Rear Extension Cost Calculator UK

How This Calculator Works

Our rear extension calculator multiplies your depth and width into a floor area, applies real 2026 spec-level build rates, then adds the knock-through, glazing and your region.

Want to compare a rear extension against other layouts before you commit? Our general extension cost calculator prices side-return, wrap-around and two-storey options on the same basis, and if garden space is tight our loft conversion cost guide shows how adding upwards instead can stack up.

What a Rear Extension Budget Actually Buys — and What It Doesn’t

The per-square-metre rate in the calculator above is a complete build cost: foundations and groundworks, the blockwork or timber-frame shell, the roof structure and covering, standard windows and external doors, first and second fix, insulation to current standards, plastering, decoration, labour and VAT. For a typical standard-spec rear extension that is where roughly two-thirds of the money goes — the shell and foundations — with glazing and the structural knock-through acting as the swing items that push a build from standard towards premium.

What the headline rate usually does not include catches most homeowners out. Budget separately for the kitchen or utility fit-out that goes inside the new space — units, worktops and appliances are a trade in their own right, so read our kitchen extension cost guide for realistic fit-out figures, and price the units, worktop and appliances themselves with our kitchen cost calculator so the fit-out sits in your budget as its own line rather than a surprise at the end. Also plan for professional fees (architectural drawings, a structural engineer for the beam and building-control charges, typically 10–15%), a party-wall surveyor where a neighbour dissents, landscaping and making good the garden, and a contingency of 10–15% for the unknowns that only surface once the foundations are dug.

The four items that move the number most

• Floor area — depth × width. Every extra square metre adds the full build rate, so a 4m depth costs materially more than 3m.
• Glazing — bifold or sliding doors add around £3,000–£8,000 fitted; a structural glass roof or slim-frame sliders can add £12,000 or more.
• The knock-through — opening the original rear wall for an open-plan layout needs a steel beam, typically £2,500–£5,000 including temporary works and making good.
• Region and access — London labour runs about 27% above the Midlands baseline; a tight side-return or no rear access also lifts groundwork and muck-away costs.

Planning Permission and Building Regulations for a Rear Extension

A single-storey rear extension often falls under permitted development, meaning no full planning application. The standard limits are 3m beyond the original rear wall on a semi-detached or terraced house and 4m on a detached house, with a maximum height of 4m. The larger home extension (prior approval) route stretches this to 6m and 8m respectively, subject to a neighbour-consultation scheme. Conservation areas, listed buildings and homes under an Article 4 direction are excluded, so always confirm your specific case on the Planning Portal extensions pages before assuming permitted development applies. If you are unsure which route you need, our guide on whether you need planning permission in 2026 walks through it.

Building regulations apply to every rear extension, whichever planning route you take. Building control signs off the foundations, the structural opening and steel beam, thermal performance, drainage, ventilation and fire safety — either through your local authority or an approved inspector. This is a separate approval from planning, and skipping it can block a future sale.

Excavating within 3m (and in some cases 6m) of a neighbour’s structure, or building up to the boundary, brings the Party Wall etc. Act 1996 into play, which is almost unavoidable on terraces and semis. You must serve notice; if a neighbour dissents you appoint a surveyor — see our party-wall surveyor cost guide and the government’s party wall works guidance on gov.uk for the notice periods and typical fees.

How Rear Extension Cost Varies by Size and Region

Because the calculator prices by floor area, the depth and width you choose drive most of the total. At standard spec a compact 3m × 5m (15m²) rear extension lands around £28,500–£39,000, a 4m × 5m (20m²) around £38,000–£52,000, and a family-sized 4m × 6m (24m²) with an open-plan knock-through around £49,000–£66,000. Push to a 6m prior-approval depth and a 30m² footprint sits closer to £57,000–£78,000 before premium glazing.

Region is the second biggest lever. London runs roughly 27% above the Midlands baseline and the South East about 13% above; the North, Wales, Scotland and Northern Ireland typically sit 4–12% below it. Set the region control in the calculator to see the multiplier applied to your own size and spec, then add the 10–15% professional fees and 10–15% contingency that a firm quote will confirm after a site survey.

Typical Rear Extension Costs (2026)

Complete build incl. foundations, shell, roof, glazing, decoration & VAT, Midlands baseline. Professional fees (10–15%) and contingency extra.

Want the full method and per-material detail? Read the complete 2026 roof replacement cost guide.

Frequently Asked Questions

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