Free House Extension
Cost Calculator 2026
Our free house extension cost calculator gives you instant 2026 cost estimates for every extension type — single storey rear, double storey, side return and wrap-around. We update all rates quarterly to reflect London market conditions. Furthermore, you can switch to Total Project Cost to see the real all-in budget including VAT and professional fees. No sign-up required.
What a House Extension Actually Costs — Total Project Budget
The builder's quote is only part of the picture. In fact, research shows total project cost typically runs 40% above the construction price. However, many homeowners plan their budget around the build cost alone and consequently face a shortfall when VAT and fees arrive. Below is a real breakdown for a typical 25m² single storey rear extension in outer London at good specification.
Build Cost Only View
Total Project Cost View
Example calculation: 25m² single storey rear extension, Outer London, good quality, 1 kitchen. Rate: £2,650 × 1.15 = £3,047/m². This all-in total is consequently the figure you should take to your bank or financial advisor — not the build cost alone.
House Extension Types — Costs & Key Facts 2026
Each extension type involves a different level of structural complexity and therefore delivers a different amount of usable space. The right choice depends on your property type, garden space and planning context. Furthermore, costs vary significantly by type — so understanding the differences helps you set a realistic budget before speaking to a contractor.
Single Storey Rear
The most popular extension type in London. It extends the ground floor into the garden, typically by 3–6m, and therefore creates a larger kitchen-diner or family living space. Furthermore, it usually falls within permitted development for terraced and semi-detached houses up to 3m depth.
Often Permitted DevelopmentDouble Storey Rear
This type extends both the ground and first floors simultaneously. As a result, it costs less per m² than a single storey because both floors share the foundations and roofing. Furthermore, it delivers a larger kitchen and an extra bedroom in one build. However, it almost always requires full planning permission.
Planning Permission RequiredSide Return
A side return fills the narrow alley alongside Victorian and Edwardian terraces. It consequently costs more per m² than a rear extension because it requires a steel goalpost frame, restricted-access logistics and complex drainage work. However, it transforms a narrow kitchen into a wide open-plan space.
Often Permitted DevelopmentWrap-Around
A wrap-around combines a side return with a rear extension, wrapping around the corner of the property. It therefore requires removal of the external corner, extensive steelwork and complex structural engineering. As a result, it creates a dramatically large, light-filled open-plan ground floor.
Planning Usually RequiredHouse Extension Cost Calculator FAQs
About This House Extension Cost Calculator — 2026
Signature Build Properties maintains this house extension cost calculator as a free tool for homeowners planning extensions across West and South West London. Over 15 years, our team has designed and built single storey rear extensions, double storey rear extensions, side return extensions and wrap-arounds across Ealing, Hounslow, Chiswick, Richmond, Hammersmith, Fulham and Putney. As a result, the rates we use reflect real-world tender returns — not national averages from surveys that rarely capture London pricing.
How We Calculate Our Rates
We update base rates quarterly against BCIS tender price data and Federation of Master Builders survey results. Furthermore, the calculator applies location multipliers: Inner London (×1.30), Outer London (×1.15) and South East (×1.06). In addition, we apply quality adjustments for standard, good and excellent specification levels. Consequently, results reflect actual 2026 market conditions rather than the outdated averages that many calculators still use.
Build Cost vs Total Project Cost
In the default view, this calculator shows the build cost only — what the contractor charges for construction. However, the true cost of any extension runs significantly higher once you add VAT, professional fees and contingency. Therefore, we built the Total Project Cost toggle to show the real all-in number automatically. Switch to that view to see 20% VAT, 10% professional fees and 10% contingency applied instantly. The calculator consequently gives you the figure to take to your bank — not just the builder's quote.
Accuracy and Next Steps
Results give you an accuracy of ±15–20%, which means they work well as a feasibility guide before you commission a formal quantity surveyor. However, for a precise fixed-price figure, you need a site visit and a written specification. Therefore, call +44 7404 931629 or use the contact form to book a free survey. Moreover, we deliver all our projects on fixed-price contracts — so the figure we agree before work starts is the figure you pay at the end.
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West London This Week
Our team will visit your property, assess the feasibility and confirm your planning position. As a result, you get a detailed fixed-price quote with no hidden costs. We cover Ealing, Hounslow, Chiswick, Richmond and all surrounding boroughs.
