations down to a lower level where the ground was stronger or more stable. The underpins were usually built of concrete with no reinforcement. This type of underpinning is called a mass concrete underpin.

In basement construction there is a structural requirement to:

  • Transfer the vertical load on the existing wall down to the new lower foundation level, and,
  • Hold back the horizontal load of the ground, hydrostatic pressure and any other surcharge loads acting on the outside walls of the basement.
The horizontal force from the ground or other surcharge from outside the basement must be considered in the permanent design of the final completed basement (the permanent condition) and at all stages of construction (the temporary condition).

1. Traditional mass concrete underpinning

Mass concrete underpins can sometimes be used to form the walls of basement structures. However, mass concrete underpins will not have sufficient strength to act as the basement walls and to retain the ground on their own.

Mass concrete underpins are quite often used to transfer vertical loads of various elements to deeper levels either temporarily or permanently.

Mass concrete underpinning can be used in conjunction with an inner reinforced concrete retaining wall as part of the permanent design. The inner reinforced concrete wall can also be part of a strong complete inner reinforced concrete box. In this option the mass concrete underpins will not be sufficiently strong to act as a retaining wall in the temporary condition, so horizontal temporary propping and a somewhat complicated construction sequence are usually needed to provide support until the inner reinforced concrete retaining wall, or box, is built and has gained strength.

This mass concrete underpin with an reinforced concrete inner lining wall is generally not seen as an efficient design but does overcome one issue that can arise from the Party Wall Act.

Video illustrating the underpinning sequence (1:35)

Underpinning and The Party Wall Act

Under the Party Wall Act, reinforced concrete underpinning, is called a 'Special Foundation'. Under the Party Wall Act the adjoining owner's agreement, 'Special Foundations Consent', is required to build reinforced concrete underpinning under a party wall.

Conversely, the Party Wall Act, allows a building owner to build a mass concrete underpin underneath the party wall without the consent of the adjoining owner. So a design with a mass concrete underpin and an inner reinforced concrete retaining wall or box avoids the need for special foundations consent.

It is now fairly rare for Special Foundations Consent not to be given. There are several reasons why the Adjoining Owner can benefit from giving Special Foundations Consent. In the few cases where consent is not forthcoming, a mass concrete underpin with a reinforced concrete lining wall can be built.

2. Reinforced concrete underpinning

Basement walls need to support the vertical loads from the existing structure and resist the horizontal forces from the ground, hydrostatic pressure and from any other surcharge loads such as the foundations of nearby structures or vehicles on roads.

A reinforced concrete underpin can be designed to achieve both of these functions in one of two main ways:

  1. A vertical cantilever with a turning moment acting around the end of the toe of the base. In other words the lower section of the underpin is held firmly in place and the vertical wall is sufficiently strong to prevent the soil outside the basement, or any other outside horizontal forces, from pushing the underpin foundation wall out of vertical, or,
  2. A vertical beam restrained at the bottom by the basement ground slab, and restrained at the top by an reinforced concrete basement roof slab, or other structural arrangement.

These sorts of reinforced concrete underpin are generally viewed as efficient underneath a building as they maximise space, can be built in a low number of construction operations and, from a structural point of view, are low risk, as they involve only one transfer of vertical loads.

3. Multi-stage underpinning

Multi-stage underpinning can be used to build deep reinforced concrete walls either as part of a deep single level basement or when constructing multi-level basements. Multi-stage underpins allow shallower individual underpin excavations which can be advantageous from a health and safety point of view, especially in unstable soils. There are several important points to note when undertaking multi-stage underpinning:

  • Temporary propping - vertical and horizontal propping must be designed and in place at all times
  • There must be reinforcement continuity between the different stages of underpinning
  • The vertical load of the building must be supported by each stage of the underpinning - the base of each intermediate stage of underpinning will often need to be thickened in order to avoid increasing the temporary foundation's bearing pressure. Any intermediate stage base thickening will usually be trimmed off later to leave a flat vertical wall
  • The underpins at each stage should be offset horizontally to avoid a continuous bottom to top vertical construction joint.

Construction methods for new build basement extensions

The section looks at two types of new build basement:

  1. Ones that have good access and lots of room without disturbance to neighbours like a flat green field site

  2. New basements which are either on a sloping site or close to neighbours that can’t be disturbed. For example, a new basement under the garden, in an urban Victorian terrace, where you want to build up to the boundary.

Shallow basements with good access

This type of construction would probably be used in a new self-build site with lots of land around the footprint of the new house. 

For most domestic basements, the required excavation depth is likely to be relatively shallow, allowing an open excavation with stable sloping sides if you have enough room and stable soil that won’t collapse back into the hole.

