Choosing a Contractor5 min

How to Vet a Building Contractor in London: The Essential Checks Before You Sign

Choosing the right building contractor is one of the highest-stakes decisions a London homeowner makes. A poorly chosen contractor can leave a project incomplete, cause structural damage, disappear with a deposit, or deliver work of such poor quality that rectification costs more than the original contract. The market for residential building work in London is large, fragmented, and largely unregulated — anyone can call themselves a builder, and the barrier to entry is extremely low. In this environment, the homeowner's protection comes entirely from their own due diligence: vetting the contractor properly before signing a contract and committing money. This guide sets out the essential checks every London homeowner should carry out before engaging a building contractor, the questions that distinguish a genuine professional from an unreliable or dishonest operator, and the red flags that should prompt further investigation or a decision to walk away.

Key Takeaways

✓Verify the contractor's legal entity before any other check. Obtain the full legal name of the business entity you will be contracting with — whether it is a sole trader (contracting in a personal name), a partnership, or a limited company. If the contractor operates as a limited company, search Companies House (companies.co.uk) for the company name. Check: that the company exists and is active (not dissolved, in administration, or struck off); the date of incorporation (a recently incorporated company may indicate a phoenix company formed after a previous company was dissolved due to financial difficulties or complaints); the directors' names (check whether the same directors have been associated with previously dissolved or struck-off companies by searching their officer history on Companies House); the filed accounts (annual accounts filed at Companies House give an indication of the company's financial health — a company with persistent losses, negative net assets, or long periods of overdue accounts may be in financial difficulty). A contractor who is unwilling to provide their Companies House number, or who cannot explain which legal entity they are contracting through, is a significant red flag
✓Verify the contractor's insurance before any payment or site access. Request written evidence (certificate of insurance) for: Public Liability Insurance (PLI) — minimum £2 million cover, ideally £5 million, per claim; confirming the insured's name matches the contracting entity; confirming the policy is current (not expired or about to expire during the project); Employer's Liability Insurance (ELI) — minimum £10 million, required by law for any contractor with employees; check that the ELI certificate is current and that the named insured matches the contracting entity. Some contractors present outdated insurance certificates, or certificates for a related but different legal entity — the certificate must show the legal name of the entity you are contracting with. Where the project involves structural work, asbestos removal, or other specialist works, verify that the contractor has appropriate specialist coverage (e.g., asbestos removal contractors should have specific asbestos liability coverage). Contract Works Insurance (CAR) — not legally required but a mark of a professional contractor; CAR covers the partially completed works against damage during construction
✓Check trade body membership and scheme registration carefully. UK residential construction has several established trade and quality assurance schemes. The most relevant for London homeowners are: Federation of Master Builders (FMB) — membership requires the business to pass an independent site inspection and to carry master builder's warranty cover. FMB members are required to follow the FMB's Code of Practice. The FMB maintains a searchable register at findabuilder.co.uk. TrustMark — a Government-endorsed quality scheme for tradespeople. TrustMark registered firms have been assessed for their technical competence, trading practices, and customer service standards. The TrustMark register is searchable at trustmark.org.uk. Checkatrade — a directory and review platform where contractors are vetted before listing (identity, insurance, and qualifications checks) and publicly reviewed by customers. Checkatrade is one of the most widely used review platforms for London residential work. Note: none of these schemes guarantee quality on every project, and membership or registration does not eliminate risk — but a contractor with verified membership of one or more of these schemes has at least passed a minimum vetting standard that an unregistered contractor has not. Always verify membership directly with the scheme (not just from the contractor's own marketing materials) by searching the scheme's register
✓Take up references — and take them seriously. Every reputable building contractor should be able to provide at least 2–3 references from recent clients for projects of a similar type and scale to yours. Before contacting references, check: that the reference is for a client (not a subcontractor, supplier, or industry contact); that the project is recent (within the last 2–3 years) and of a comparable type and value to your project; and that you have the client's direct contact details (not a reference through the contractor's own office). When speaking with the reference: ask whether the project was completed on time and on budget; ask whether there were any issues during the project and how the contractor resolved them; ask whether the finished work matched the contractual specification; ask whether they would use the contractor again; and ask whether they would recommend the contractor without reservation. A reference who is evasive, vague, or unenthusiastic is as informative as a glowing reference — and a reference who actively warns you about specific problems is essential information. The most reliable references are those you find independently (through a neighbour or local Facebook group who used the same contractor) rather than those provided by the contractor themselves
✓Agree the contract structure before any money changes hands. A professional building contractor will be comfortable with a written contract. The minimum requirements for a residential building contract in the UK include: a detailed scope of works (what is included in the contract price); the contract price (fixed price or remeasured); payment terms (never pay more than 10–15% as a deposit; progress payments should be linked to measurable stages of work completion, not calendar dates); a programme (start date, expected completion date); variation procedure (how changes to the scope are agreed and priced); defects rectification provisions (the contractor's obligation to remedy defects identified within a reasonable period after practical completion — typically 6–12 months); and dispute resolution provisions. Many reputable contractors and consumer protection organisations recommend using the FMB's standard form of contract or the JCT Minor Works Building Contract (or its associated Homeowner/Occupier Contract) for residential projects. A contractor who refuses to sign any written contract, or who insists on an entirely verbal arrangement, is operating outside normal professional standards and should be avoided

