Loft Conversion Guide — in depth information on how to successfully tackle a loft conversion

This guide provides everything you need to know about loft conversion design considerations, costings, planning and building regulations. A loft conversion is an ideal way to gain more space. There may be scope for one or two extra bedrooms possibly even with a bathroom or home office.

There is no doubt that it's easier to extend than to move. If you've got some unused space, it's worthwhile thinking about building into it to create that in demand extra space. This guide is the first in a series about getting the most from your property — going up, going down, going in and going out.

This article will be useful to anyone undertaking a loft conversion throughout England. If you are in Cambridge, St Albans or anywhere in East Anglia and London areas then we are local architects who can provide specific hands on help with loft conversions. You can contact us here.

An overview of this article, What to examine when considering a loft conversion, was featured in the August issue of i-build. 



Loft conversion by Harvey Norman Architects in Cambridge

Common design considerations you are likely to face and will need to resolve are:

  • Ceiling height
  • Access
  • Services
  • Lighting issues
  • Planning permission
  • Building regulations related to floor strength and fire escapes

All these points are covered in further detail within this article. Despite these design considerations a loft conversion can be life changing and a cost effective solution to lack of space or facing up to the challenges of moving house. 


Establishing if your property is suitable for a loft conversion

Not all lofts are suitable for a loft conversion. Before going too far into planning your yourloft conversion project, we suggest you carry out a brief survey that checks the following:

  1. Roof structure — Is it  traditional cut rafter and purlin roof or a trussed roof? (See diagram on the left and read about roof structures below).
  2. Height — Is there enough height within your loft? Note the minimum height for a traditional roof is 2.2 to 2.4 meters and the minimum height for a modern trussed roof is 2.4 to 2.6 meters.
  3. Space — Is the loft space large enough to provide a usable room?
  4. Chimneys or services — These don't pass through the loft space and will not need moving
  5. Felt — Your roof has felt under the tiles or is fully weather tight. Note that if you don't have felt you will see the back of the roof tiles and the battens they are fixed to. If you have felt you will most likely see a black bituminous paper under the tiles and battens. Most modern houses will have felt. 

If you are confident about all these checks then you have passed the first stage of assessment. Your property may well be suitable for a loft conversion and you can proceed to assessing what kind of loft conversion you could have.

Do not despair if one of these five points highlights a problem. This doesn't necessarily mean you have to throw your loft conversion plans out the window. There is always an answer and we can help you find it.


get help understanding your roof structure and the options you have available

3 types of loft conversions to consider

What type of loft conversion is most suitable for you will be influenced by three main factors:

  1. The type of roof you have
  2. What you want to use your loft for
  3. Your budget

There are three types of loft conversions to consider. These are internal loft conversions, dormer loft conversions and loft conversions that require a full removal and build.

Internal loft conversions are usually the cheapest and require minimum building intervention. However, dormer loft conversions are the most common type of loft conversions because of the additional space they can provide with relatively simple building works. Full removal and build loft conversions will give you the most flexibility but they are the most complex and most expensive. 

1. Internal loft conversions

Roof Lights

These are the most cost effective loft conversion as they involve converting the existing loft space. Very little alterations to the roof space are needed. Additions include windows set into the existing roof slope, insulation and strengthening of the floor.

2. Dormer loft conversions

With dormer loft conversions, dormer windows are added to increase the volume of the roof space while providing full head height. Dormers are usually added to the rear, but subject to planning permission, they can be added to the side or front of your property. 

Following a loft conversion, the additional space can be used as an additional bedroom or two, a study or home office, a separate bathroom or a nursery. Or you can consider adding an en-suite or separate dressing room attached to a master bedroom. 

Single Dormer
These are favoured by planners in
conservation areas. If permitted, two of these can be used to increase space and add symmetry.

Side Dormer
A side dormer is often used to increase
head height for houses with a hipped roof, where access to the loft is located under that hip.

Involves one or more of the hips being replaced with a gable wall (where
the roof slopes in from the side(s) as well as the front and back). The roof is then extended over these gables to add extra space with full headroom.


Full Width Dormer
This type of loft conversion will really maximise space and achieve a completely different feel to any other type of loft space. 

L-Shape Dormer
These are only suitable on certain properties such as Victorian properties with an addition to the rear. The L-Shape Dormer provides a signifcant amount of additional space.

