Extension Cost Calculator
Get a realistic 2026 cost estimate for your house extension. Enter your extension type, dimensions, extras, spec level, and location to get a build cost range with a full breakdown. These estimates are based on current UK market rates — not vague guesses.
Typical 2026 extension costs
Build costs per m² vary by extension type, specification, and region. These figures are the starting point before regional adjustment, add-ons, and professional fees.
Rates shown are before regional adjustment and do not include kitchen fit-out, bathroom, underfloor heating, or specialist finishes.
Estimate your build cost
Work through each step below. The calculator will give you a cost range with a full breakdown when you click “Calculate estimate”.
Not sure? A single-storey rear extension goes out from the back of your house at ground floor level. A two-storey adds a first floor room above. A side return fills in the gap beside your kitchen. A wraparound combines rear and side.
Width (side to side)
Depth (how far out)
Measure the footprint of the extension — not including the existing house. For a two-storey, enter the footprint of one floor; the calculator automatically doubles it.
These are fitted-out cost add-ons on top of the build shell. Leave all unchecked if this is a shell-only build or if you will fit these out separately.
Basic: block and render walls, standard tiles, Velux windows. Mid-range: brick skin to match, quality flooring, roof lantern. High-spec: aluminium windows, structural glazing, stone worktops, premium kitchen and bathrooms throughout.
Labour costs vary significantly by region. Inner London rates are typically 40% above the national average; North East is around 10% below.
Related guides
Full breakdown of extension costs including design, structural work, build, fit-out, and planning. Includes case study cost examples for different sizes and specs.
Free GuideClass A permitted development rules for rear extensions including depth limits, height rules, the Larger Home Extension scheme, and when a full planning application is needed.
Free GuideAlmost every extension needs building regulations approval regardless of planning permission. Learn what’s involved, what it costs, and how to apply.
Free GuideFull breakdown of planning costs including application fees, architect fees, structural engineer, building regulations, and party wall surveyor costs.
What affects the cost of an extension?
Ground conditions
Clay soil, trees nearby, or a sloping site can all increase foundation costs significantly. A soil investigation (trial pit) typically costs £500–£1,000 and is worth doing before finalising your budget.
Structural connections
Knocking through between the new extension and the existing house requires structural steelwork (RSJs). This typically adds £3,000–£6,000 depending on the number and size of openings.
Access and site
Difficult access (narrow side passage, terrace with no rear access, basement excavation) adds to labour and skip costs. Budget an extra 5–10% for restricted access sites.
Conservation areas & listed buildings
Conservation area extensions often require matched brickwork, specific window styles, and additional planning fees. Listed buildings require listed building consent and specialist contractors. Budget 15–25% more.