Extension Builder London 2026: How to Choose the Right Company for Your Project

Choosing the right extension builder in London is one of the most important decisions you will make in your home improvement project. The quality of your builder determines whether your extension is delivered on time, on budget and to a standard that will add significant value to your home. This guide sets out what a London extension builder does, how to choose the right company for your project and what to look for in a shortlist. LCCL Construction has been building extensions across London and Hertfordshire for over 20 years as an award-winning design and build company.

LCCL Construction — award-winning extension builders across London and Hertfordshire

What Does an Extension Builder Do in London?

An extension builder in London handles all construction work to add extra floor space to your property. This includes groundworks and foundations, structural steelwork to remove existing walls, superstructure construction with brickwork or blockwork, roof construction, external finishes, windows and doors installation, first fix electrics and plumbing, insulation, plastering, second fix and internal finishing. A full design and build extension builder — like LCCL Construction — also handles architectural design, planning permission, building regulations, structural engineering and party wall agreements as part of one integrated service.

Types of Extensions London Builders Construct

London extension builders typically construct four main types of extensions. See individual guides for each:

rear extensions from £35,000,

side return extensions from £35,000,

wraparound extensions from £60,000 and double storey extensions from £75,000.

See our full home extension cost guide for a complete cost breakdown.

How to Choose the Right Extension Builder in London

Seven things to look for when choosing an extension builder in London:

1. Track record and years of experience. Look for extension builders with at least 10 years of continuous trading and 100+ completed projects. This tells you they have delivered under multiple market conditions.

2. Verified reviews. Look for 100+ verified reviews across multiple platforms — Google, Trustpilot, Houzz, Reviews.io and Yell — with a consistent average rating of 4.7 stars or higher.

3. Industry accreditations. LABC Registered Partner status, National Federation of Builders membership, LABC or similar structural warranty provider affiliation. These are independent validations of quality and reliability.

4. Recent industry awards. LABC Building Excellence Awards, Federation of Master Builders awards and Best of Houzz Service awards are the most credible. Multiple recent wins signal current top-tier performance.

5. In-house team. Design and build companies with in-house architects, structural engineers, project managers and builders — no subcontracted teams — give you a single point of accountability throughout the project.

6. Transparent pricing. Extension builders should provide a fully itemised quote covering design, planning, construction, materials, contingency and VAT. Avoid vague estimates or square-metre rates without detail.

7. Insurance and warranty. Full public liability and professional indemnity insurance are essential. Optional 10-year LABC structural warranty is a strong signal of build quality confidence.

Extension Builder London Costs 2026

London extension builders typically charge £2,400 to £3,800 per square metre for a single storey extension and £2,600 to £3,800 per square metre for a double storey. Total project costs range from £35,000 for a small rear extension to £165,000 for a large double storey with premium specification. Prices include design, planning, construction and building regulations but exclude kitchen and bathroom fittings which are typically client-supplied.

LCCL Construction — Extension Builders London and Hertfordshire

LCCL Construction is a design and build extension builder based in Finchley N3 covering all of London and Hertfordshire. We are LABC Building Excellence Award winners 2024 for Best Residential Extension in London, hold 200+ verified reviews at 4.7/5 across Google, Trustpilot, Houzz and Reviews.io, and are members of the National Federation of Builders and LABC Registered Partners. Our in-house team of architects, structural engineers and builders handles every stage of your extension project. See our complete design and build service.

Frequently Asked Questions About Extension Builders in London

How much does an extension builder charge in London?

Between £2,400 and £3,800 per square metre for a full design and build extension. Total costs range from £35,000 to £165,000+ depending on size, type and specification.

How long does an extension builder take to complete a London extension?

Typically 12 to 16 weeks on site for a single storey rear and 18 to 24 weeks for a double storey. Design and planning add another 12 to 16 weeks upfront.

Should I choose a design and build company or a separate architect and builder?

Design and build gives you a single point of contact, one contract and one company accountable for the result. Separate architects and builders can work — but coordination risks and finger-pointing between the two are common problems.

To book a free consultation with LCCL Construction call 020 8037 7332 or get an instant quote online.