How to Choose a Builder in West London: 12 Questions to Ask Before You Sign

Choosing a builder is the decision that decides whether your extension or loft conversion is a good experience or a story you tell for years. In West London there is no shortage of firms – a search for “builders Hayes” or “builders Ealing” returns hundreds – but the difference between a good one and a bad one is rarely visible on a website. This guide from GSM Builders Ltd sets out the questions we would ask if we were hiring a builder ourselves, the paperwork that should exist before a spade goes in, and the warning signs that mean walk away.

Before you ask for quotes

• Know what you want. A sketch, a Pinterest board, and a rough budget. A builder cannot price “an extension”; they can price “a 4 m rear extension with a lantern, sliding doors and a kitchen”.
• Get the constraints checked first. Is the house in a conservation area? Is there a manhole at the back? Is the loft ridge over 2.2 m? A good builder checks these at the first visit; you can check the council’s conservation map yourself in five minutes.
• Ask three builders, not eight. Three itemised quotes are enough to see the range; more than that and you spend weeks comparing apples with oranges.

The 12 questions to ask every builder

1. Can I see the last three jobs like mine?

Not the portfolio – the last three. Ask for addresses (you can look at an extension from the street), and ask whether you can call the owners. A builder who has just finished a loft in Ruislip and an extension in Hounslow will say yes without hesitation.

2. Who exactly will be on site?

Is the person quoting the person managing the job? Are the trades employed or subcontracted? Neither answer is wrong, but you want to know who to call at 8 am when the plumber has not turned up.

3. Are you insured, and can I see the certificate?

Public liability of at least £2 million and employer’s liability of £10 million are standard. Ask for the certificate, not a promise. Check the expiry date.

4. What is included in the price – and what is not?

A proper quote is itemised by stage: groundworks, structure, roof, glazing, first fix, plaster, second fix, decoration, Building Control. It should say what is excluded (kitchen units, sanitaryware, VAT, fees) so you can compare like with like.

5. Is the price fixed, and what happens if something changes?

Ask how variations are handled. The honest answer is: a fixed price for the specified work, a written variation with a price before any change is made, and a contingency you hold rather than the builder.

6. What are the payment stages?

Staged payments tied to completed work – foundations, structure, roof, first fix, and so on – are normal. A large deposit before anything is delivered is not. Ten per cent at signing to secure a start date is reasonable; forty per cent is a warning sign.

7. Who handles planning, Building Control and the Party Wall Act?

The best answer is “we do, and it is in the price”. Some builders leave the paperwork to you, which is fine if you know what you are doing and a disaster if you do not. Ask specifically whether the Building Control completion certificate is included – you will need it when you sell.

8. Who does the structural design?

Steel beams, foundations near trees, and knock-throughs need an engineer’s calculations. Ask which engineer they use and whether the calculations are included.

9. How long will it take, and when can you start?

A realistic programme is 8–12 weeks for a single-storey extension and 6–10 weeks for a loft. A builder who can start next week in the middle of summer is either very lucky or very quiet – ask why.

10. What warranty do you give?

Twelve months on workmanship is the minimum you should accept; structural work often carries longer. Ask whether it is written into the contract and what the process is if something goes wrong.

11. Will we have a written contract?

Yes, always. For most domestic projects the JCT Home Owner Contract or the FMB contract is straightforward and fair to both sides. It should cover the price, the programme, payment stages, variations, insurance, and what happens if either side needs to end the agreement.

12. How do you keep the house liveable during the work?

Dust screens, floor protection, a working kitchen until the last fortnight, site tidied every evening, no radios before 8 am. Ask; the answer tells you how the builder thinks about you.

Paperwork that should exist before work starts

Red flags

• Cash-only pricing or “no VAT if you pay cash”
• No fixed address, or an address that is a mailbox
• A quote on a single page with one number and no exclusions
• Deposit of more than 10–15% before materials are on site
• Reluctance to show insurance, references or previous jobs
• Pressure to sign today for a “special price”
• Reviews that are all five stars, all recent, all in the same style – or none at all
• No mention of Building Control on an extension or loft

How to check a builder in ten minutes

• Companies House – search the company name: incorporation date, filed accounts, directors. A company that was dissolved and reformed under a similar name is worth a question.
• Google Business Profile – reviews with replies, photos of real jobs, and an address that matches the website.
• Checkatrade / TrustATrader / FMB – vetted membership and verified reviews.
• The website’s address and phone should match everywhere you find them. Mismatched numbers usually mean a lead-generation front, not a builder.
• Drive past a recent job. Brickwork that matches, neat pointing, straight guttering – the outside of an extension is a good proxy for the inside.

What you should expect from GSM Builders Ltd

We are a family-run building company at Leven Way, Hayes UB3, and we work the way this guide describes because it is how we would want to be treated: a free site visit and an itemised written quote; drawings, engineer’s calculations, planning, Building Control and Party Wall notices handled by us and included in the price; a JCT contract with staged payments; public and employer’s liability insurance you can see; a dedicated site manager; and a 12-month workmanship warranty with the Building Control completion certificate handed over at the end. See our house extensions, loft conversions and refurbishments, or check what we do in Hayes, Ealing, Hounslow and across West London.

Choosing a builder FAQs