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  • Double-storey extensions from £44,000 nationally
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✅ Reviewed by the BestBuilders editorial team on 17 June 2026. All cost ranges, brand pricing, regulatory references and step-by-step processes verified against current Q2 2026 UK market data and regulator publications. Editorial standards: /editorial-standards.
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Extension Types & Costs

How Much Does a House Extension Cost in the UK?

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Single-Storey Extension
Rear or Side-Return Extension
£22,000 – £67,000

The most popular choice for UK terraces and semis. Adds a kitchen-diner or extra living space at ground level. Typically 3–5m deep, completed in 10–14 weeks.

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Double-Storey Extension
Full Double-Storey Rear Extension
£44,000 – £121,000

Maximum space gain in a single build — adds a ground-floor room and a bedroom above. Strong ROI in most UK property markets. Requires full planning permission.

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Side Extension
Side-Return or Infill Extension
£14,500 – £50,500

Ideal for terraced homes with a narrow side alley. Transforms a cramped kitchen into a full-width open-plan space. Often combined with a rear extension.

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Your Project
Get an Accurate Quote

Costs vary by region, size and specification. Compare 3 quotes from vetted local extension builders for your exact project.

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Regional pricing note: Extension costs vary significantly by region. London and South East prices are typically 25–40% above the national average due to higher labour costs and stricter planning requirements. Use our local cost guide or enter your postcode above for town-specific ranges.

Why Build an Extension?

House Extensions — What You Need to Know

A house extension is one of the most effective ways to add space and value to your UK home without the cost and upheaval of moving. Whether you need a larger kitchen, an extra bedroom, a home office or a new family living space, an extension lets you reshape your home around how you actually live.

The key to a successful extension is choosing the right builder. BestBuilders connects you with FMB and CIOB-qualified extension specialists who understand local planning policies, UK building regulations and the structural demands of different property types — from Victorian terraces to post-war semis.

Every builder in our network is verified for public liability insurance, trade qualifications and genuine customer reviews before they receive a single lead from us.

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How It Works

1
Tell us your extension type and postcode
Takes 60 seconds. No drawings or plans needed.
2
We match you with extension specialists
Up to 3 vetted builders in your area, within 24 hours.
3
Compare quotes, reviews and timelines
No obligation to proceed with any builder.

Every Extension Builder We List Is:

✓ Minimum £2m public liability insurance — verified before listing
✓ Holds relevant trade qualifications (FMB, CIOB)
✓ Has a minimum of 5 verified customer reviews before joining
✓ Actively monitored — removed for sustained negative feedback
Planning & Regulations

Permitted Development & Planning Permission for House Extensions

🏡 Permitted Development

Many single-storey rear extensions qualify under Permitted Development (PD) rights in England — meaning no formal planning application is required, as long as they stay within size, height and materials limits. PD rights do not apply in Scotland, Wales or Northern Ireland in the same way. Your matched builder will advise on PD eligibility for your specific property.

📋 Full Planning Permission

Double-storey extensions, extensions on flats, and properties in conservation areas or with an Article 4 Direction typically require a full planning application. The process takes 8–10 weeks on average. Your builder will advise on what's required for your local authority and can recommend architects who handle extension planning regularly.

🔧 Building Regulations

All extensions — including those under Permitted Development — require Building Regulations approval. This covers structural safety, fire safety, energy efficiency, ventilation and drainage. Building control inspections happen at key stages of the build. Your builder manages this process and provides the completion certificate at the end.

Party Wall Act: If your extension is close to or on the boundary with a neighbouring property, you may need to serve a Party Wall Notice and potentially commission a party wall surveyor. This applies to rear extensions going deeper than 3m (semi-detached) or 6m (detached) under the Neighbour Consultation Scheme. Always check early — the process can add 1–2 months to the pre-construction phase.

2026 Cost Comparison

House Extension Cost by Region (2026)

Live cost ranges sourced from Best Builders quote data across all 519 UK towns we cover. Figures are mid-range finish; exclude VAT, kitchen and bathroom fit-out, and architectural fees. London prices reflect Zone 1–6 averages.

