Extension Cost Per Square Metre Calculator UK 2025/26

Extension costs in 2025/26 range from £1,500/m² for a budget single-storey to £4,500/m² for premium-spec work. Standard single-storey extensions typically cost £2,000–2,800/m². Double-storey extensions cost around 1.6x the ground floor rate per m². These figures include groundworks, structure, roofing, windows, plastering, and first fix, but exclude furniture and landscaping.

Under Permitted Development Rights, single-storey rear extensions up to 3m (attached house) or 4m (detached house) deep, and up to 4m high, generally don't need planning permission. Larger extensions (up to 6m for attached, 8m for detached) require a Prior Approval application. Side extensions and anything in a conservation area usually require full planning permission.

Planning permission controls whether you can build (size, appearance, impact on neighbours). Building regulations ensure the build is structurally sound, energy-efficient, and safe. Most extensions need building regulations approval even if they don't need planning permission. Building regs approval is obtained from your local authority's building control department or an approved inspector.

Yes, in almost all cases. A structural engineer designs the foundations, steelwork (RSJs), and structural connections. Typical fees are £500–£1,500 for a standard extension. Building control will require structural calculations before approving the build. Your builder may have a preferred structural engineer, but you are free to appoint your own.

The Party Wall etc. Act 1996 requires you to notify adjoining owners if you are: building on or near a shared boundary, excavating within 3–6m of a neighbour's foundations, or working on a party wall. Notice must be given 1–2 months before work starts. If neighbours don't agree, both parties appoint a surveyor. Party wall agreements typically cost £700–£1,500.

A standard single-storey extension of 20–30m² typically takes 10–16 weeks: groundworks and foundations (2–3 weeks), blockwork and structure (2–3 weeks), roofing (1–2 weeks), windows and doors (1 week), first fix (plumbing/electrics) (2 weeks), plastering, second fix, and finishes (3–4 weeks). Planning and building regs approval add time before work starts.

Extensions to existing homes are standard rated at 20% VAT. New builds and conversions of non-residential buildings attract 0% or 5% VAT. DIY builders can claim back VAT on new builds (HMRC VAT431 scheme). Always check whether your builder's quote is inclusive or exclusive of VAT — some small builders are not VAT-registered and charge no VAT.

A wrap-around extension combines a rear extension with a side-return extension (infilling the narrow side passage beside a Victorian or Edwardian terraced house). They are common in London and typically add 15–30m² of floor space. Wrap-arounds tend to be among the highest-value extensions in dense urban areas but are complex and require full planning permission.

Get at least 3 quotes from VAT-registered builders with public liability insurance. Check Federation of Master Builders (FMB) or TrustMark membership. Ask for references from completed extensions and visit if possible. Avoid paying large upfront deposits (no more than 10%). A contract should detail scope, payment schedule, timeline, and variation procedures.

A main contractor manages all trades and carries overall responsibility — costing 10–20% more but saving management time and carrying professional liability. Self-managing (procuring individual tradespeople) is cheaper but requires significant time and project management skills. Most homeowners use a main contractor for extensions to avoid coordination stress.

Budget spec (£1,500–2,000/m²): standard brick or block construction, basic uPVC windows, simple flat or lean-to roof, standard finishes. Standard spec (£2,000–2,800/m²): better-quality windows, insulated roof, underfloor heating option. Premium spec (£3,000–4,500/m²): structural glazing, architectural roof lanterns, bespoke joinery, high-end kitchen-quality finishes.

A well-executed extension typically adds 10–15% to property value, and up to 20% for a kitchen-diner extension. A loft conversion adds 15–20%. Value added depends strongly on location — in high-demand areas the return on investment can exceed 100%. In lower-value areas, the cost may exceed the value added. Check local sold prices of extended vs non-extended comparable properties.