3. roof off Loft conversions requiring removal and rebuild

Mansard conversions

This type of loft conversion involves one or both slopes of the roof being replaced with a new structure with very steep sloping sides (almost as steep as the walls). An almost flat roof is placed over the top. This design is used where the original roof had little or no headroom and creates sufficient volume for an additional storey. Mansard conversions normally require planning permission.

Pre-fabricated loft

Where vertical space inside a roof is limited, the existing roof structure can be replaced with a larger one. “Room in the roof” trusses can be craned in place to form the shell. The roof can then be constructed around them. But living without a roof for over a month is no fun! But a pre-fabricated unit can be watertight in a couple of days.



Get help designing the ideal loft conversion to suit your property, needs and budget


The best loft conversion for the most space

The most popular type of loft conversion is the L-Shape conversion with owners of Victorian terraced houses and properties of a similar style. It involves constructing two dormers – one over the roof of the main house and a second above the rear extension. In the majority of cases, the second dormer will be constructed above what is usually the existing kitchen or bathroom. The two dormers meet to form a right angled ‘L’ shape (how this type of conversion came to be named).

The advantage of an L-Shape conversion is that it allows you to almost replicate your first floor in terms of space and design. It gives you the possibility for even three or four new rooms and no other type of loft conversion will give you as much additional space.

A loft conversion p lan of an L-shaped dormer 

A loft conversion plan of an L-shaped dormer 

An illustration of a L-shaped dormer loft conversion

An illustration of a L-shaped dormer loft conversion

Of course this is just an example and there are many different types of properties with varying loft conversion options resulting in different space advantages. 



Let's discuss your property and find out what additional space you could have with a loft conversion 


Loft conversion costings and budgets

What you pay for a loft conversion depends on the type you undertake. At current prices (August 2015) we would offer the following guide price per square meter. Note this excludes VAT and fees.

loft conversion Guide price per square meter

  • Simple rooflight conversion — £1,200 to £1,500
  • Dormer conversion — £1,680 to £2,400

Professional fees for your loft conversion

  • Architects fees — a typical £30,000 to £40,000 loft conversion would be in the region of £1,200 to £2,400 for planning drawings
  • Building regulations fees — Under 40m2 is £385 and 40 to 60m2 is £460
  • Engineer’s structural design fees — in the region of £600 to £1,800
  • Planning and certificate of lawful development fees — If you cannot carry out your loft conversion under Permitted Development Rights then a householder planning application costs £172. If you’re using your Permitted Development Rights, we advise you obtain a certificate of lawful development for £86. This certificate takes away any uncertainty and you can produce it when selling the property
  • Building Control fees — in the region of £960 top £1,200
  • Party wall arrangement fees — Budget about £850.00 per neighbour

Loft conversion building works

  • Heating — Radiators are typically £30 per m2 and underfloor heating is £45 per m2 
  • Boiler — If a new boiler is required with more capacity typical costs are usually between £1,800 and £3,500
  • Bathrooms — Budget in the region of £4,500 to £11,000
  • Decorating — Set aside £77.00 per square meter for plastering or dry lining and paint.
  • Flooring — Plan for £24.00 per square meter upwards


Get a more accurate idea of costs based on your specific loft conversion


planning permission and your loft conversion

You can extend your roof space by up to 50m3 (or 40 m3 for terraced housing) under Permitted Development Rights. This is providing the allowance hasn’t previously been removed or used up. This can save you the hassle of gaining planning permission, but there are strict limits to follow.

For example, no additions are allowed at the main elevation beyond the plane of the existing roof slope. Also, you can only use materials similar in appearance to the existing house.
Permitted Development Rights are removed for loft conversions exceeding the 40 m3 to 50 m3 space allowance, in conservation areas and in other designated zones, so you’ll always need full planning consent in here. 



Get help with planning permission and Permitted Development rights



Building Regulations, party walls and safety

Building regulations

Loft conversions always need approval under Building Regulations (irrespective of whether they need planning permission). It pays to adopt the full plans application approach and have a detailed scheme approved before you find a builder. Having an approved design will take much of the risk out of the work and also mean the builder has a chance to give you a fixed quotation, rather than a vague estimate.

Your Building Control officer will inspect the work at various stages. On the final inspection they should issue you with a completion certificate — don’t settle any final accounts until you’ve received the certificate.

Party Walls

If your house is semi-detached or terraced don’t forget to notify your neighbour(s) of your proposals. This requirement usually falls under the Party Wall Act 1996.


