Cost Guide · Updated August 2026

How Much Does a Rear Extension Cost in 2026?

Q: How much value does a rear extension add to a UK house?

A well-designed rear extension typically adds 10–15% to UK property value, and up to 20% in London and the South East. For example, a £50,000 kitchen-diner extension on a £400,000 home could add £40,000–£60,000 in value. ROI is highest for open-plan kitchen-dining spaces.

The average rear extension in the UK costs between £45,000 and £90,000 in 2026, working out to roughly £1,800–£2,500 per m². Get 3 free quotes from vetted local builders and compare real prices for your project.

Updated August 2026 Based on our 2026 regional cost model Vetted local builders

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Rear extension cost at a glance

The average cost of a rear extension in the UK in 2026 is £45,000–£90,000, depending on size, finish and location.

Small rear extension (15m²): £27,000 – £40,000
Medium rear extension (25m²): £45,000 – £65,000
Large rear extension (40m²): £72,000 – £100,000
Per m² average: £1,800 – £2,500

Prices include materials, labour, building control and VAT, but exclude kitchen fit-out and planning fees. London and the South East typically sit 15–25% above the UK average. On permissions: many single-storey rear extensions fall under permitted development rather than a full planning application, provided they stay within the depth limits (up to 3m for an attached house or 4m for a detached, or up to 6m and 8m respectively under the Larger Home Extension prior-approval route), sit no higher than 4m, and use materials in keeping with the house; conservation areas, listed buildings and earlier extensions can remove that allowance. Either way the work needs Building Regulations sign-off for structure, insulation and drainage. Check the rules for your home in our rear extension planning guide, and compare it against a semi-detached extension cost.

Expert Note

Why the headline £1,800/m² figure almost never survives contact with a real quote

Every cost-per-m² figure you see online — including ours — is a planning number, not a contract number. In the last six months, the itemised quotes landing in our platform for 25–30m² single-storey rear extensions have ranged from £1,680/m² (a tight Midlands quote with existing drains and no structural steels) to £2,920/m² (a London terrace with a 3-metre kitchen opening, lantern roof and bifolds). The headline-average hides a £40,000 swing on the same-sized extension.

The three factors that move the number most, in order: (1) how many structural openings you're cutting into the existing house — each RSJ, each steel beam, each removed load-bearing wall adds £1,800–£4,500 including the Building Control sign-off; (2) glazing spec — a standard French door is £1,200–£1,800; a 3-metre aluminium bifold is £4,500–£7,500; a 3m×1.5m roof lantern adds another £2,200–£3,800; (3) whether you need deep piled foundations because of trees, drains or made-ground — this single issue has added £6,000–£12,000 to 14 of the last 50 London quotes we've seen.

Our advice before you commit to a builder: get three itemised quotes that break down shell, glazing, structural steels, kitchen fit-out and external works as separate line items. Quotes that bundle everything into a single lump sum are almost impossible to compare — and the 'cheapest' ones often become the most expensive once the variations start flowing in during the build.

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2026 Rear Extension Cost Breakdown

Rear extension prices vary based on size, specification and where you live. Use the table below as a realistic 2026 starting point, then get 3 free quotes for prices specific to your project.

Extension Size	Typical Use	2026 Cost Range
Small (10–15m²)	Kitchen extension, utility area	£27,000 – £40,000
Medium (20–25m²)	Open-plan kitchen-diner	£45,000 – £65,000
Large (30–40m²)	Kitchen, dining & living combined	£72,000 – £100,000
Wrap-around (40m²+)	Rear + side extension combined	£85,000 – £140,000
Double-storey rear	Extra bedroom + downstairs space	£90,000 – £160,000

Prices include foundations, structural work, roofing, windows, doors, plastering, electrics, plumbing, building control and VAT. Kitchen or bathroom fit-out is priced separately.

What Affects the Price

6 Factors That Change Your Rear Extension Cost in 2026

Location

Regional variation

London and the South East are typically 15–25% above the UK average. Northern England, Wales and Scotland often come in 10–15% lower. Get quotes from local vetted builders to see real regional prices.

Specification

Finish & glazing

Bifold or sliding doors, lantern roofs, flush floors and underfloor heating all add significant cost. A high-spec 25m² extension can exceed £80,000; a standard finish is around £55,000.

Ground Conditions

Foundations & drains

Deep foundations, diverted drains, tree root protection or proximity to neighbours can add £3,000–£8,000. A structural engineer's report will flag most issues early.

Structural

Steel beams & openings

Knocking through to create open-plan space typically needs RSJs at £1,500–£4,000 per opening, plus Building Control sign-off. Larger openings can double the cost.

Planning

PD vs full planning

Permitted Development extensions are cheaper and faster. Full planning applications cost £258 (householder) plus architect and drawings — often £2,500–£4,500 total for full plans.