Region Single-Storey Rear Side Return Double-Storey
London£33,000 – £67,000£21,500 – £50,500£67,000 – £121,000
South East£30,000 – £60,000£19,500 – £45,000£60,000 – £108,000
Southern£27,000 – £55,000£17,500 – £41,500£55,000 – £99,000
Eastern£27,000 – £54,000£17,500 – £40,500£54,000 – £97,000
West Midlands£25,000 – £50,000£16,500 – £37,500£50,000 – £90,000
East Midlands£25,000 – £50,000£16,500 – £37,500£50,000 – £90,000
Yorkshire£23,000 – £47,000£15,000 – £35,500£47,000 – £85,000
North West£23,000 – £47,000£15,000 – £35,500£47,000 – £85,000
South West£23,000 – £46,000£15,000 – £34,500£46,000 – £83,000
North East£22,000 – £45,000£14,500 – £34,000£45,000 – £81,000
Scotland (North)£23,000 – £46,000£15,000 – £34,500£46,000 – £82,000
Scotland (South)£23,000 – £46,000£15,000 – £34,500£46,000 – £82,000
South Wales£22,000 – £44,000£14,500 – £33,000£44,000 – £79,000
Merseyside & N. Wales£22,000 – £44,000£14,500 – £33,000£44,000 – £79,000

Source: Best Builders 2026 Cost Index, aggregated across 519 UK towns. See full cost index → Ranges reflect mid-spec finishes; premium glazing, structural steel and high-end kitchens move costs above the upper bound. See full cost breakdown →

Customer Reviews

What Homeowners Say About Our Extension Builders

★★★★★ 4.8 / 5
★★★★★

Got 3 quotes within 24 hours for our single-storey rear extension. All three builders were knowledgeable about local planning rules. The one we chose was brilliant — on time, clean site and the quality is excellent.

Sarah J.
Chorlton, Manchester
House Extension
★★★★★

We used BestBuilders to find a builder for our double-storey rear extension. The matching was fast and all three quotes were detailed and professional. Saved thousands compared to quotes I'd got independently.

Raj K.
Wembley, London
Double-Storey Extension
★★★★☆

Side return extension in our Victorian terrace — a complex job. The builder BestBuilders matched us with had done dozens of similar projects in our area and handled planning, Party Wall and build. Highly recommend.