From April 2015 homeowners are now responsible for safety on their building projects – big or small. All projects will require a health and safety plan and you will be required to manage that plan.



get help with building regulations, party walls and safety requirements


Technical issues to be aware of when undertaking a loft conversion

The Roof Structure

The existing roof of your home is designed to keep out the rain and snow and to cope with light loft storage loading. After a loft conversion your roof will have to cope with
significantly different loadings. A new floor structure will be required. It is also likely that structural elements will need to be altered to allow for circulation within the room and roof windows. Roofs can generally be divided into two types.

  1. Trussed Rafter Roofs — These have been common since the 1970s and roofs of this type are difficult to convert. Roof trusses are complex pieces of engineering and they should not be altered without the advice of a structural engineer. When converting this type of roof it is common for a series of beams to be installed to provide support to the new floor and to strengthen the rafters. This allows the bracing sections of the trusses to be cut out to create a clear floor area.
  2. Traditional Roofs — These are generally made up from a series of rafters and purlins spanning between load bearing walls. These roofs are less complicated to convert than Trussed Rafter Roofs. However, beams are often required to provide support to the new floor structure and the existing purlins. A Structural Engineer’s design will be required for all but the simplest conversions.

Accessing your Loft Conversion

If you want to convert your loft for habitable use you will need to install a staircase. Careful design of this can be critical to the success of your loft conversion. If there is enough headroom it is often best to continue the stairs in the existing stairwell. This saves space and gives a feeling of continuity within the home. Alternatively part of a room will have to be partitioned off to accommodate the new staircase. Wherever the stairs are installed it should be designed in accordance with the following guidance:

  1. Fire Precautions — Fire precautions are a major concern for the Building Regulations. The most dangerous fires generally occur at night when everyone is asleep. When converting your loft you will need to ensure you have mains powered, interlinked smoke detectors in the hall and/or landing areas of every floor.

  2. Escape windows — These are windows large enough to allow people to escape or be rescued through them. They need to have a clear opening area of at least 0.33m2 and a clear width of at least 450mm. The bottom of the opening light should be no more than 1100mm above floor level and they should allow people to escape to a place free from danger. Escape windows need to be fitted with escape hinges that allow the window to fully open. Some of the standard hinges fitted to UPVC windows do not achieve this. It is wise to check this with your glazing supplier.

  3. Two Storey Houses — The Building Regulations requirements for fire precautions in two storey housing are quite simple. It is generally felt that if you couldn’t get out down the stairs you could jump or be rescued from a first floor window. If you are converting the loft of a bungalow you will need to ensure that you have mains powered interlinked smoke detection at ground and first floor level and that all habitable rooms at first floor level have an escape window.

  4. Three Storey Houses — When you convert the loft of a house and create a third floor the Building Regulations require you to look at the fire precautions a lot more seriously. Mains operated smoke detection need to be fitted. Due to the height of your new floor, you can no longer rely on escaping through the windows so the only safe way out is down the stairs. It is therefore vital that the stairs are protected from fire. To protect the stairs all of the doors that open onto the stairs need to be half hour fire doors and the stairs should discharge into a hall with a door direct to the outside. Alternatively, the stairs should deliver into a space with access to two external doors, separated from each other by fire resisting construction and fire doors. Generally, unless a sprinkler system or alternative escape stairway is provided, stairs cannot discharge into other rooms in three storey properties.

  5. Four Storey Houses — If your house already has three storeys, loft conversions become more complicated. Careful design and planning is required to ensure that fire risks are minimised. You are likely to need to install a sprinkler system or a second escape stairway. This type of project will need specialist design. 

Bathrooms in loft conversions

You may want to include a bath or shower room in your loft conversion. The best place for this is generally directly above your existing bathroom. This should ensure you can connect into the existing drainage and water supplies without the need for excessive pipework. Any bath or shower room will also need to be fitted with an extractor fan to improve ventilation. We recommend you decide on the location of any bathrooms at an early stage in your space planning process.

If you are going to fit in a bathroom in your loft conversion then ensure you follow these tips:

  • Place a shower where there is full headroom
  • A WC and a washbasin ideally need full headroom
  • A bath can be tucked under the eaves 
  • A wetroom can be a space-efficient option, but needs full tanking
  • Use the voids in stud walls for concealed shower and tap mixers
  • Concealed cisterns in metal frames for building into studwork are ideal
  • Good lighting and large wall-to-wall mirrors create the illusion of space
  • Wall-mounted sanitaryware helps make a small bathroom appear more spacious

Using a macerator system

If you want to add a WC in your new loft, but there is no suitably located existing soil stack, you will need to add a new one or, if this is too impractical or expensive, you can install a pumped macerator system.

