TOOL 01 · COST

How much does an extension cost in London?

Name: Vision + Design
Price range: ££

Set your floor area, specification and borough for an indicative build cost, with professional fees, contingency and VAT included rather than added later. Free, and no email required.

Indicative build cost

London Extension Cost Calculator

Build rate includes demolition and strip-out, structure and steels, foundations, envelope and roof, windows and doors, first and second fix services, plastering and standard finishes, and contractor preliminaries and overheads.

Project basics

Project typeLondon zoneNew extension area · 40 m²Assumed build rate: £2,700 to £4,500 per m²Internal refurbishment area · 0 m²Assumed refurbishment rate: £1,300 to £1,700 per m²Existing floor area being refurbished rather than newly built: finishes, partitions, doors, rewiring, replumbing, decoration. Kitchen, utility and WC fit-out is priced separately below, so it is not included in this rate.

Rooms and fit-out

Specification tierNew kitchenWCs / cloakroomsUtility roomBoot room / larder / storageExternal worksExternal works area (m²)

Fees, risk and VAT

Professional fees15% of construction cost

Statutory costs and surveys3% of construction cost

Contingency10% of construction cost

VAT treatment

Cost breakdownRates as at Q3 2026

New build (extension)£108,000 – £180,000

Internal refurbishment (existing areas)£0 – £0

Kitchen, utility, WCs and boot room / storage allowance£67,000 – £67,000

External works (landscaping, patios, boundary walls)£0 – £0

Professional fees (~15%)£26,000 – £37,000

Statutory costs and surveys (~3%)£5,000 – £7,000

Contingency (10%)£18,000 – £25,000

Subtotal, fees and risk£49,000 – £69,000

Subtotal (ex VAT)£224,000 – £316,000

VAT (20%)£45,000 – £63,000

Total project cost (inc VAT)£269,000 – £379,000

VAT treatment varies by project and eligibility for relief.

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Indicative estimate only. Final costs depend on scope, planning, structural complexity, site access, and procurement route. Rates based on publicly available market data (Q3 2026).

02 · HOW THE FIGURES ARE DERIVED

What sits behind the number.

The build rate covers demolition and strip-out, foundations, structure and steels, envelope and roof, windows and doors, first and second fix services, plastering and standard finishes, and the contractor's preliminaries and overheads. It does not cover the kitchen, sanitaryware, bespoke joinery or landscaping, which are priced separately in the tool so you can see them rather than discover them later.

Refurbishment of existing rooms is rated at roughly 40 to 50 per cent of the new build rate, because it is finishes, partitions, services and decoration rather than new structure. Professional fees run at 15 per cent, statutory costs and surveys at 3 per cent, and contingency at a minimum of 10 per cent, which on a Victorian or Edwardian property is the line most likely to be tested.

A rate per square metre is a planning tool, not a quotation. It tells you whether a project is affordable before you commit to design fees. What it cannot account for is your ground, your existing structure, your access or your specification, and those are what move a real number.

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