How to Price a House Extension in the UK in 2026

House extensions are one of the highest-value residential construction projects — and one of the most competitive to price. Getting the estimate right means winning work at the right margin rather than winning it and losing money.

Extension Types and Ballpark Costs

The four main residential extension types each carry a different cost band in 2026. A single-storey rear extension — the most common type — prices at £1,500 to £2,500 per square metre, with the wide range driven by specification rather than size. A two-storey extension runs £1,800 to £3,000 per square metre because of the additional structural, first-floor and staircase work involved. A side return extension (typically in Victorian terraced properties with a narrow side passage) sits at £1,600 to £2,800 per square metre. An over-garage extension, where the first floor extends above an integral garage, runs £1,500 to £2,500 per square metre. For a typical 20 square metre single-storey rear extension, the all-in cost — groundworks, structure, roof, glazing, internal finishes and M&E — sits between £30,000 and £50,000 for most of the UK, rising to £40,000 to £65,000 in London and the South East.

What Drives Extension Pricing

More than any other factor, groundworks and foundation specification drive extension cost variance. A straightforward strip foundation on stable clay might cost £5,000 to £8,000 for a 20 square metre extension. A raft foundation on made ground, or piled foundations on shrinkable clay near trees, can add £8,000 to £25,000 to the programme before the first block is laid. Roof specification is the second biggest variable: a flat roof with GRP or EPDM membrane is significantly cheaper than a pitched tiled roof matching the existing house, which requires additional structural work, rafter construction and tiling. Structural steel requirements — particularly for large open-plan rear extensions removing a load-bearing wall — add £3,000 to £12,000 depending on span. Glazing specification has an enormous impact on final cost: standard double-glazed French doors versus a full-width bi-fold set with aluminium frames and a rooflight above can add £8,000 to £18,000. Kitchen or utility fit-out, party wall agreement costs (surveyor fees of £1,000 to £3,000 per adjoining owner), and planning and building regulations fees are additional cost layers that are frequently omitted from initial ballpark quotes.

Labour Costs for Extension Builders in 2026

Main contractors and project managers typically apply a 15 to 25 per cent management margin on the total project value, covering programme management, subcontractor co-ordination, site supervision and commercial risk. For trade labour direct to a self-build or supply-and-fix arrangement, current 2026 rates are: bricklayer £200 to £350 per day, carpenter £180 to £300 per day, groundworker £180 to £280 per day, electrician £200 to £320 per day, plumber £180 to £300 per day, and plasterer £180 to £280 per day. Programme length for a single-storey rear extension is typically 8 to 16 weeks from groundworks start to completion, depending on specification and the number of trades working simultaneously. Two-storey extensions run 14 to 22 weeks. These timescales assume no planning delays, no unforeseen groundworks conditions and a complete design package before work starts.

Structuring an Extension Quote

A well-structured extension quote starts with a site survey — photographs of existing conditions, measurements of the proposed footprint, inspection of the existing structure and foundations where accessible, and a review of any existing drawings or planning documents. The quote itself should be itemised by trade or work package: groundworks and foundations, superstructure (blockwork and brick), structural steel and beams, roof structure and covering, windows and doors, internal first fix (electrical, plumbing, insulation), drylining and plastering, floor finishes, second fix (electrics, plumbing, joinery), decoration, and external works. A contingency of 10 to 15 per cent should be stated explicitly — clients who see a contingency line understand that unforeseen conditions are a normal part of construction, rather than suspecting the builder of padding the price. Exclusions should be clearly listed: structural engineer's design fees, party wall surveyor costs, planning application fees, and anything requiring specialist input not covered by the main contractor scope. A staged payment schedule — deposit on contract signing, payments tied to milestone completion (foundations, wall plate, roof tight, first fix complete, second fix complete, practical completion) — protects both parties and improves your cash flow through the programme.

Common Extension Pricing Mistakes

The most expensive pricing mistake in extension work is quoting without a structural engineer's input on foundation and beam specification. Guessing foundation type on a visual inspection and then encountering tree roots, made ground or a high water table after the contract is signed can cost more than the entire quoted profit margin. Underestimating groundworks is the single most common reason extension projects run over budget. Not including mechanical and electrical co-ordination — the time and cost of ensuring the electrician, plumber and MVHR installer are sequenced correctly and their works integrate — adds invisible cost to fixed-price contracts. Forgetting skip hire and spoil disposal (foundation excavation from a 20 square metre extension generates significant volume), site hoarding, scaffolding and temporary weatherproofing are all common omissions that erode margin. Quoting without any contingency transfers all risk to the contractor — and construction always has unforeseen elements.

Trade2Base for Extension Builders

Trade2Base supports the entire commercial cycle of an extension project from initial survey to final payment. AI quote drafting lets you build a structured, itemised quote from your site survey notes in minutes rather than hours — pulling in your saved trade rates and standard line items automatically. Photo survey notes attach directly to the job record so your quote references the same conditions you inspected. Digital milestone sign-off lets clients approve each project stage from their phone, creating an auditable record of progress and reducing disputes at final account. Stripe stage payments are requested automatically when you mark a milestone complete — keeping cash flowing through the programme without manual invoice creation at each stage. Google review automation triggers on practical completion, capturing the moment clients are most satisfied with the result.