Extension Builders London: 6 Things to Check Before You Appoint

Finding the right extension builders in London is one of the most important decisions you will make when planning a house extension. London has hundreds of builders offering extension services but quality, reliability and value vary enormously. Knowing what to look for before you appoint anyone will save you significant time, money and stress.

LCCL Construction — specialist extension builders based in Finchley, North London

Here are 6 key things to check when looking for extension builders in London — and why LCCL Construction meets every one of them.

6 Things to Check When Choosing Extension Builders in London

1. Verified Reviews Across Multiple Platforms

Reviews are the most reliable indicator of a builder's quality and reliability. Look for extension builders with a minimum of 50 verified reviews at 4.5 or above across independent platforms including Google, Houzz, Trustpilot and Reviews.io. Be wary of builders with very few reviews or a sudden surge of recent five-star reviews — these can indicate artificial reviews rather than a genuine track record. LCCL Construction has over 200 verified reviews at 4.7 out of 5 built over 20 years.

2. Professional Accreditations

The most relevant accreditations for extension builders in London are LABC Registered Partner status and National Federation of Builders (NFB) membership. Both require independent vetting and ongoing compliance with professional standards. LABC Registered Partners can also offer a 10-year LABC warranty on completed projects — valuable when selling your property. LCCL Construction holds both LABC Registered Partner status and NFB membership, and won the LABC Award 2024 for Best Residential Project.

3. A Full Design and Build Service

The most efficient way to build a house extension in London is through a design and build company that handles architectural design, planning permission, building regulations, structural engineering, party wall agreements and construction under one roof. This eliminates the risk of communication gaps between a separately appointed architect and builder. LCCL Construction provides a complete design and build service — one company, one estimate, one point of contact.

4. Experience with Your Specific Extension Type

Different extension types require different expertise. A builder with strong experience in rear house extensions may have limited experience with complex wraparound extensions or double storey extensions. Ask specifically about recent projects of your extension type and ask to see finished project photos. LCCL Construction has completed hundreds of all extension types across London.

5. A Detailed Written Estimate with No Hidden Costs

Any professional extension builder should provide a detailed written estimate covering every element of the project — foundations, structure, roofing, glazing, electrics, plumbing, plastering and decoration. Never accept a verbal quote or a vague one-line estimate. Hidden exclusions such as structural steelwork, scaffolding or building regulations fees are a common source of budget overruns and disputes. Ask builders to provide a like-for-like scope so you can compare prices accurately.

6. A Dedicated Project Manager on Your Build

Poor communication is the most common complaint about builders. Before appointing extension builders, test their responsiveness during the quotation stage. A builder who is difficult to reach before you appoint them will be significantly harder to deal with during a 12-week construction project. LCCL Construction assigns a dedicated project manager to every project who provides weekly updates and coordinates all trades on site.

How Much Do House Extensions Cost in London in 2026?

Rear extension: from £35,000.

Side return extension: from £35,000.

Kitchen extension: from £40,000.

Wraparound extension: from £60,000.

Double storey extension: from £80,000.

All costs are estimates — see our complete guide to house extension costs in London for a full breakdown.

Frequently Asked Questions About Extension Builders in London

How do I find reliable extension builders in London?

Look for builders with 50+ verified reviews at 4.5 or above, LABC Registered Partner or NFB membership, a full design and build service and a detailed written estimate. LCCL Construction meets all of these criteria.

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

A design and build company is generally more efficient, faster and better value. One company is responsible for everything from design to completion, which reduces communication gaps and cost uncertainty.

To get a free consultation and detailed cost estimate from LCCL Construction, call 020 8037 7332 or book online.