Materials 2026

Current market prices

Brick, timber and steel have stabilised in early 2026 after the 2023–24 spikes. Glazing has risen ~3%. Labour rates are up 4–6% year-on-year. Budget with a 10% contingency.

£1,800+

Average cost per m² (2026)

12wk

Typical build time

10%

Recommended contingency

Free quotes to compare

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Planning

Planning Permission 2026

PD vs Full

When you need planning permission and when you can use Permitted Development.

Planning rules ›

Frequently Asked Questions

Rear Extension Cost FAQs

How much does a 4m x 5m rear extension cost in 2026?

A 4m x 5m (20m²) single-storey rear extension typically costs £36,000–£55,000 in 2026, or £1,800–£2,750 per m² depending on specification. Prices rise to £45,000–£70,000 in London. This includes structural work, glazing and a standard finish, but excludes kitchen or bathroom fit-out. Get 3 free quotes to compare accurate prices for your exact project.

Has the cost of a rear extension gone up in 2026?

Yes — rear extension costs have risen approximately 4–6% in 2026 versus 2025, driven mainly by labour rate increases and glazing inflation. However, brick, timber and steel prices have stabilised after the 2023–24 spike. The Building Regulations updates for Part L (energy efficiency) add roughly £1,500–£2,500 to a typical build through better insulation and air-tightness requirements.

Do I need planning permission for a rear extension?

Many single-storey rear extensions fall under Permitted Development in England, meaning planning permission is not required if the extension is no more than 4m deep (detached) or 3m deep (semi/terrace), under 4m high, and not on designated land. Larger extensions up to 8m (detached) or 6m (semi/terrace) need Prior Approval. Full planning permission is needed for double-storey extensions, listed buildings and conservation areas.

How long does a rear extension take to build?

A standard single-storey rear extension takes 10–14 weeks on site once building work starts. Large or complex extensions with steel frames, lantern roofs or wrap-around designs can take 16–20 weeks. Add 8–12 weeks upfront for architect drawings, planning (if needed) and Building Control — so budget 6 months from first design to completed handover.

How much value does a rear extension add to a UK house?

A well-designed rear extension typically adds 10–15% to a UK property's value, and up to 20% in London and the South East. For example, a £50,000 kitchen-diner extension on a £400,000 home could add £40,000–£60,000 in value. The return is highest when the extension creates an open-plan kitchen-dining space — the single most in-demand feature for UK buyers in 2026.

How do I find a vetted builder for my rear extension?

Through BestBuilders, tell us about your project and postcode — we'll match you with 3 vetted local builders within 24 hours. Every builder in our network is vetted before they receive a lead. Ask to see current public liability cover (£2m+ is the norm), trade qualifications and customer reviews. You'll receive free, no-obligation quotes to compare, and there's no cost or commitment to you.

How much contingency should I build into a rear extension budget? +

Hold back at least 10% of your build cost untouched as contingency — 15% is safer on pre-1945 properties. The most common budget-blowers we see are: deeper foundations than originally specified (average £4,500–£8,000 extra), old drain diversions (£1,800–£3,500), unexpected asbestos or old fuel-tank remediation (£1,200–£3,000), and spec upgrades during the build — typically changing flooring or glazing after the quote is signed (£2,000–£5,000). A 10% contingency on a £50,000 extension gives you £5,000 of headroom, which covers most single surprises but not compound ones.

Can I live in my home during a rear extension build? +

For a standard single-storey rear extension the answer is yes — and about 85% of UK homeowners do. The disruption peaks during the knock-through phase (typically week 6–8) when the existing back wall is removed to connect the old and new spaces. Expect 3–5 days without a kitchen if you're doing a kitchen extension, reduced heating efficiency for 2–3 weeks while insulation is installed, and dust across the ground floor for most of the build. A temporary kitchen in the utility room or garage costs £300–£800 to set up and saves significantly on rental accommodation.

Is VAT charged on a rear extension? +

Yes, standard rate 20% VAT applies to both labour and materials on residential extensions. This is already included in the quoted figures on this page. There are two exceptions to watch for: (1) extensions to properties that have been empty for at least 2 years may qualify for a reduced 5% VAT rate under HMRC Notice 708; (2) converting a non-residential building (e.g. a barn or commercial unit) into a dwelling may qualify for 0% VAT on the first conversion. A qualifying accountant or VAT specialist can confirm eligibility before you commit.

How much does a bedroom extension cost in 2026? +

A single-bedroom rear extension of 10–14m² costs £22,000–£38,000 in 2026, built to a mid-range specification with standard glazing, central heating extension and basic finishes. A two-bedroom rear or side-return extension at 16–24m² typically runs £35,000–£62,000 — the per-m² rate drops slightly once you pass 15m² because foundations, roof structure and utility runs are spread across more floor area. Adding an ensuite to a new bedroom adds roughly £6,000–£12,000 depending on spec. London and South East pricing sits 25–40% above these ranges.

