How to Choose the Right Builder in London — 12 Questions to Ask Before You Appoint

The difference between the highest and lowest quote for the same project in London can be 30–50%. The cheapest quote is almost never the right choice. Low quotes are produced either because the contractor has missed scope, underestimated complexity, uses poorly skilled labour, or intends to make up the shortfall through variations once you are committed to them.

A contractor who gives you a precise, well-structured quote that reflects what the drawings actually show — including structural works, M&E, and Building Control fees — is demonstrating competence and attention to detail before they've picked up a tool.

Any builder taking on works above a few thousand pounds should be a registered company (Ltd or LLP) or a sole trader with a clearly documented business identity. Check Companies House (companieshouse.gov.uk) for company registration number, date of incorporation, filing history, and whether the company is active. A recently incorporated company with no accounts filed warrants further scrutiny.

Public liability insurance covers damage or injury to third parties (your neighbours, passers-by, your property) caused by the contractor's work. Most reputable residential builders carry £2 million PLI as a minimum; many carry £5 million or £10 million. Ask for the insurance certificate — not just a verbal confirmation — and check the expiry date, the level of cover, and what it covers.

Contracts works insurance (also called contractors all risk) covers damage to the works themselves — for example if the shell of a loft conversion is damaged in a storm before the roof is finished. Ask whether the contractor carries this.

A guarantee (or warranty) on construction works is only as good as the company behind it. A 10-year guarantee from a company that closes 18 months after completion is worthless. Ask:

• •What guarantee is offered and for how long?
• •Is the guarantee insurance-backed (i.e., underwritten by an insurer so it is valid even if the contractor ceases trading)?
• •What does it cover? Structural defects? Workmanship? Materials?

RCB Design & Build offers a 25-year structural warranty on all structural works. Ask for this in writing as part of any contractor appointment.

Reputable builders in the UK are often members of recognised trade bodies, which carry out vetting, have codes of conduct, and provide dispute resolution mechanisms:

• •FMB (Federation of Master Builders) — vetting of members, code of practice, deposit protection
• •TrustMark — government-endorsed quality scheme for tradespeople working in and around homes
• •Checkatrade — verified reviews from homeowners; background-checked members
• •NHBC — for new home builders
• •LABC Warranty / Premier Guarantee — for structural warranties

Membership of one or more of these is a positive signal. Ask to see membership certificates and verify them directly with the trade body.

Ask for a portfolio of completed projects — specifically projects similar to yours in type and scale. Ask for addresses you can view externally. Ask to speak to previous clients. A contractor who cannot provide references from recent clients, or who provides references only from projects completed many years ago, is a concern.

For loft conversions and extensions, look specifically at: quality of brickwork (pointing, courses, alignment), quality of roof tiling (straight lines, proper overlaps), plastering (flat, even, no hollows), and the quality of joinery (straight lines, tight mitres, consistent reveals). These are reliable indicators of overall site quality.

The person who quotes you is not always the person who manages your project. Ask:

• •Who will be the site manager or project manager for your project?
• •Will there be a consistent team on site or will subcontractors change?
• •How often will the PM visit site?
• •Who do you call if there is a problem?

For a loft conversion or extension, you want a dedicated site manager present at critical stages — foundation pour, structural steels installation, roof structure, first-fix inspections — and available by phone every day.

• A competent contractor should be able to explain clearly:
• •Whether your project needs planning permission or falls within Permitted Development
• •What Building Control route they use (local council or Approved Inspector)
• •Who coordinates the structural engineer
• •When Building Control inspections occur during the build

A contractor who says 'the council will sort all that' or who cannot explain the inspection process is not in control of the process.

Request an itemised quote, not a single lump sum. A well-structured quote will list: groundworks/foundations, structural steels, roofing, windows, first-fix (M&E), insulation, plastering, second-fix (M&E), internal joinery, bathroom installation, decoration, external works. Each section should have a line value.

• Key things to check:
• •Are Building Control fees included or extra?
• •Are structural engineer fees included?
• •Are scaffolding costs included?
• •Are skip hire and waste disposal included?
• •Is the Party Wall Surveyor fee included?
• •Are windows and doors included, and to what specification?
• •Is the bathroom suite included (or are you supplying it)?
• •Are floor finishes included?

Never pay 100% upfront. Never pay a large sum before work starts. A reasonable payment schedule for a standard loft conversion or extension might be:

• •Deposit on signing: 10–15%
• •Mobilisation / scaffold / groundworks: 15–20%
• •Structural shell complete: 20–25%
• •First-fix complete: 20%
• •Second-fix / decoration: 15%
• •Practical completion / snagging resolved: 5–10% retention

Any contractor demanding 40–50% upfront before starting work should be regarded with serious caution. A retention (typically 2.5–5%) held for 3–6 months after completion is standard practice.

A reputable contractor will use a written contract that defines the scope of work, the price, the programme, the payment schedule, what constitutes a variation, and the dispute resolution mechanism. A verbal agreement or a one-page letter is not sufficient protection for a project of £30,000+.

For residential works, the JCT Minor Works Building Contract (MW) or the JCT Homeowner Contract are the most commonly used forms. Some contractors use their own standard terms. Ask to see the contract before appointment. If a contractor does not use a written contract, that is a serious red flag.

Ask for a programme in weeks or a Gantt chart. A loft conversion should take 8–14 weeks; a rear extension 10–16 weeks; a large wraparound or double-storey extension 20–28 weeks. Be sceptical of very short programmes (cut corners) and very long programmes (contractor is overcommitted). Ask specifically when they intend to start and whether they have other projects running concurrently that might affect your site.

Variations (changes to the agreed scope during the build) are inevitable on any project. The question is how they are managed. Ask:

• •Will variations be priced and agreed in writing before being carried out?
• •Who authorises variations — you or the site manager?
• •Is there a dayworks rate for small variation items?

A contractor who carries out additional work without written agreement and then invoices for it at project completion is a problem you can avoid by asking this question upfront.