How to Choose a Builder in London: Complete 2026 Buyer's Guide

You're about to spend £50,000 to £200,000 on a renovation. The single biggest variable in whether it goes well isn't your architect, your budget, or your design taste. It's the builder. Get the choice right and the project finishes on time, on budget, and to a standard that adds value to your home. Get it wrong and you spend two years in mediation with a half-finished extension and a contractor who's gone bust.

This guide covers the four decisions a London homeowner has to make before signing anything: how you structure the project, where you find the builder, what contract you use, and how you shortlist. Each decision links to a deeper article that goes through the trade-offs in detail. By the end you should know which route suits your project, how to vet the firms you talk to, what questions to ask, and what red flags to walk away from.

What to expect from this guide

• Decision 1: Project route. Design-and-build, architect-led, project manager, or direct hire — the four delivery models, who carries the risk on each, and which fits your project size.
• Decision 2: Where to find a builder. Checkatrade and other directories versus architect referrals versus direct hire of a design-and-build firm.
• Decision 3: Contract structure. Fixed-price versus cost-plus, what each actually means in practice, and how variations get handled.
• Decision 4: Shortlist criteria. The verifiable signals that separate a serious firm from a flash-in-the-pan operation.
• Vetting checklist, red flags, and interview questions to use on the firms you actually meet.

Decision 1: How you structure the project

There are four ways to deliver a London renovation, and the first thing to decide is which one suits your project, your budget, and your appetite for risk.

Design and build. One firm does the architectural design, builds the work, and signs off the Building Control. You have one contract, one project manager, and one company to chase if anything goes wrong. The cost is the most predictable of any route. The design ceiling is "very good" but not "Architectural Digest" — design-and-build firms are competent designers, not always world-class ones. Best for kitchen extensions, loft conversions, bathroom refits, side return extensions, and full residential renovations up to about £200k.

Architect-led with separate contractor. A RIBA-chartered architect designs the work, you tender to several contractors, the winning contractor builds it, and the architect supervises through a contract administration role. The design ceiling is the highest of any route. The cost is the least predictable because you have two contracts (architect plus contractor), and any design changes mid-build cost more because they touch both parties. Best for listed buildings, conservation area work where design quality matters, basement digs, and any project north of £250k where the architectural ambition justifies the cost.

Project management firm. A PM coordinates separately-hired trades on your behalf. You hold the contracts with each trade, the PM holds your hand through the process. Costs roughly the same as design-and-build for the same scope but with weaker risk transfer (you carry overrun risk on each trade contract, not the PM). Best for homeowners who want the appearance of professional management but couldn't find a design-and-build firm that takes the project on.

Direct hire of trades. You hire the electrician, plumber, plasterer, joiner, and labourer directly. You are the project manager. Costs the least on paper, more in practice once you account for the wasted weekends, the trade clashes, and the times you got the order of work wrong. Best for very small jobs (under £15k) and for homeowners with construction industry experience.

The detailed breakdown of these four routes, with cost ranges, timelines, and verdict by project profile, is at Design and build vs architect-led.

Decision 2: Where you find them

Once you know the route, you need to find the firms or trades. There are seven main channels in the UK and each has different mechanics, fees, and risk profiles.

Checkatrade. A directory of vetted contractors. Members pay an annual fee (around £799 to £1,200 plus VAT depending on plan, as of 2026). Vetting includes ID checks, insurance verification, and a sample of reviews. Customers leave reviews after the job. Works for jobs from £500 to about £50k. Above that, the fee model means contractors are paying significant overhead that flows into your quote.

MyBuilder. Job-board model — you post a job, builders quote on it. MyBuilder takes around 5-8% of completed jobs (as of 2026). Reviews are tied to actual completed jobs. Better quality control than the directory model but the pool skews towards smaller jobs.

TrustATrader. Directory model similar to Checkatrade. Membership-fee based, similar fee structure.

Rated People. Lead-based model — builders pay per lead (£15 to £40 per lead typically, as of 2026). Means the builders quoting your job have paid to be there, which they recover through higher quotes. Works for genuinely competitive bidding but the economics push smaller-margin trades to the top.

