Do I need planning permission for a house extension?

Many single-storey rear extensions can be built under Permitted Development rights, but it depends on the size, height, design, and whether your property has any restrictions such as being in a conservation area or having had previous extensions. Best Builders will advise you at the initial consultation, work with your architect, and help you secure planning permission or lawful development certificates where required.

How long does a typical house extension take from start to finish?

As a general guide, a standard single–storey rear extension will usually take around 8–12 weeks on site once all permissions and designs are in place. Two–storey or more complex extensions can take 12–20 weeks depending on the scope of work, structural requirements and weather. We provide a clear programme before starting and assign a dedicated project manager to keep your extension on schedule.

Can you manage the whole extension project, including design and building control?

Yes. Best Builders can provide a full design-and-build service for your extension, or work seamlessly with your existing architect and structural engineer. We can assist with planning applications, building control submissions, structural calculations, party wall matters and all on-site construction, giving you a single point of contact throughout the project.

Will I have to move out while my extension is being built?

For most standard rear and side extensions, clients are able to remain in their home while we build. We carefully plan phases of work to minimise disruption, maintain services where possible and keep your home secure and weather-tight. For larger, more intrusive structural work there may be a short period where temporary relocation is advised, and we will discuss this clearly with you during the planning stage.

What areas do Best Builders cover for house extensions?

Best Builders carries out house extensions across London and the surrounding areas. If you are unsure whether your property is within our coverage, contact our team with your postcode and we will confirm availability and arrange a free initial consultation where possible.

What guarantees and insurance do you provide on extension work?

All extension work carried out by Best Builders is fully insured and completed in line with current UK Building Regulations. We provide written guarantees on structural work and key components, and we are happy to discuss additional warranty options at the quotation stage so you have complete peace of mind.

How do I get a quote for a house extension?

You can request a quote for your house extension by contacting Best Builders via our website enquiry form or by phone. We will discuss your ideas, review any existing drawings, arrange a site visit and then provide a detailed written quotation that clearly sets out the scope of work, timelines and costs before you commit to anything.

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Q: How much does a house extension cost in the UK?

Typical house extension costs in the UK usually start from around £1,800–£2,500 per m² for a high–quality finish, with total project costs varying depending on size, specification, structural changes and access. At Best Builders we provide a detailed, fixed-price quotation after a site visit and drawings review, so you know exactly what your extension will cost before work begins.

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Extension Types & Costs

How Much Does a House Extension Cost in the UK?

Full cost guide →

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

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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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Every Extension Builder We List Is:

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✓ 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.

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

How long does a house extension take to build?

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.

Do I need an architect for a house extension?

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.

How much value does a house extension add?

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.

What's the difference between a rear extension and a side return extension?

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.

Can I stay in my home during the extension build?

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.

How many quotes should I get for a house extension?

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.

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Extension Builders Across 519 UK Towns

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How Much Does a House Extension Cost in the UK?

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Permitted Development &amp; Planning Permission for House Extensions

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House Extension FAQs

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