How To Calculate The Cost of a Home Extension
Top tips from an experienced architecture firm
There is no quick way to calculate the cost of a house extension or refurbishment. Also, you will never truly know how much the cost of an extension on your dwelling will be until you have a full set of plans and a detailed Scope of Works drawn up with 3 to 4 tender quotes from contractors! But that’s not all.
You will also need to research and choose a new kitchen, bathroom finishes, sanitary ware, as well as many more variables. You may already have to pay £1000’s to architects and design consultants and hours of time to get to that stage.
We would always advocate paying a Quantity Surveyor (QS)/Cost consultant to produce a cost estimate.
All this sounds rather expensive and time-consuming. Is there a quick way to anticipate your refurb or extension costs for free? Yes, and we’ll explain more in this article.
How Much Does An Extension Cost Per m2?
In short, build costs per m2 will vary wildly. This cost of house extension guidance is by no means definitive, but it will give you a ballpark figure in which to help you move forward.
Let’s start by saying that this article is aimed mainly towards projects in London and the South East. Labour costs and some materials will vary depending on where you are in the country – we can only advise on our patch!
However, those of you outside London and the South East will still benefit from reading this, and we’re sure you will gain some valuable tips on what your house extension or refurbishment cost will be. Later in the article, we will show some real-world examples to help you piece together the information.
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House Extension Cost Overview – Estimates (2022 UPDATES!)
The table below gives an overview and estimated price range for how much your house extension or refurbishment cost may be based on our experience. These figures should be used as guidance only.
New Extension (single storey)
£2,000 – £3,000
Per m2, additional floor area
The lower range would be a basic construction with uPVC windows/doors. The higher range would be a more complex construction with specialist glazing. Note that small extensions like side-return extensions will likely have a much higher £/m2 rate.
New Extension (additional storey)
£1,200 – £1,600
Per m2 additional floor area
When building a second storey, you no longer have the groundwork costs, which can be considerable
Major Structural openings (streel frame)
£3,000 – £10,000
When creating an open-plan kitchen/living area, for example, this cost would be relevant to removing the house’s existing ground floor rear wall.
£1,500 – £2,500
Per m2 of additional floor space (above 1500mm ceiling height)
The cost of this will rise with the complexity of your existing roof structure or if its a new extension completely. If your roof and proposed layout are complex, it may be worth considering removing the existing structure completely and installing a new roof.
Basement (under existing house plus underpinning)
£3,000 – £4,500
Per m2, additional floor space
Site accessibility and logistics will play a large part in your basement extension hence the wide range in cost.
Basement (under garden)
£2,500 – £3,500
Per m2, additional floor space
This can be a great option in conjunction with walk-on roof lights to let light into your new basement space.
Internal refurbishment (including minor structural works, e.g. new door openings)
Up to £500 – Cosmetic ¦ £500-£1,000 – New wiring/plumbing and back to brick ¦ £1,000-£1,400 – including layout changes and minor structural work
Per m2 refurbished floor area
Back to brick? New wiring/plumbing? Or just a cosmetic uplift? This cost will vary greatly.
£2,000 – £5,000
Depending on height, new structural details, site access, ownership, type of construction.
New Bathroom (3-piece fully tiled)
£3,500 – Shell exc Finishes below ¦ Sanitary ware and tile Finishes: £2,000 – basic ¦ £5,000 – mid-range ¦ £10,000+ – high-spec
‘Shell’ would include carpentry, plumbing, and electrics.
We have given an indication of how much you are likely to spend depending on the level of finish.
£6,000+ for basic spec ¦ £15,000+ for mid-range ¦ £40,000+ for high-spec
Generic 20 kitchen units including appliances
Assumptions: Basic – e.g. Howdens/Magnet, basic appliance spec, laminate worktops Mid-range – e.g. Wren/IKEA, better appliance spec (e.g. siemens), stone worktops High-spec – bespoke/designer manufacturer, top appliance range (e.g. Miele), stone worktop.
What Will Affect My House Extension Cost?
Drainage Location and Diversions
If you’re building over or near a Thames Water sewer that is classified as ‘Public’, you will likely need to submit relevant drawings and details to apply for a Thame Water Build-Over Agreement. We explain more about these in our blog.