The typical range of construction materials for the external walls could be:

  1. EPS insulated formwork
  2. In-situ concrete
  3. Precast concrete
  4. Masonry
  5. Solid brick or block walls
  6. Reinforced filled hollow blocks
  7. Reinforced cavity

Basement construction where minimum disruption to surrounding area is required

There are two techniques commonly used in this situation:

  1. Excavations with temporary support
  2. Excavations with permanent retaining walls formed before the main excavation

Excavations with temporary support

Temporary support is usually used where access is limited and the extent of the basement is constructed right up to the boundary. An often used solution is to use steel sheet piles driven or vibrated into the ground around the edge of the intended basement. These are then propped with temporary supports to prevent collapse as earth is excavated.

Permanent walls are then constructed, and once in place the steel sheets are removed and resultant voids filled. This method can result in subsidence, so this approach may not be suitable where there are adjacent buildings that might be affected.

Sectional precast concrete

Mostly used where depth is relatively shallow, and sequential alternate sectional excavation can be achieved. The temporary support is often achieved with steel sheets supported by diagonal bracing or ground anchors. Reinforced concrete upstands sections are then placed, with the sectional wall panels connected and secured to them. Again, the temporary support is removed, and the gap left behind back-filled.

Permanent retaining walls formed before main excavation

The most common options available in such a situation are

  1. Bored piles
  2. Diaphragm walls

In-situ bored cast piles can be an efficient and cost-effective means of constructing temporary or permanent retaining walls. They are typically used near existing structures requiring restraint and where working space is limited. The technique avoids extreme excavation and help control movement in the ground. Three types of bored pile that are used:

Contiguous piles

Contiguous plies.jpg

These are suitable to retain stiff and cohesive sub-soils and where the groundwater level is lower than the final excavation depth. Diameters can range from 450mm to 900mm centred between 500 and 1000mm respectively. This thereby leaving gaps between the piles of between 15 and 100mm.

Interlocking piles

This type of construction is similar to the contiguous piles, except the space between the principal piles are filled with a subordinate ‘soft pile’ consisting an unreinforced weak concrete mix, constructed to a depth just below the final excavation depth. The secondary piles are constructed followed by a sequence of primary piles cutting into the secondary piles. These are taken to the full design depth and then reinforced as usual. This form of construction ensures that water entry into the excavation is significantly reduced.

Secant piles

Similar to interlocking piles, the secondary pile is not soft but concrete as the primary pile is reinforced in a similar way to the primary pile. Heavy duty CFA augers with rotary cutting heads are used to construct secant piles. Secant piles can be a cost effective alternative to diaphragm wall construction.

Diaphragm walls can be constructed from either concrete, metal sheet or masonry, and built in a deep trench excavation dug between temporary sheet piling. The walls range from 500-1,500 mm in thickness and can be taken to 50m in depth and over. Trenches are excavated by rope-suspended mechanical or hydraulically-operated grabs.

The excavation stability is maintained by the use of a drilling fluid, usually a bentonite slurry. This is a controlled mixture exerts a pressure more than the earth and hydrostatic pressures on the sides of the excavation.

Diaphragm walls can be expensive, making them uneconomic if they can’t be included as part of a building structure. As a result, they are not generally suitable for residential buildings and better suited to deep constructions such as car parks.

waterproofing basement conversions and basement extensions

Basements fall into one of three types of waterproofing construction methods:

  1. Tanked structures
  2. Waterproof concrete structures
  3. Drained structures

The British Standard BS8102:2009 define these three basement wall types in more detail.

Type A (barrier) protection

Protection against water ingress which is dependent on a separate barrier system applied to the structure.

Type B (structurally integral) protection

Protection against water ingress which is provided by the structure.

Type C (drained) protection

Protection against water into usable spaces which is provided by the incorporation of an appropriate internal water management system.

The following Youtube videos show the installation of a cavity drain system and pump:

Which type will be suitable for your project will depend on the constraints of the site and your project objectives. The following flow chart (from BS8102) sets out a procedure to follow with options and investigations that need to be gathered in relation to your particular site context:

The specification of waterproofing protection is a specialised task. We would always recommend that, once the design team has given the system some thought, an appropriate specialist should be contacted for early advice and help on waterproofing design. The following chart is a good starting point:

waterproofing systems

There is a range of waterproofing systems that can be used with the above three waterproofing construction methods:

waterproofing systems for existing basements

Many older buildings that have basements tend to have problems with damp. Before any damp proofing work is specified, the cause of existing damp needs to be identified. The use of waterproofing systems is not suitable if the main cause of damp in basements is leaking pipes or condensation.

Since existing basements are already constructed it is either not possible or would be very costly and difficult to waterproof them externally. Therefore, as a rule, the damp proofing of existing basements needs to be carried out internally. Damp proofing systems for existing cellars include:

  • Drained cavities
  • Mastic asphalt
  • Cementitious renders
  • Self-adhesive membranes
  • Liquid applied membranes
  • Ventilated dry linings

Whichever system is chosen, special attention needs to be given to the details at reveals, thresholds, partitions, ceilings and points where services are fixed. This will help ensurethat these do not become weak points that will encourage localised water seepage.

