How much does it cost to build a house or extension — estimating costs for domestic building projects


Why it's impossible to give a straight answer to the question, "How much does it cost to build a house or extension?"

Whether its a new self build house loft conversion, house extension or internal alterations, estimating building projects can be a complex process because build costs depend on so many factors. The reality is you will only get an accurate idea of the price for your project when all drawings and schedules of work have been completed. These outline your particular requirements, the type of house construction and any other factors specific to your project.

The problem is, people want to know about building costs before they know about the project specifics or have a clear project brief! This is quite understandable, so we created a tool you can use to help you estimate the cost of your building project in the early stages.

However, remember it is difficult to be accurate on build costs at an early stage of a project idea. This is because details of what is being included and what is not is usually quite vague at initial stages of the project. So, although rough estimates may be useful, we recommend you are careful of ball park figures that may be thrown at you early on as these can vary significantly.

You can only begin to get some price certainty when you have a building design and plan, and the project is split into its various elements, which can then be measured and costed out. 

This article explains how we help clients through this difficult first stage of building projects. The advice will be useful to anyone undertaking an extension, loft conversion or new build 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 guidance on costings for your house extension, loft conversion or new build. You can contact us here.


To answer the question, "How much does it cost to build a house or extension?" you must understand that cost certainty comes from design certainty

Construction projects start with an idea in the client’s mind but very little else in terms of key information to take the project forward. However, cost certainty only happens as a result of design certainty, and to get to that stage a project will have to progress through several stages.

At the inception providing costings for an ‘average’ build cost can be tricky — since there really is no average! Issues such as ground conditions, access to the site, location and proximity of services can be a huge factor in the build budget. Remember the cost process evolves, and is only as reliable and accurate as the inputs.


The eternal triangle and how it affects the question, "How much does it cost to build a house or extension

From the client’s perspective, there are usually only three criteria on their agenda — cost, time and quality. The client always wants the highest quality, at the lowest cost, in the shortest time. Unfortunately, this is not always possible. Compromise, based on priorities will be needed.

If cost, quality and time are kept in balance, the triangle will appear equilateral. However, if circumstances dictate that one of the factors must take precedence, the other two will “suffer”.

  • If quality is paramount, adequate time must be allowed for the design, specification and construction to be perfected and cost will rise on all three counts
  • If speed of completion is paramount, quality may reduce and cost may go up
  • If lowest cost is the priority, time may not be prejudiced, but quality will suffer
The effect of the different priorities placed on cost, time and quality is relative. There is no reason why, with proper planning and management, those elements with a lower priority cannot be adequately controlled. It is therefore of vital importance that the cost, time, quality strategy is discussed at the earliest opportunity.

How to establish a budget that will answer the question, "How much does it cost to build a house or extension?"

Initially, it is usual to base the pricing of building estimates on historical cost data gleaned via experience and from the analysis of the costs of similar buildings, and adjusted to take account of any fluctuations in price or for project idiosyncrasies.

The required levels of accuracy will vary at the different stages of the project. Design decisions made during the very early stages of a project are more global and tentative than those made at later stages. The accuracy of a cost estimate will always be a reflection of the quality and detail of the design information available at the time of estimation.

Cost plans develop, in detail and accuracy, as the project progresses. This is because

  • More information is added
  • The nature of the design is confirmed
  • Actual prices provided by specialist builders and suppliers come in

1. Initial cost appraisal

Initial cost appraisals which we call Kick Starts are produced when there is little or no design information available. It is usual to base such an estimate on a historical net area cost / m2 data from similar past projects, adjusted with known inclusions and exclusions, based on the experience of the estimator, and taking into account known variables of the particular project. Examples of these variables might include  

The site context and site constraints, for example, ground conditions and proximity to neighbouring buildings

  • The type of existing building and state of repair
  • Access for builders
  • Existing drainage and services
  • Demolition requirements
  • Level of internal alteration required
  • Level of upgrade required to services, heating, etc
  • Level of external works required
  • Fixtures, fittings and equipment and related specification levels
  • Fees for consultation, planning, building control, etc
  • VAT

Arriving at an extension and renovation build cost estimate using our Kickstart service

Below is a simple guide intended only as a starting point. The actual eventual cost will depend on a myriad of factors such as access, availability of contractors, complexity of the works and the client's specification. Harvey Norman Architects have developed the diagram and budget descriptions as a guide to the possible costs per square meter for overall construction costs excluding VAT.

Note that house refurbishment and renovations costs are harder to estimate on a square meter basis than new builds because different areas of the house are likely to require different levels of attention. Furthermore, the home-builder cannot be sure of the exact condition of an existing house until work begins and the structure and services are uncovered. 

The diagram shows a way to break down the areas on a typical terraced house. The higher costs relate to the extension where the build is new, from the foundations to roof to fit out. The next highest cost relates to associated areas where the existing structure is being adapted for an open plan layout and new plastering, flooring and services for a relocated kitchen will be required. Along the front of the house refurbishment work is less intensive although new services will be fitted along with making good to existing rooms. 

Note that construction prices will vary considerably depending on the build route you take and quality level you insist on.

Important note — the figures quoted above are not universal, they are indicative only for the purposes of this article. Each project is different, subject to its own constraints.