1. Questions to Ask Every Contractor at First Contact
2. The Red Flags That Expose Unreliable Contractors
3. The Right Payment Structure for a London Building Project
4. Frequently Asked Questions

Questions to Ask Every Contractor at First Contact

Before inviting a contractor to submit a price, the following questions will quickly distinguish a professional operation from an unreliable one:

How long have you been trading, and under what company name? A professional contractor will answer this clearly and confidently. Ask for the Companies House number if they are a limited company. A vague or defensive answer is a red flag.

Are you VAT registered? A contractor with an annual turnover above the VAT registration threshold (currently £90,000 per annum for 2024–25) is legally required to be VAT registered. A contractor carrying out a significant London renovation project who is not VAT registered, or who suggests arrangements to avoid VAT (cash-in-hand pricing below what a formal VAT-registered price would be), is raising a significant compliance and quality risk signal. VAT-unregistered work is not necessarily dishonest for smaller contractors, but for a project above £30,000–40,000, the contractor should either be VAT registered or clearly operating below the threshold.

Can I see your current insurance certificates? A confident, professional contractor will produce these immediately and without hesitation. A contractor who says they will "bring them on site" or "email them later" but repeatedly fails to do so is raising a red flag.

Who will be on site every day, and who is my point of contact? On a professional residential project, there should be a named site supervisor or lead tradesperson who is present on site consistently. A contractor who proposes to manage the project remotely without a consistent on-site presence is a risk on quality control and programme.

What is your process for variations to the scope? The answer to this question reveals whether the contractor has a structured process (ideally written variation orders approved before work proceeds, with agreed additional costs) or an ad-hoc approach (verbal agreements that may lead to disputes later).

Have you carried out a project similar to this in the last 12 months, and can I speak to that client? A contractor who cannot point to a comparable recent project should be asked why — and the references provided should be verified directly.

The Red Flags That Expose Unreliable Contractors

The following patterns of behaviour should cause a homeowner to investigate further or walk away:

Excessive deposit demands: A contractor who demands 30–50% of the project value as an upfront deposit before any work begins is either in financial difficulty (needing your money to fund ongoing commitments or materials for other projects) or is intending to disappear. A reasonable deposit for a London residential project is typically 10–15% of the contract value. For a small project (under £5,000), a deposit of up to 25–30% may be reasonable given the material costs relative to contract value, but even then, the deposit should be linked to materials being ordered rather than just to the contractor's availability.

Pressure to decide quickly: "I have another job starting next week — if you don't confirm by Friday I'm not available." Time pressure is a classic sales manipulation tactic used to prevent the homeowner from taking the time to carry out proper due diligence. A reputable contractor with a full order book does not need to pressure clients into hasty decisions.

Unwillingness to provide a written contract or detailed scope: "We'll sort out the details as we go" is not acceptable for any project above a minor maintenance job. A written scope protects both parties — a contractor who resists this is typically seeking to maximise informal scope creep and charge extras for items that a homeowner reasonably assumed were included.

Cannot provide insurance certificates immediately: As noted above, current insurance certificates should be available immediately on request. A contractor who cannot produce them promptly has either inadequate insurance or no insurance.

No landline, no company address, no online presence: While sole traders and smaller contractors may legitimately operate without a full company infrastructure, a contractor who has no verifiable business address, no landline number, no Companies House presence, and no online footprint is operating without accountability structures.

Cash-only pricing with significant discounts for cash: While cash payment is not inherently suspicious for small jobs, a contractor who offers substantial discounts for cash payment on larger projects is typically operating outside HMRC's tax system, which means they are also likely operating without proper insurance and outside the regulatory framework — creating risk for the homeowner.

Previous company names or dormant companies: A search on Companies House for the director's officer history may reveal multiple previously dissolved or struck-off companies. Multiple company dissolutions over a short period, particularly in the same or related trade, is a strong indicator of a "phoenix" operator who repeatedly incorporates new companies to avoid debts from failed predecessors.