Houzz. Design-led platform with contractor discovery. Higher fee model for contractors. Stronger on design inspiration than on builder vetting.

Architect referral. Your architect recommends contractors they've worked with on past projects. Free for you, biased towards builders who don't make life hard for the architect. Works well when the architect has been honest about who delivers on cost and time, less well when the architect is loyal to a builder who happens to be a friend.

Direct hire of a design-and-build firm. You skip platforms entirely and contact the firm direct. No platform fee means no padding in the quote for membership costs. Works best when the firm has a website that demonstrates the work, public reviews on Google (not just internal testimonials), and verifiable accreditations.

The detailed comparison of all seven channels, with fee mechanics and the project sizes each suits, is at Checkatrade alternatives for London renovation.

Decision 3: Contract structure

Once you've shortlisted firms, the quote will come back in one of two contract structures, and which one you accept matters as much as the price.

Fixed-price (lump-sum). The builder commits to a total figure for a defined scope. Cost overruns are theirs. Variations (scope changes you request mid-build) are quoted separately, in writing, before work proceeds. Most reputable design-and-build firms work this way on residential renovations.

Cost-plus (open-book). You pay actual labour and materials cost plus a percentage markup (typically 10-25%) or a fixed management fee. Overruns are yours. Works for listed buildings and projects with genuinely unknown ground conditions where pricing the unknowns under fixed-price either inflates the quote or sets the builder up to lose money.

The headline figure on a fixed-price quote is usually higher than a cost-plus estimate because a sensible contractor builds in a 5-10% contingency to cover their own risk. On a job that runs smoothly, cost-plus comes out cheaper. On a job with material price rises, weather delays, or an awkward delivery, fixed-price comes out cheaper because the contractor absorbs the overrun. The question isn't which is cheaper on day one. It's which protects you if things go wrong.

The detailed breakdown of fixed-price versus cost-plus, with PC sums, variation procedures, and red flags in fixed-price quotes, is at Fixed-price vs cost-plus contracts.

Decision 4: How you build the shortlist

Before you even contact a firm, work out what category of firm fits your project, then look for verifiable signals that the specific firm is real.

The four main categories of project delivery firms for London residential work:

• Design-and-build firms. Single contract, single PM, fixed-price typical.
• Architect-led project management. RIBA architect supervises a separately-tendered contractor.
• Dedicated project management firms. PM-only role, often chartered (RICS).
• Construction management firms. Coordinate sub-contractors on cost-plus basis, typically for larger projects.

A roundup of the four categories, with evaluation criteria for each, is at Best London construction project management firms.

Vetting checklist (apply to every firm)

Whatever route, whatever channel, the verification checks are the same. Apply this to every firm before you let them quote, never mind sign.

1. Companies House registration. Look up the company number at companieshouse.gov.uk. Check filed accounts (proves the business is real and trading), check director history (look for short-lived previous companies that went into liquidation), check confirmation statement date (active filing means active business).
2. Google reviews. Not internal testimonials. Real Google reviews under the company's Google Business Profile. Look at the count, the average rating, and the response pattern from the company. A firm with five 5-star reviews from accounts created the same day is faking it. A firm with 30+ reviews averaging 4.3+ with detailed text and the occasional 2-star with a measured response is real.
3. Trade accreditations. Match accreditations to scope. NICEIC for electrical work to BS 7671. FENSA or Certass for replacement windows. Gas Safe for boiler and gas appliance work. CHAS or SafeContractor for health and safety. RICS for surveying and PM. RIBA for architecture. All of these have public registers you can search by company name.
4. Contract structure. Ask whether they work fixed-price or cost-plus. The answer tells you about their confidence in scoping work. Fixed-price suggests they've done enough projects of this kind to price the risk. Cost-plus on a standard renovation should make you ask why.
5. Single project manager assigned. Will one person run the project from survey through handover, or are you handed from sales to delivery to snagging? Single-PM models produce better outcomes because there's no handover loss of context.
6. Building Control sign-off. Confirm they take responsibility for Building Control liaison, fees, and final certification. Some builders leave this for the homeowner to chase, which is a hidden cost and a source of disputes at completion.
7. Public liability insurance. Ask for the certificate. Standard residential cover is £2m. £5m is better for larger projects. Ask the date it expires.
8. Direct access to the principal. On a £100k+ project, you should be able to call the company director, not just a project coordinator. If you can't, the company is too big for the project (you'll be a small account) or has structured itself to insulate decision-makers from problems.

