Cost Guide · 2026
How Much Does a House Extension Cost in 2026?
Real 2026 price ranges for single-storey, side return, wrap-around, double-storey, and kitchen extensions across London and the South-East. Based on live project data from RCB's build programme.
Price Ranges
House extension costs by type — 2026
Extension costs in London and the South-East vary significantly by type and size. The table below gives honest 2026 ranges for the most common extension types, based on a mid-specification finish, proper structural engineering, and Building Regulations compliance. These are not builder website minimums — they are what projects actually cost.
| Extension Type | Typical Cost Range |
|---|---|
| Single-Storey Rear (4×5m) | £40,000 – £60,000 |
| Side Return Extension | £30,000 – £45,000 |
| Wrap-Around Extension | £55,000 – £80,000 |
| Double-Storey Extension | £70,000 – £110,000 |
| Kitchen Extension | £45,000 – £65,000 |
These are ballpark figures only, provided for high-level budgeting. Not a formal quotation. Actual costs vary depending on design, compliance, procurement, and site complexities. A detailed estimate requires a site visit and full drawings.
For context: a good single-storey rear extension in an accessible London location, designed to Building Regulations, with structural steels, bi-fold or patio doors, and a properly finished internal connection to the existing property, is realistically a £45,000–£55,000 project in 2026. Projects quoted materially below this are either missing scope or carrying risk.
Cost Drivers
What affects the cost of a house extension?
Size and floor area
The single biggest cost driver. Extension cost broadly scales with floor area, though not linearly — fixed costs (groundworks, roof structure, connections to the existing building) are spread across the total m². A 30m² extension is not 50% more expensive than a 20m² one.
Foundations and ground conditions
Standard strip foundations work on most London clay soils. Where ground conditions require deeper or wider excavation, raft foundations, or pile caps, costs increase significantly. Drain locations and sewer diversions also affect foundation design and cost.
Structural openings
The bigger the structural opening into the existing house — to create the open-plan space most clients want — the more steelwork is required. Twin steel beams on padstone pockets, with structural calculations and Building Control inspection, are the norm for wide openings. Heavily loaded spans or old lintels add complexity.
Glazing specification
Bi-folding doors, aluminium sliding doors, and roof lanterns can move an extension budget by £8,000–£20,000 depending on specification and number of panels. Budget aluminium is significantly cheaper than premium-brand thermally broken systems. The quality difference is visible for 20+ years.
Drainage and utilities
Extensions that cross existing drains, require sewer diversions, or need new drainage runs add real cost — drainage engineers, CCTV surveys, and sometimes Thames Water involvement. Gas and electrical diversions add further. These are rarely visible from a desktop assessment.
Party wall and planning
Permitted Development extensions are faster and avoid planning fees. Extensions requiring full planning permission add time, professional fees, and uncertainty. Party wall obligations on attached properties add legal and surveyor costs. Conservation areas add design constraints that affect both cost and programme.
What You Get
What's included in the price
Not Included
What's NOT included
RCB separates the structural build cost from client-specified finishes so you can see exactly where the money goes. The following are costed separately, client-supplied, or excluded:
Budget Planning
Additional costs beyond the build
A complete project budget for a house extension includes professional fees and statutory costs on top of the build contract. Budget for:
For a typical £50,000 single-storey rear extension, expect to add £7,000–£12,000 in additional costs above the build contract. RCB provides a full all-in project budget at the review stage so you know the real number before committing.
Get an accurate quote for your extension
Send your address and brief — we'll come back with an honest assessment within one working day.
FAQs
Common questions about house extension costs.
How much does a house extension cost in 2026?
A house extension in London and the South-East costs between £30,000 for a smaller side return and £110,000+ for a full double-storey extension. The most common project — a 4×5m single-storey rear extension — typically costs £40,000–£60,000 including structural works, roofing, and internal fit-out to first-fix stage.
Do I need planning permission for a house extension?
Many single-storey rear extensions fall under Permitted Development rights and do not require a planning application, provided they meet size limits (generally up to 4m for detached, 3m for semi-detached and terraced) and material conditions. Double-storey extensions, side extensions visible from the street, and properties in conservation areas usually require full planning permission. Building Regulations approval is always required.
How long does a house extension take to build?
A typical single-storey rear extension takes 10–16 weeks from site start to completion. Double-storey extensions run 16–24 weeks. This excludes the pre-construction phase — design, planning, and Building Regulations submissions — which typically adds 8–20 weeks depending on your local authority.
What is a party wall agreement and do I need one?
If your property shares a wall with a neighbour and you are carrying out excavation within 3 metres, or building on or close to the party wall, you are legally required to serve a Party Wall Notice under the Party Wall Act 1996. This is not optional. Typical professional fees for a party wall agreement are £900–£1,500 per adjoining owner.
How much value does a house extension add?
A well-designed, properly built house extension typically adds 10–20% to a property's market value in London and the South-East. Kitchen extensions that open up to garden space and double-storey extensions adding bedrooms and bathrooms tend to deliver the strongest returns in the current market.
Trusted. Accredited. Verified.
Independently vetted by the UK's leading construction accreditation bodies
FMB
Federation of Master Builders
TrustMark
Government Endorsed Quality
Checkatrade
Vetted & Reviewed
TrustATrader
Trusted Trader Listed
Ready to discuss your extension?
Book a free project review or send your drawings for a fully itemised estimate. No lump sums — clear scope and clear costs.