How to Choose a Building Contractor in London: A Homeowner's Checklist

• **Step 1 — Establish what you are looking for:**
• Before approaching contractors, be clear about what your project requires:
• •A contractor capable of managing the full project (principal contractor), or individual trades?
• •A design-and-build contractor (who manages design and construction together), or a build-only contractor working from an architect's drawings?
• •A company with employed direct labour, or a company that manages subcontractors?

For an extension or loft conversion in London, a principal contractor with in-house project management and a managed supply chain is almost always preferable to self-managing individual trades — the coordination, sequencing, and accountability burden on the homeowner is substantially reduced.

• **Step 2 — Source contractor names:**
• Reliable sources for building contractor names in London:
• •Personal recommendation from a friend, family member, or neighbour who has done a similar project
• •Architect recommendation (where you have an architect, they are familiar with contractor quality)
• •Trade body directories: a recognised trade body, accredited quality schemes, verified contractor review platforms, NFRC (National Federation of Roofing Contractors) for specific roofing works
• •Local neighbourhood groups (Nextdoor, local Facebook groups) — with appropriate caution around volume rather than recency of reviews

• **Step 3 — First contact and qualification:**
• Before inviting a contractor to price your project, establish the basics:
• •Are they currently taking on projects of your type and scale?
• •What is their typical project programme lead time (when could they start)?
• •Can they provide examples of similar recent projects in London?
• •Are they VAT registered? (Absence of VAT registration on a project above the VAT threshold — currently £90,000 — is a commercial red flag)

**Step 4 — Invite to price:** Provide every contractor with exactly the same information to price from: drawings (planning and technical), a scope of works brief, specification notes, and any other relevant project information. If you give different information to different contractors, the prices are not comparable.

• **Insurance:**
• A building contractor working on your property in London must carry:
• •**Public Liability Insurance:** minimum £2 million (£5 million+ for larger projects). Covers bodily injury to third parties and damage to third-party property during the works.
• •**Employers' Liability Insurance:** minimum £5 million. Legally required where the contractor has employees. Ask for the certificate — it should be current.
• •**Contract Works (All Risks) Insurance:** covers the works themselves against fire, flood, theft, and accidental damage. This protects you and the contractor if the half-built extension is damaged before handover.

Ask for copies of current insurance certificates before appointing. A contractor who cannot or will not provide insurance certificates should not be appointed.

• **References:**
• Ask for at least two references from similar recent projects. Contact the references directly — a reference provided by the contractor who does not respond, or who gives vague answers, is not a satisfactory reference. Ask specifically:
• •Did the project come in on the quoted price? If not, why not, and how were variations handled?
• •Was the contractor's communication and responsiveness good?
• •Would you use them again?

**Trade body membership:** Membership of the a recognised trade body or accredited quality schemes requires the contractor to meet minimum standards and carry appropriate insurance. It is not a guarantee of quality but it is a positive signal. Many London contractors are members of verified contractor review platforms, which verifies trading history and insurance and displays verified customer reviews.

**Company financial stability:** A basic Companies House check (free at companieshouse.gov.uk) shows when the company was incorporated, its filing history, and whether there are any county court judgements. A company that was incorporated 6 months ago, or that has filed inconsistent accounts, or that has CCJs registered against it, is a higher-risk appointment.

**VAT registration:** For projects above the VAT threshold (currently £90,000), a contractor billing without VAT is likely to be trading illegally or to be misrepresenting the project scope. Verify VAT registration numbers at the HMRC VAT checker (hmrc.gov.uk).

Comparing contractor quotes requires more care than simply comparing the total number. The most common mistake is choosing the lowest price without understanding what is included — and what is not.

• **What a good quote should include:**
• •A clear scope of works (what is included)
• •Clear exclusions (what is not included)
• •Material specification (what materials are allowed for; what the client supplies)
• •Programme (expected start and end dates, or programme duration)
• •Payment terms (stage payment schedule)
• •Variation procedure (how changes to scope will be handled)
• •Retention (if applicable)

• **Red flags in a quote:**
• •A single lump sum with no breakdown — you cannot check what is priced or compare against other quotes
• •No exclusions listed — the contractor has not thought about what they are not including
• •No payment terms specified — in practice this means you will be asked for large upfront payments
• •Unusually high initial deposit required (more than 10% upfront is unusual; requests for 30–50% upfront before any work has started are a serious red flag)
• •No programme dates — a contractor who will not commit to a start date may already be overcommitted

• **The price gap:**
• If one quote is substantially lower than the others (more than 20–25%), investigate why. Common reasons:
• •The contractor has missed scope items
• •The contractor is using substandard materials
• •The contractor intends to recover the difference through variations during the project
• •The contractor is financially struggling and is underpricing to win work

The right question when a quote is low is not 'can I get it even lower?' — it is 'what is missing from this price?'

A written contract between you and your contractor is essential for any project above a few thousand pounds. It protects both parties by creating clear, documented expectations on scope, price, programme, and procedure.

• **What a good contract should include:**
• •Parties (the correct company name and registered address of the contractor)
• •Description of the works (by reference to drawings and specification)
• •Contract price (lump sum or schedule of rates)
• •Payment schedule (interim valuations or stage payments with dates/triggers)
• •Programme (start date, expected duration, completion date)
• •Variation procedure (how changes are agreed and priced before being instructed)
• •Defects liability period (typically 6–12 months; the contractor returns to rectify defects at no cost)
• •Insurance requirements
• •Dispute resolution procedure

**JCT Minor Works:** The JCT Minor Works Building Contract (JCT MW) is an industry-standard short-form contract suitable for domestic renovation projects. It is available from the JCT website and provides a balanced framework that protects both homeowner and contractor. Many reputable London contractors use it or a close equivalent.

**Avoid verbal contracts:** A verbal agreement — regardless of how clear it seems at the time — is extremely difficult to enforce if a dispute arises. If your contractor resists signing a written contract, this is itself a red flag.