Emma W.
Islington, London
Side Return Extension
Common Questions

House Extension FAQs

Most single-storey extensions take 10–14 weeks on site once work starts. Double-storey extensions typically take 14–20 weeks. Add 8–12 weeks for planning permission if required, and allow time for builder availability — popular builders are often booked 4–12 weeks in advance. Your matched builder will provide a detailed programme before work begins.
For straightforward extensions under Permitted Development, many builders can work from design-and-build drawings produced in-house or by a draughtsperson rather than a fully qualified architect — which can save cost. Where planning permission is required or the design is complex (e.g. structural alterations, conservation area), engaging an architect is advisable. Your matched builder will recommend the right approach for your project.
Studies consistently show well-built extensions deliver strong ROI in the UK. A single-storey kitchen extension can add 5–8% to property value; a double-storey extension adding a bedroom and bathroom can add 10–20% depending on the area. ROI is highest in markets where the cost of moving is high relative to extension costs — which currently describes most of the UK. A well-matched builder who understands local buyer expectations is key.
A rear extension projects into the garden from the back of the house. A side return extension fills in the narrow alley that runs alongside many Victorian and Edwardian terraced properties, typically 1–2m wide. Combining both creates a wrap-around or L-shaped extension, which is the most common choice for terrace houses looking to maximise kitchen and dining space. Both types can fall under Permitted Development if they meet the relevant criteria.
In most cases, yes. Good extension builders are experienced at working in occupied homes and will protect the rest of the house, maintain access to key rooms and plan the sequence of work to minimise disruption. The most disruptive phase is typically when the existing wall is broken through to connect the extension — this usually takes 1–2 days. Your builder will discuss sequencing with you before work starts.
Always get at least 3 quotes for a project of this size. Extension quotes can vary by 20–40% for the same scope of work, and comparing quotes helps you understand what's included, identify any outliers, and assess each builder's communication and professionalism. BestBuilders provides up to 3 matched quotes from a single form submission — the right number to compare with confidence.
In 2026, single-storey extensions in the UK average £1,800–£2,800 per m² for a mid-range finish (excluding VAT and kitchen fit-out), with London projects typically £2,500–£3,400 per m². Our 2026 cost data across 519 UK towns shows the largest variance comes from regional labour rates and ground conditions, not from headline material prices. Get 3 free quotes to see real per-m² figures for your project.
They are separate consents. Planning permission decides whether you can build the extension; Building Regulations check that the construction is safe, weather-tight, energy-efficient and accessible. Many single-storey rear extensions fall under Permitted Development and skip planning, but all extensions still need Building Regulations approval. Apply via your local authority or an Approved Inspector — your builder usually handles submissions. Typical Building Control fees: £700–£1,800.
A Party Wall Agreement is required when building work affects a shared wall, structure, or boundary with a neighbouring property. For extensions, this applies if you're excavating within 3m of a neighbour's foundation, building on the boundary line, or cutting into a shared wall. Serve a Party Wall Notice at least 2 months before works start. If your neighbour disagrees, surveyor fees of £700–£2,500 per side apply, usually paid by you.
Yes — most UK extension projects are financed through a further advance on your existing mortgage, remortgaging to release equity, or a secured home improvement loan. Typical further-advance rates in 2026 sit around 4.5–6.5%, with arrangement fees of £300–£1,000. Lenders usually require build cost quotes, planning evidence and a chartered surveyor valuation showing the post-extension property value. Speak to a mortgage broker familiar with home improvement lending — they often find better rates than going direct.
Standard-rate VAT (20%) applies to most extension work — both labour and materials. A few exceptions reduce VAT: properties that have been empty for 2+ years qualify for a reduced 5% rate; converting non-residential buildings into homes can also qualify; some listed-building adaptations have reduced rates. Builders registered for VAT add it to invoices automatically. Always ask whether quotes include or exclude VAT before comparing.
The snagging period is the time after practical completion when your builder fixes minor defects — cracked plaster, sticking doors, paint touch-ups, sealant gaps. Industry standard is 12 months from completion, with most snags raised in the first 6 weeks once heating cycles have run through the building. Withhold 2.5–5% of the contract value as a retention, released at the end of snagging — this is standard practice and protects you. For jobs over £40k, consider an independent surveyor for the formal snagging inspection.
After project size, the biggest cost drivers are: (1) Ground conditions — clay, water table, made ground or trees within 5m can add £8,000–£25,000 to foundation costs; (2) Glazing — bifolds, sliders and structural glass can swing the budget by £6,000–£20,000; (3) Roof type — flat with steel beams vs pitched with tiles. Site access (no rear access) typically adds 10–15%. Kitchen and bathroom fit-out level can double headline cost. To control budget, fix the build envelope first, then adjust fit-out level.
Insurance: tell your home insurer before work starts — they may add an endorsement or require Contractor's All-Risks (CAR) cover during the build. Most builders carry their own public liability and CAR cover, but always check certificates. After completion, update your sum insured to reflect the higher rebuild cost. Council Tax: banding is only reassessed on sale or a "material change" — most extensions don't trigger reassessment for current owners. Adding a self-contained annexe with kitchen and bathroom may.
Best Builders matches homeowners with vetted, insured extension builders across all 519 UK towns covered by our network — London, the Home Counties, Midlands, North, Scotland and Wales. Each town page shows the verified builders local to your postcode plus typical regional cost ranges. If your postcode falls outside main coverage areas, we'll still send your enquiry to qualified builders willing to travel — but availability may be limited.

Our sources for this guide

Cost ranges, timelines and regulatory information on this page are compiled from:

Editorial standards & how to reach us

This page is maintained by the Best Builders editorial team. Cost figures are reviewed quarterly against active quote data from our verified UK builder network. Regulatory information (planning, Building Regs, VAT, Party Wall) is checked against current UK government guidance at each refresh. We do not accept payment for editorial coverage; builder listings are earned through our vetting process, not bought.

If you spot an error, have feedback, or want to suggest content improvements, email editorial@bestbuilders.co.uk.

Last updated: 23 May 2026 · Next scheduled review: 23 August 2026

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Extension Builders Across 519 UK Towns

Each town page shows local extension builders, town-specific cost ranges and nearby areas.

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