A macerator is a small box which sits behind the toilet. It can be hidden behind a wall, although you will need access to it. The basic concept of a macerator is it uses rotating blades to chew up waste and reduces it so it can comfortably fit through your normal pipe work. A macerator can also be used to pump away water from your shower or sink.

While a macerator is a great way to get around tricky plumbing problems it does come with some drawbacks:

  • You must stick to the 'basics' and avoid flushing anything complex like sanitary items
  • It can be noisy and will continue to do its work for about 30 seconds after the flush
  • There have been some reports of blockages and smells coming from the unit 

However, there are recommended ways to use a macerator that will minimise the risk of these drawbacks.

Thermal Insulation

CO2 emissions are a major concern in today’s environment and you will need to provide a high level of insulation to your roof as part of your loft conversion. 

The most common way of achieving this is to place a high performance insulation board in between and below the rafters. Unless your roof has a breathable felt you will need to leave a void above the insulation and ensure that you have effective roof ventilation to prevent the build-up of condensation.

Sound Insulation

To reduce unwanted noise the walls and floor around bedrooms will need to be insulated to reduce sound transmission. This is generally achieved by placing 100mm of sound deadening quilt in the floor void and in the partitions around the bedrooms. If you are converting the loft of a semi-detached or terraced property you will need to ensure that the sound resistance of the Party Wall is upgraded so that sound transmission to your neighbours is reduced.


To maximise the usability of the room you will probably want to install heating. In most instances the most effective way of doing this is to extend the existing central heating system. You will need to check with your plumber or heating engineer to ensure that your existing boiler has sufficient capacity to serve any additional radiators. New radiators should be fitted with thermostatic valves to control the room temperature. If it is not possible to extend the existing system or if you prefer an alternative method of heating, (e.g. electric panel heaters), careful consideration should be given to how these can be controlled to ensure they function efficiently.


You are likely to require some electrical alterations as part of your loft conversion. Depending
on the age and condition of your existing electrical system it is sometimes possible to extend existing circuits. Sometimes new circuits and even a new distribution board will be required. It is a good idea to get advice from a competent electrician at an early stage. When appointing an electrician please ensure they are able to issue you with BS7671 test certificates when they have completed their installation. This will be required before your Building Regulations Completion Certificate can be issued and you will incur additional costs if the test
certificates have not been provided. 

Windows and Ventilation

Any new habitable rooms will need to be ventilated. Generally this is achieved by providing an opening window or roof light equivalent to 1/20th of the floor area of the room with a trickle vent at high level. All new windows must be fitted with highly efficient double glazed units. In bath or shower rooms an extractor fan should be fitted. Rooms without opening windows require extractor fans that are triggered by the light switch with overrun timers. This allows the fan to remain on after the light is switched off.


In a successful interior lighting scheme, different light sources should be combined. Light sources include:

  • Ambient lighting — substituting for daylight
  • Task lighting — for reading and working
  • Accent lighting — to add atmosphere

Good lighting options on sloping ceilings (which you will have in a loft conversion) include down lights and track lighting. A section of flat ceiling beneath the ridge or within a dormer window is the ideal surface for down lights. Where the ridge is higher, it may be possible to suspend pendants or use a track lighting system. Ambient lighting can be achieved with floor and table lamps, providing they are on a switched lighting circuit so that they can be controlled, and ideally dimmed from the main wall switches.


Conclusions about building a loft conversion

A successful loft conversion is a definite asset to your home. It can provide useful extra space
and add value to your property. However a poorly converted loft can reduce your property’s value and in some cases compromise your safety and the structural integrity of your home. 

As you can see there are many elements that you ought to consider when planning a loft conversion. You can use this guide and manage a loft conversion yourself. However, if you want to make sure it's done right, its generally a good idea to hire a professional Architect to ensure you have a quality loft conversion that stands the test of time and adds value to your property.

A recommended start to any loft conversion is our Kickstart Consultation which quickly gets to grips with design, costs and planning. we mainly work with clients in cambridge, st albans and throughout east anglia and london.

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