How much does a house extension cost in the UK?
The short answer
There is no single reliable figure for a house extension, because the price is driven by decisions that have not been made yet. Size, ground and structural conditions, specification, services, access and the level of finish all move the number substantially, and regional labour rates move it again. The most useful thing you can do early is define the work precisely, so the quotations you receive are comparable rather than guesses.
Key points
- Cost per square metre figures found online are averages, not quotations for your property.
- The specification — what materials and standards you are asking for — often matters as much as the floor area.
- Groundworks and structural work are the most common source of unpleasant surprises.
- Professional and statutory costs sit outside the builder's price and are easy to forget.
- A contingency is not pessimism; it is part of a realistic budget.
- Undefined scope produces uncomparable quotes and disputes later.
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Build your extension budget in five parts
Use columns for item, amount, included in builder quote, confirmed or allowance, and who supplies it. This helps prevent counting an item twice or overlooking it. Review your total whenever the scope changes.
- Construction: the written builder price for the agreed scope, with VAT treatment stated.
- Separate costs: design, surveys, approvals and specialist advice where needed; check which are already included.
- Your purchases: kitchen, appliances and finishes excluded from the builder price, including delivery and installation.
- Living arrangements: any temporary accommodation, storage or additional travel.
- Contingency: a separate allowance for unresolved risks and a record of agreed changes.
Why extension costs vary so much
A builder is pricing labour, materials, plant, preliminaries, risk and profit against a defined piece of work. Change any of those inputs and the price changes. Two homeowners describing 'a 20 square metre kitchen extension' can be describing jobs that differ by a very large margin.
The main cost drivers
Size and shape
Floor area is the obvious driver, but shape matters too. More external wall, more corners, more roof junctions and more glazing all add cost relative to a simple rectangle.
Location
Labour rates and subcontractor availability vary considerably around the UK. Parking, congestion charges, restricted delivery hours and skip permits also differ by area.
Specification
The written specification decides whether you get standard or premium windows, basic or high-performance insulation, a simple or complex roof build-up, and so on. This is the single biggest area where quotes drift apart, because each builder is otherwise free to assume their own standard.
Structural work
Foundations, ground conditions, trees, drainage runs, retaining structures, steel beams and openings into the existing house are all significant. Some of this cannot be fully known until work is opened up, which is why contingency exists.
Finishes
Flooring, joinery, tiling, kitchens, sanitaryware, decoration and ironmongery cover an enormous price range for exactly the same floor area.
Services
Heating alterations, hot water, electrical distribution, lighting design, ventilation, drainage connections and any relocation of existing services all add cost, and often need a competent person or Building Control sign-off.
Access and site constraints
Rear gardens with no side access, terraced properties, sloping sites and limited storage space all slow work down and increase labour time.
Professional and statutory costs
Design, structural engineering, planning application fees, Building Control charges, party wall matters, surveys and utility company charges are usually separate from the builder's figure. Budget for them explicitly rather than absorbing them into a guess.
Contingency
Set aside a sensible allowance for the unknowns that appear once work starts. Agreeing in advance how variations will be priced and approved is as valuable as the money itself.
Define the work before you compare prices
If three builders price three different interpretations of your project, the cheapest number usually reflects the smallest assumption rather than the best value. A written scope of works, with exclusions stated, turns three unrelated figures into a genuine comparison.
It also protects you during the build: when everyone agreed in writing what was included, a variation is a conscious decision rather than an argument.
Things to check
- Is every quotation pricing the same scope, or has each builder assumed their own?
- Are professional fees, statutory fees and utility charges accounted for outside the build cost?
- What is excluded from each quotation, and who pays for those items?
- How will variations be priced and approved once work starts?
- Is there a contingency in your budget, and do you know what would trigger it?
- Are provisional or provisional sum items clearly identified as not yet fixed?
Start by defining the project, not the price
The KEZARIQ Planning Assessment helps you set out what your project actually involves and what still needs to be resolved, so later cost conversations are based on a defined scope. KEZARIQ does not provide construction cost estimates or guarantee any price you are quoted.
Assessment Project Report — £10.99
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Important
This guide is general information for UK homeowners. It is not legal, planning, structural or professional advice, and it does not take account of your individual circumstances. Rules change and vary between England, Wales, Scotland and Northern Ireland, and by local authority.
KEZARIQ provides guidance and documents you complete and review yourself. It does not manage your project, carry out professional services, or replace an architect, surveyor, structural engineer, planning consultant or other competent professional where one is required. Nothing here guarantees planning permission, Building Regulations approval, prices, contractor performance or any project outcome. You remain responsible for your own project and decisions.