Red flags to walk away from

The following are not always disqualifying but they're warning signs serious enough that you should ask follow-up questions, and if the answers aren't convincing, walk away. A bad builder will cost you ten times more than the time spent finding a good one.

• The quote is materially below the others. For a London extension in 2026, anything below about £2,400 per sqm warrants questions. A £200k project quoted at £140k means either corner-cutting or under-quoting that will turn into mid-project arguments about variations.
• Heavily front-loaded payment. A 40% deposit on a £100k job before any materials arrive is a warning sign. Reasonable structures: 10-15% deposit at contract signing, then milestones tied to verifiable work stages.
• Vague scope. "Re-tile bathroom" with no PC sum for the tiles. "Plaster and decorate" with no specification of finish or paint grade. Every undefined item is a future argument.
• No written variations procedure. If they can't explain how mid-build changes get priced and signed off, they'll handle it badly when the inevitable happens.
• No mention of Building Control, planning fees, or structural engineer. These costs exist on every notable project. If they're not in the quote, they appear later as extras. A proper quote names the planning fee (£103 LDC or £206 full planning), Building Control fee (£400-700 typical), and any structural engineer pack (£600-1,200 typical).
• No filed accounts at Companies House. Either brand new (acceptable if you can verify experience another way) or in trouble.
• Only testimonials, no Google reviews. Anyone can paste a testimonial. Google reviews are tied to a Google account, can't be removed by the company, and accumulate over time. Their absence is a signal.
• The director is hard to find. If you can't speak to whoever owns the business on a £100k project, you're an account, not a client.

Questions to ask in the first meeting

Use these on the on-site survey. The answers tell you more than the eventual quote.

• How many projects of this size and scope have you finished in the last 12 months?
• Who will be my project manager and how often will I hear from them?
• What's your variations procedure when I change my mind about something mid-build?
• What's included in the quote for planning fees, Building Control fees, and structural engineer calculations?
• Can you give me two references from finished projects in the last year, both in writing and with phone numbers?
• What's your typical timeline from contract signing to start on site? (Anything under three weeks suggests the builder is desperate for work; anything over twelve weeks suggests they're too busy to give your project proper attention.)
• How do you handle Building Control inspections and final certification?
• What happens if I'm not happy with something during the snagging phase?
• Is there a retention held at completion? (5% held for 3-6 months is reasonable on larger jobs.)
• Are you NICEIC, FENSA, Gas Safe and CHAS accredited? Can I see the certificates?

Timeline expectations

From first contact to project completion, here's what to expect for a typical £80,000 to £130,000 London renovation:

• Week 1-2: Initial enquiry, on-site survey, written quote within 48-72 hours of survey. If quotes take longer than that, the builder is overloaded or disorganised.
• Week 2-4: Quote review, references checked, contract negotiated. Don't rush this. Take the longest a builder will tolerate without pressure.
• Week 3-5: Contract signed, deposit paid, planning applications submitted where needed (Lawful Development Certificate at £103, full planning at £206), structural engineer calculations commissioned, party wall notices served if required.
• Week 5-10: Planning approval (LDC is 6-8 weeks, full planning is 8 weeks plus). Party wall agreement reached (2 months from notice service). Structural engineer pack received. Building Control application submitted.
• Week 10-14: Start on site. Demolition and strip-out. Structural shell.
• Week 14-22: First fix electrical and plumbing, plastering, joinery, second fix, decorating.
• Week 22-26: Final fix, kitchen and bathroom fitting, snagging, Building Control final inspection, certificates issued.

This is the realistic timeline. Any builder promising a £100k extension finished in 8 weeks from contract signing is either lying or hasn't accounted for the planning, structural engineering, and Building Control process.