Statutory approvals for basement conversions and basement extensions

Planning requirements

Retrofitting an existing basement from storage to habitable space will be Permitted Development and does not require planning permission (unless your property is in a Conservation Area or specially designated area, or your home is listed). However, if you need to lower the floor level to get adequate head height, this is considered a basement extension and planning permission may be necessary.

Planning can be about interpretation and as such planning policy on basements by individual local authorities varies, but even so it may be difficult for a local authority to find reasonable justification for not granting permission, particularly if the work does not substantially alter the appearance of the building. Much has been made in media coverage recently about “iceberg basements”, going down several stories in certain areas of Central London and as a result a lot of neighbours are complaining about construction disturbance.The upshot is that a lot of Local Authorities are currently reviewing related policy.

Building regulations

Building Regulations approval is necessary for the construction of a new basement habitable space. Because of the specialist nature of basement conversions and basement extensions, we would advise a Full Plans Application rather than a Building Notice. Particular issues to be aware of are:


Tanking systems and cementitious renders should have BBA or similar independent accreditation. Any other system that is used should have a suitable third party insurance guarantee. If a liquid applied waterproofing product is used it must have adequate loading, so it stays on walls.

Means of escape in case of fire

New ceilings may need to have a fire resistance. Sometimes basement should have an FD20 fire door between it and the rest of the house. 

Especially when accessed from ground level, an escape window or a door giving access directly to an external area should be provided, allowing anyone escaping from a window or door to be able to get to the outside of the house at ground level simply. Steps with a pitch of 42 degrees should be provided. An escape window must have a minimum area 0.33m² with no dimensions less than 450mm, with the cill and no higher than 1100mm.

Smoke alarms

Linked mains operated smoke detectors should be provided in the ground floor hallway and first-floor landing and a heat sensor should also be fitted in any basement kitchen area.


Requirements are purge ventilation (an opening which is not less that 1/20th of the floor area) and background ventilation of 8000mm² for habitable rooms and 4000mm² for kitchens, bathrooms and utility rooms should


New stairs to the room should have a 42 degrees pitch with 2.0m headroom and suitable handrails.

Guarding to new windows or light / access wells

If there is a change of levels of more than 600 mm, guarding of 1100mm high will need to be provided.


Any insulating material should be compatible with the tanking. All new doors and windows should have a "U" value of 1.6 Wm²/k.

New guidance for basement conversions and basement extensions

In Dec 2013 a new document The Building Regulations 2010 - Basements for Dwellings - Guidance Document was published

This new document gives guidance on the design and construction of basements for dwellings and describes the means of meeting the relevant performance requirements of Schedule 1 of the Building Regulations 2010 on the creation of basements to houses.

Party walls

Very often, particularly when turn of the century properties are concerned, and party walls will be affected you have a duty to serve notice on your neighbours and follow the legislative procedures. See our party walls blog for more detail. 

We would also draw your attention to the notes above under underpinning and party walls plus the effect constructing basements will have on your insurance. See our blog on insurance.

Costs of basement conversions and basement extensions

The cost of basements is affected by many things. The cost of basement work relative to local property values will depend on whether a cellar conversion is financially viable. If you are in a sought-after urban area and have an existing cellar, our opinion is that it is most likely to make financial sense to add lower ground floor space.

The potential to add value with basement conversions and basement extensions needs careful analysis and estate agents should be consulted to give you an idea of local market conditions.

Bear in mind that a high quality, well-lit habitable basement with access directly onto the garden will be worth more than a dark, converted damp cellar space with low headroom.

If you are trying to decide to go for a loft conversion or a basement conversion, remember an uncomplicated cellar conversion is about the same cost, but creating a new basement from scratch is twice the cost per m². However, you may get more useable floor space in the new basement than a loft conversion.

If there is adequate headroom, retrofitting an existing basement into habitable space wil be similar to a simple loft conversion, at approximately £1,150-1,450/m². However, once you start to get involved with new complex substructures the costs begin to get expensive.

Guideline costs for basement conversions and basement extensions

  • Refurbishment conversion of an existing cellar with adequate headroom – £1,150-1,450/m²
  • Retro-lowering the floor level and underpinning an existing cellar – £1,500-2,500/²
  • Digging new basement space and underpinning under an existing building – £2,000-4,000/²
  • Excavating a new basement space under the garden – £1,750-2,250/²
  • Creating a light well/external access – £5,500-9,500 each

How long it takes to carry out a basement conversion or basement extension

A simple refurbishment conversion by converting a single-room cellar using a membrane lining system, including digging out the sump and fitting the pumping system can take just two or three weeks. Converting and extending the cellar beneath an entire house, involving underpinning of the existing structure, and depending on the size of the property, the amount of excavation and building work required and its complexity is likely to take 12 – 20 weeks.

Considering a basement conversion, extension or new build with a basement? Book your kickstart consultation today.

This consultation will help you get started right with expert advice early on in the project — quickly get to grips with design, costs and planning, and minimise the likelihood of stress, time wasting and escalating costs. We mainly work with clients in Cambridge, St Albans, and throughout East Anglia and London. 

Like this post? Consider sharing it or saving it for later.