The Right Payment Structure for a London Building Project

The payment structure for a building project is one of the most important contractual protections for the homeowner. The fundamental principle is: never pay significantly in advance of work carried out.

Recommended payment structure for a standard London renovation or extension: Deposit: 10–15% of the contract value, payable on signing of the contract. This covers the contractor's initial mobilisation costs and materials ordering. The deposit should not be paid before a written contract is signed.

Stage payments: typically 4–6 stage payments (for a project with a contract value of £30,000–100,000) linked to specific, verifiable stages of work completion. For example: structural stage complete (foundation, steels in, roof structure up); first-fix trades complete (first-fix electrics, plumbing, and carpentry); plastering complete; second-fix and decoration complete. Each payment should be made within 7–14 days of the stage being confirmed as complete — the homeowner should physically inspect (or have their architect or project manager inspect) the completed stage before authorising payment.

Retention: typically 5% of the contract value retained until practical completion (the date when the project is substantially complete and ready for use), and a further portion (sometimes the full 5%, sometimes split 2.5%/2.5%) retained for 6–12 months after practical completion to cover any defects that emerge during the defects period. The retention is then released when the contractor has remedied all defects notified during the defects period.

Final payment: the final payment (releasing the balance of retention after the defects period) is the contractor's financial incentive to respond to and remedy defects. Releasing the full balance before the defects period is over removes this incentive.

Practical enforcement: The payment structure only works as protection if the homeowner actually withholds payments at the appropriate stage. A homeowner who pays ahead of the contractual schedule (because the contractor says they need it for materials, or because the site is going well) loses the leverage the payment structure is designed to provide. Stick to the contract payment schedule.

Frequently Asked Questions

Should I always get three quotes for a building project?▼

The conventional wisdom of getting three quotes is a useful starting point but is not an end in itself. Three quotes are valuable because they give you a market reference for the price range for your project, expose significant outliers (a quote that is 40% below the other two deserves investigation — it may reflect a misunderstanding of the scope, a plan to claim back the cost in variations, or unsustainable pricing from a contractor in financial difficulty), and provide a basis for discussing scope and value with each contractor. However, comparing quotes is only meaningful if all three contractors are quoting for exactly the same scope. Where quotes are received on a different basis (different specifications, different inclusions, different exclusions), direct comparison is misleading. A better approach is to prepare a detailed scope of works (ideally with the help of an architect or quantity surveyor) and send the same document to all three contractors, asking them to price it on a like-for-like basis. Any deviations from the specified scope should be clearly identified in each contractor's submission.

Is Checkatrade a reliable way to find a contractor?▼

Checkatrade is a useful starting point for finding contractors in London — the platform carries out identity and insurance verification checks on listed contractors, and the customer reviews (which are verified by Checkatrade as genuine customer feedback) provide useful qualitative information about contractors' reliability, communication, and quality of work. However, Checkatrade is a commercial directory (contractors pay to be listed) and the vetting process is not as rigorous as the FMB's site inspection or TrustMark's technical competence assessment. The most reliable uses of Checkatrade are: searching for contractors who have a significant number of verified reviews specifically for projects of the type and scale you require (10+ reviews for loft conversions, for example, rather than 30 reviews for general handyman work); reading negative reviews carefully — how a contractor responds to complaints is as informative as the complaint itself; and using it as a starting point to identify candidates who then undergo your own full vetting process (insurance verification, Companies House check, reference calls) before being invited to quote.

What should I do if the contractor has gone quiet during the project?▼

A contractor who has gone quiet — not responding to calls or messages, not appearing on site, or providing vague excuses for delays — is a serious warning sign. The first step is to review the contract: what are the contractor's contractual obligations in relation to site attendance, communication, and programme? Write a formal letter (or email) to the contractor's registered address stating the specific concern, the contractual obligation that is not being met, and requesting a specific response or a site meeting within a defined period (typically 7–14 days). If the contractor does not respond, or the response is unsatisfactory, the contract may give you the right to terminate for contractor default and engage another contractor to complete the work — with the original contractor liable for the additional cost of completion. Before terminating, seek legal advice (ideally from a solicitor experienced in construction disputes, or via the FMB if the contractor is an FMB member) — wrongful termination of a building contract can expose the homeowner to a counter-claim from the contractor. Keep all correspondence and records of what has been completed and paid.

Important Note

This guide is for general information only. Building regulations, planning rules, and legal requirements change regularly and vary by local authority. Always seek professional advice specific to your project and location. RCB Design & Build offers free initial consultations — book your free survey.

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