How All Well runs renovations

All Well Property Services is a design-and-build firm covering 25 South East London boroughs from an office at Unit 1 Limes Avenue, Anerley SE20 8QR. Founded 2020 by director Richard Pryce. Companies House registration 12721034. NICEIC, FENSA, Gas Safe and CHAS accredited. 57 verified Google reviews averaging 4.5/5.

Every project runs on a fixed-price contract. The process is a free on-site survey at your property, a written fixed-price quote within 48 hours, and a single project manager assigned from survey through handover. The quote covers labour, materials, all planning fees, Building Control fees, FENSA registration for new glazing, structural engineer calculations, party wall surveyor coordination where required, and Listed Building Consent or conservation area applications where the property needs them.

Services covered: kitchen extensions, bathroom fitting, side return extensions, loft conversions, painter and decorator, full property renovation, damp proofing, garage conversions, and end-of-tenancy painting. Typical project size £30,000 to £200,000. Building Control sign-off included on every project.

Frequently asked questions

How many builders should I get quotes from?

Three is the standard answer. Two is too few to spot the outlier. More than three is wasted effort once you've established the realistic price range. The point of multiple quotes isn't to find the cheapest, it's to confirm the market rate so you can identify the best value across price, scope clarity, contract structure, and how comfortable you feel with the firm.

How long should it take to get a quote?

For a properly priced fixed-price quote, 48-72 hours after the on-site survey is reasonable. Anything faster than 24 hours probably means the builder hasn't costed it properly. Anything slower than a week means they're overloaded and your project won't get attention. A vague verbal estimate at the survey itself ("around £80k") is not a quote and shouldn't be treated as one.

Should I accept the cheapest quote?

Not without understanding why it's cheapest. Cheap quotes usually mean one of three things: the scope is less complete than the others (re-read the spec), the builder is underestimating costs (they'll either lose money and walk off mid-project or pad it with variations later), or the firm has a structural cost advantage you can verify (maybe they own their own joinery shop, or have a long-term materials deal). The third case is rare. The first two are common.

What if I want to change my mind about something during the build?

This is what a variations procedure is for. On a proper fixed-price contract, the change gets quoted in writing before any work happens, you sign it off, then the builder proceeds. The cost is added to the final invoice or settled at the variation. Don't accept verbal agreements on phone calls. Don't accept "we'll just absorb it" — small concessions add up to disputes at completion.

How do I check a builder is actually who they say they are?

Companies House for the company itself (registration, directors, accounts filed). NICEIC, FENSA, Gas Safe and CHAS registers (each searchable by company name) for the accreditations. Google reviews under the firm's actual Google Business Profile, not testimonials hosted on their own site. References from finished projects, ideally in writing and with phone numbers. None of these checks take long and they collectively confirm whether the firm exists, trades, holds the accreditations they claim, and has a track record of finished work.

What's the contingency I should hold on a renovation budget?

10-15% on top of the contract price on a fixed-price job, 20-25% on a cost-plus job. The fixed-price contingency is for variations you decide on (upgraded tiles, extra rooflight, additional power sockets you didn't think of at the design stage). The cost-plus contingency is for variations you decide on plus contractor overruns. Plan for it before the project starts. Mid-build is the worst time to realise you're £15k short.

What's the realistic price per sqm for a London extension in 2026?

£2,400 to £3,500 per sqm for a single-storey rear extension to a Victorian or Edwardian terrace in South East London, including foundations, structural steel, glazing, plumbing and electrical first and second fix, plastering, and decorating. Higher for conservation areas or listed buildings. Lower for more remote London boroughs. Anything below £2,000 per sqm warrants questions about what's been excluded. Anything above £4,500 per sqm should also warrant questions unless the spec is genuinely premium.

Get a free quote

If you're planning a kitchen extension, bathroom refit, loft conversion or full renovation in South East London, All Well offers a free on-site survey and a written fixed-price quote within 48 hours. Every quote includes planning fees, Building Control fees, FENSA registration where applicable, structural engineer calculations, and a written variations procedure. Call 020 3920 9617 or use the contact form to book a survey. Unit 1 Limes Avenue, Anerley SE20 8QR.