2. Budget Elemental Cost Plan

Assuming approval has been given by the client to move into the design development phase, as a minimum the following information should be produced

Sketch plans

  • 1:100 plan of each floor
  • 1:100 elevations
  • 1:100 sections
  • 1:200 site plan showing extent of external works and location of external services

Brief notes relating to the extent and / or standard of the

  • Substructure
  • Frame
  • Upper floors
  • Roof
  • Stairs
  • External walls
  • Windows and external doors
  • Internal walls and partitions
  • Internal doors
  • Wall, floor and ceiling finishes
  • Fittings and furnishings
  • Internal services such as sanitary fittings, disposal, water, heating, electrical, gas, security, media, etc
  • External services such as disposal, water, electrics, gas, telephone, cable, etc
  • External works such as roads, pavings, walls, fences, gates, water features, etc

The elemental cost plan is broken down into these series of standard building elements. Initially, it will simply be the total construction cost for the project divided into elements on a percentage basis. As the design develops, the plan will be 'measured', based on the actual quantities of work and materials required. 

The elemental cost plan is prepared in a spreadsheet format that is easy to read. The plan should also set out any assumptions made and should identify any exclusions. It might also include a list of non-standard items, to help inform any value engineering.


3. Detailed Elemental Cost Plan

A Detailed Elemental Cost Plan is simply an updated Budget Elemental Cost Plan, now including fully measured quantities. This is particularly useful to do if the design was not 'fixed' at the previous Initial Cost Appraisal and Budget Elemental Cost Plan. At this stage, Building Regulations drawings are usually available so the final version of the cost plan can be produced. 


4. Traditional Bill of Quantities (BOQ) or APPROXIMATE BILL OF QUANTITIES (ABOQ)


The use of Bills of Quantities is the oldest construction procurement practice in the UK and possibly the most widely understood. They include definitive measurements of the major elements of a building. Provisional sum allowances for both undefined and defined works can also be included, plus provisional sum allowances for works to be executed by ‘named’ specialist sub-contractors, to then be employed as ‘Domestic sub-contractors.' It has a range of advantages

  • Estimating risk is lower
  • Competition is fairer, as all contractors are tendering on the same basis
  • Allows the total price to be to be analysed in great detail based on the same information
  • Provides the best means of controlling the cost of variations on the contract
  • Assists project management on site, particularly for the ordering and management of subcontractors and materials
  • Measurement before the tender is a useful test as to whether what has been drawn and specified is complete and helpful in identifying problems not apparent at first sight
  • If required it can include Design and Build and Performance Specified works

Its disadvantages are

  • For the work to be measured, the design must be substantially complete
  • The process of measurement can be time-consuming
  • Errors in the BOQ are required to be corrected at the client’s risk

On balance, the advantages of the traditional measurement of quantities are overwhelming but there are alternatives that can be used to obtain a tender at this stage of the project. These are

Approximate Bill of Quantities (ABOQ)

Bills of Approximate Quantities are an alternative to a Full Bill of Quantities. They are usually produced when one or a mixture of the following possible advantages are required

  • The need for costs before a firm design is available
  • A quick start on site and an early appointment of a contractor is required
  • The proposals are unusual, and / or complex,
  • Design and Build and performance specifications, if needed, can be incorporated
  • The project involves the alteration of an existing building

This type of estimate is prepared on the basis of individually priced approximate quantities that are inclusive of all labours. Only the main item of work need be measured. Wherever possible items having similar measurements, e.g., hardcore, blinding, damp proof membranes, insulation, reinforcement and concrete slabs, can all be grouped together. Relatively low-cost complicated elements of work can be itemised and individually priced.

However, the disadvantages are that they do not set a fixed cost for the works when the contractor is appointed, resulting in less price certainty. This is because the actual cost is only known when the design is available and detailed re-measurements have been made. Thus, the client proceeds to the construction stage at greater risk.


Schedule of Works

Schedule of Works are instructional lists of the main sections and heads of work without quantities. They can be arranged on an elemental basis (for example groundwork, concrete, masonry, etc.) or on a room by room basis.

The Schedule of Works must be read together with the specification and the drawings and are priced, on a lump sum basis, by the contractor. Quantities if given, are for guidance only, and the contractor will make his own measurements as part of his tender.

Often as well as the Schedule of Works, the contractor is requested to price a schedule of rates relating to material, which can help in the valuation of variations.


  • Risk to client can be avoided, as the contractor does his own measurements
  • Provides a firm lump sum price subject to any later variations
  • Design and Build and performance specifications can be incorporated
  • Provisional sums can be included as can ‘named’ specialist sub-contractors


  • For the work to be measured, the design must be substantially complete
  • Can be difficult to agree on a basis of variations
  • Tenders are not readily comparable to each other
  • Use is not particularly appropriate where change can be predicted

What a Quantity Surveyor offers as part of the design team

In most cases, the support you will need to understand and identify costs, as well as examine and challenge the costs received from the builders will come from a quantity surveyor. Bringing one on board proves invaluable in terms of cost advice, appraisal of incoming information and establishing the framework for consistent and reliable costing. The savings in terms of efficiency, accuracy and fixed-price certainty with the avoidance of unforeseen extras, usually justifies the fees. Harvey Norman Architects have a panel of quantity surveyors which we work with on a regular basis and are happy to recommend to clients. Some of the example documents for this article have been provided by Trinity Place Consultants who we work with on a regular basis.

Contact us on 0800 014 1732. We can help you take your first steps towards planning and costing your new build, house extension or loft conversion in Cambridge, St Albans or throughout East anglia and london

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