This will affect the type and depth of your foundations. Up to 25% of your overall budget may be spent in the ground! So doing your homework on ground conditions can really help nail down a realistic budget. A soil investigation will be required, or your structural engineer will advise on the likely ground conditions.
Tress absorb groundwater and can affect the shrinkability of the soil surrounding your house. This may require deeper foundations – therefore, adding extra cost. For example, Leylandii trees in a London clay soil close to your proposed extension can require 2.5m deep foundations and, in certain scenarios, make trench foundations unfeasible. We recommend that you find out which trees are in your garden, and how deep your foundations may need to be using the handy NHBC Foundation Depth Calculator.
Period and older properties will often bring unexpected costs and uncover hidden problems that were unforeseen. Therefore, if you are carrying out building works on older properties, ensure you have a healthy contingency in your budget and ensure you carry out a full structural survey of the property as early as possible.
This is where a good engineer can save you money. There will be many ways to design your supporting structure, and a good engineer will design a lean scheme with due regard to an easy installation on-site and good detailing in conjunction with your architect.
This is another topic in itself that relates to the level of project management you wish to undertake yourself, the level of risk, your building contract, and many other factors which will form the topic of another useful article from AURA Architecture & Interiors.
Having a good relationship with your neighbours can keep your project moving swiftly, especially when your works require Party Wall Agreements in compliance with the Party Wall Act 1996.
Health & Safety
You have obligations as the homeowner under the Construction, Design, and Management Regulations (CDM) 2015. Your contractor will need to be aware of these as they will have the greatest responsibilities during your build project. Incorrect health and safety on-site can cause delays and even fines. We recommend that your contractor has sufficient costs in his tender to fulfil his duties.
What Professional and Statutory Fees Form Part Of My Refurbishment Cost?
Fees – Structural Engineer
This will vary depending on the complexity of the design. Guide costs below (VAT not included):
|Simple loft conversion or single-storey rear extension||£1,000-1,500|
|Combination of the above, side and rear extension||£1,500-2,000|
|Combination of the above + internal structural alterations||£2,000-3,000|
|Major works to 4+ bedroom property||£3,000+|
Fees – Party Wall Surveyor
You will need to notify your neighbours of any relevant works being carried out under the Party Wall Act 1996. More information can be found here, but in general, if you’re installing foundations within 3m of your neighbour’s property or putting steel beams within party walls, you will need to notify your neighbour and potentially draw up Party Wall Agreement. This can be a large part of your house extension cost if surrounded by leasehold flats. All leaseholders and freeholders need to be served a Party Wall Notice.
Cost: To serve notice, circa. £200. From £900 up to £2,000+ Per Award (if needed)
Fees – Drainage CCTV survey
If you do not know the routes and flow direction of your property’s drainage, we’d suggest paying for a CCTV survey. This will give us the location and direction of drainage, manhole invert (depth) and cover levels, and detail whether there is any damage to the pipes.
Fees – Thames Water Build-over Agreements
If you’re building over or near a Thames Water sewer that is classified as ‘Public’, you will likely need to submit relevant drawings and details to apply for a Build-over Agreement. Most residential applications will be for pipes up to 160mm in diameter. There will be additional work for us to generate the design documents and manage the application on your behalf.
Fees – Planning Permission
|Householder Planning Application Fee||£206|
|Certificate of Lawful Development||£103|
|New Dwellings||£462 for each house|
Other fees may apply. A full Planning Fee Schedule can be downloaded from the Planning Portal.
Fees – Building Control
We will generally recommend instructing a private Approved Building Control Body instead of the Local Authority Council (*our preference only). A Building Control application is split into two sections.
This is where our drawings and the engineer’s calculations are submitted to the inspector, and a desktop study is carried out to check compliance with the Building Regulations Approved Documents. Payment for this is generally required on the submission of the application.
These are carried out at various stages of the build to check progress on site. The inspector will check compliance with the Building Regulations and comment on any alterations needed. Payment for this is generally required following the first inspection.
|Simple loft conversion or single-storey rear extension||£300||£350|
|Combination of the above, side and rear extension||£400||£500|
|Combination of the above plus major internal structural alterations||£500||£700|
|Major works to 4+ bedroom property||£600||£900|
Note: The above house extension costs are for guidance only. Your local authority website will have a full fee schedule for you to download. Alternatively, an Approved Inspector will provide quotes on a project-by-project basis.