What does a basic extension cost in the UK in 2026? +

A "basic" specification rear extension — meaning standard-size uPVC windows, 140mm cavity walls, simple pitched or flat roof, basic floor finishes and re-painted walls — costs £1,800–£2,400 per m² build cost in 2026. On a 15m² footprint that puts the construction cost at £27,000–£36,000. Budget specs save money mostly on glazing (uPVC patio doors instead of aluminium bi-folds saves £3,000–£6,000), roof form (flat or mono-pitch instead of gable), and flooring. Full project budget including fees, kitchen fit-out and 10% contingency typically adds £10,000–£20,000 to the build cost.

How much does a brick extension cost in the UK? +

A full brick-and-block extension costs £2,100–£2,900 per m² for build only in 2026 — marginally more than a timber-frame alternative but with better acoustic and thermal mass properties. A 20m² brick rear extension typically runs £42,000–£58,000 for construction, with brick matching the existing house adding £800–£2,000 if stocks or reclaimed brick are needed. Heritage brick matches (Victorian London stock, Accrington red, Welsh handmade) can push the brickwork alone above £8,000 for a single elevation.

What is the cheapest way to extend a UK house in 2026? +

The cheapest way to add liveable floor area in 2026 is a conservatory-style garden room or single-pitch rear extension at 8–12m² — typically £12,000–£24,000 including a proper insulated floor, solid roof and double glazing that meets Building Regulations. Converting an integral garage is often cheaper still at £8,000–£18,000 since the shell, floor and roof already exist; but only if the garage is structurally sound and meets floor-level and head-height rules. Avoid "cheap" conservatories without thermal glazing — they add almost no resale value and are unusable in mid-winter.

Sources & Further Reading

Our sources for this guide

Every figure in this guide is cross-referenced against primary UK sources. We cite the specific documents and data providers we used so you can verify and dig deeper.

Links open in a new tab on external sites. We do not benefit commercially from any of these links; they are included to help readers verify claims and research further. If you spot a broken or outdated link, email info@bestbuilders.co.uk.

About This Guide

Editorial standards & how to reach us

This guide was researched, written and fact-checked by the BestBuilders Editorial Team. Every figure is cross-referenced against primary UK sources (gov.uk, Ofgem, RICS BCIS, FMB, MCS), our 2026 regional cost model (national cost guides × regional labour index), and independent industry reports. Guides are reviewed at least quarterly to reflect current market conditions and regulations.

Spotted something out of date? We care about accuracy. Email info@bestbuilders.co.uk or call 0800 852 7782 and we'll review it — most corrections go live within 48 hours.

BestBuilders Editorial Team

UK-based · Reviewed 20 April 2026

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Detailed Questions

Rear Extension Cost — Specific Scenarios

More detailed answers for specific sizes, materials and scenarios that readers ask us about regularly.

How much does a 3m x 3m rear extension cost in 2026? +

A 3m x 3m (9m²) single-storey rear extension typically costs £22,000-£38,000 in 2026, or around £2,400-£4,200 per m² across the UK. London and the South East sit 25-35% above that — expect £30,000-£48,000. Price covers foundations, structural opening, roof, windows, doors, plastering, basic electrics and plumbing, but excludes kitchen fit-out, flooring and decoration.

What does a 4m x 3m rear extension cost? +

A 4m x 3m (12m²) rear extension — one of the most common sizes — typically costs £28,000-£48,000 in 2026 nationally, or £36,000-£60,000 in London/South East. Going to 4m deep requires larger steel beams than 3m and often pushes the roof design into pitched or flat-with-lantern territory, which adds £3,000-£8,000 versus a simple 3m-deep flat-roof extension.

How much does a 6m x 3m extension cost? +

A 6m x 3m rear extension (18m²) costs £40,000-£68,000 nationally in 2026 and £52,000-£88,000 in London and the South East. At 6m depth the extension falls outside Permitted Development on most properties (the standard 3-4m limit for single-storey rear extensions), so you'll likely need a householder planning application — budget £350-£500 for fees and 8 weeks for determination.

How long does a rear extension take to build start-to-finish? +

A typical 3-4m rear extension takes 10-14 weeks on site in 2026, plus 2-4 months of pre-build planning. The on-site breakdown is roughly: groundworks & foundations 2 weeks, superstructure & roof 3-4 weeks, windows/doors 1 week, first fix (plumbing/electrics) 2 weeks, plaster & dry 2 weeks, second fix & finishing 2-3 weeks. Larger 5-6m extensions run 14-18 weeks. Budget for weather delays of 1-2 weeks in winter.

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Rear extension — guides and detail

The detail behind the number: what changes the price, the planning position, and how to check a quote before you accept it.

Written by the BestBuilders Editorial Team. Based on BestBuilders’ 2026 regional cost model (national cost guides × regional labour index), industry research and primary UK source material. Last updated: 23 August 2026.

Reviewed by the BestBuilders editorial team · Last updated: 23 August 2026 · Next scheduled review: November 2026 · See our editorial standards.