Dont forget! Add 20% VAT and 10-15% on top of your builders quote for Professionals like AURA!
Our build cost analysis of 100’s of projects suggested an average of 12% in professional and statutory fees above the base build costs from your builder
Start your home extension journey by Creating Your Design Brief below…
Build Cost – Example Case Studies
Case Study 1 – Large single-storey rear house extension costs (with internal reconfiguration and cosmetic upgrade)
The table below gives an overview and estimated price ranges for how much your house extension may cost.
|GF – New works||Basic rectangle, high-end doors/glazing||36.4||£2,500 per m2||£91,000|
|GF – Major structural opening||unit||£10,000||£10,000|
|GF – Remodel||minor structural alterations, new elec/plumbing, cosmetic upgrade||65||£750 per m2||£48,750|
|FF – Internal works and additions||minor structural alterations, new elec/plumbing, cosmetic upgrade||30.3||£750 per m2||£22,725|
|FF – Cosmetic upgrade||Fresh paint, new carpet, new rads||31.2||£250 per m2||£7,800|
|2F – Bathroom remodel (mid-range)||4-piece mid-high spec, existing shell||unit||£10,000||£10,000|
|2F – Cosmetic Upgrade||Fresh paint, new carpet, new rads||33.5||£250 per m2||£8,375|
|Kitchen and Joinery||£45,000|
Case Study 2 – Side-return Extension and Kitchen Refurbishment
This type of project is most common in London and often misleading when taking a £/m2 value because of the large number of engineering works needed to create a small amount of additional floor area in order to create that stunning open-plan kitchen/living space.
Based on our guidance above, a value of £3,000/m2 would be taken here for the extension works. The additional floor area is approx. 15m2, so the guideline cost of this structural work is £45,000. Taking a figure of £750/m2 for the refurbishment of the remaining 20m2 = £15,000, plus approx.. £10000 for the major structural opening. But this would fall short of what this could be built for (!) because of the large number of works needed in just a small area. It skews the figures significantly.
The below breakdown gives a better reflection of what you should expect for this type of side return extension, bespoke glazing and large rooflights.
|GF – Side Return Extension||Roof glazing, high-end doors.||15||£3,000 per m2|
|GF – Major structural openings||unit||£10,000|
|GF – Refurbish||new elec/plumbing, a cosmetic upgrade||20||£750 per m2|
|GF Side Return Extension||Our experience shows this cost is likely to be more in the region of||£100,000-120,000*|
|*Excludes Kitchen and VAT||The kitchen could be anywhere between £10,000 – £80,000+ (as is the case with this Tom Howley Kitchen shown)|
We’d suggest budgeting circa. £120,000 – £150,000 for this side-return and refurbishment to include VAT and Professional Fees.
Case Study 3 – Rear and Side-Return Extension, Loft pod extension and Full Refurbishment
The table below gives an overview and estimated price ranges for how much this house extension may cost.
|GF – New works – Rear/Side Extension||Basic rectangle, high-end doors/glazing||45||£2,500 per m2||£112,500|
|GF – Major structural opening||Larger than normal structural spans||unit||£15,000||£15,000|
|GF – Remodel||Minor structural alterations, new elec/plumbing, back-to-brick||84||£500 per m2||£42,000|
|FF – Internal works and additions||Minor structural alterations, new elec/plumbing, a cosmetic upgrade||84||£300 per m2||£25,200|
|FF – New bathrooms/utility||4-piece principal bathroom (£10,000), 3-piece bathroom (£3,000), Laundry (£2,000)||unit||£15,000|
|2F – Loft Extension||Plus 3-piece mid-range en-suite (£3,000)||16||£2,500 per m2||£43,000|
|2F – Internal works and additions||Minor structural alterations, new elec/plumbing, a cosmetic upgrade||60||£300 per m2||£18,000|
|2F – New Bathrooms||2x 3-piece mid-range (£6,000)||unit||£6,000|
|Kitchen and Joinery||£45,000|
Add 12% in professional and statutory fees (circa. £40,000) and we’d suggest budgeting circa. £425,000 for this large home transformation.
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