Comparing building quotes like for like in London: how to stop three prices meaning three different jobs

Three quotes for a London building project mean nothing if they are not for the same job. One contractor prices a watertight shell; another includes kitchens, flooring and decoration; a third leaves out scaffolding, skips and VAT. This guide shows how to force quotes onto a like-for-like basis, which documents to issue first, and the red flags that separate a careful firm from a cowboy. Sources were checked on 28 October 2026.

Issue the same pack to every contractor

Before you ask for prices, assemble a single briefing pack:

• Drawings you own (or a clear sketch with dimensions), including structural engineer information if you have it.
• A written specification: finishes, windows, insulation standards, who supplies kitchens and bathrooms, who does decoration.
• Assumptions about access, parking, working hours and whether the house will be occupied.
• A request that every quote separate labour, materials, preliminaries, provisional sums, contingencies and VAT.
• A deadline and a request for programme length and insurance details.

If you only say "extend the kitchen by three metres", you will receive three incompatible numbers. HomeOwners Alliance and Checkatrade both stress itemised quotes against a defined scope; published extension bands (Checkatrade about £1,800 to £3,000 per m² nationally in the June 2026 type table; HomeOwners Alliance citing London single-storey from about £2,300 to £3,400 per m²) only help once the scope is fixed.

Build a comparison table, not a gut feel

Create a simple spreadsheet with a row for each cost head and a column for each contractor:

1. Preliminaries (site setup, welfare, scaffolding, skips, parking permits).
2. Substructure and drainage.
3. Superstructure, roof and waterproofing.
4. Windows and external doors.
5. First fix and second fix (electrics, plumbing, heating).
6. Plastering, flooring, decoration.
7. Provisional sums and client-supply items.
8. VAT and proposed payment stages.

Where a cell is blank, ask the contractor to fill it or to confirm it is excluded. A low total that hides missing scaffolding is not a saving. Citizens Advice advises getting more than one quote and agreeing the details in writing before work starts - the comparison table is how you make that advice practical.

Provisional sums and "PC sums" explained

A provisional sum is an allowance for work that is not yet fully designed. A prime cost (PC) sum is often an allowance for items you will choose later, such as tiles or sanitaryware. Both can be legitimate; both are abused. Ask:

• Is the sum for supply only, or supply and fit?
• What happens if the final choice costs more - who keeps any saving if it costs less?
• Are labour and waste included?

Large unexplained provisional sums are where thin quotes hide. Prefer fixed prices for defined work and small, named allowances for genuine choices.

Programme, attendance and exclusions

A twelve-week programme with the house empty is not the same job as a twenty-week programme with children at home and restricted hours. Ask each firm to state working hours, which trades they employ directly, which they subcontract, and who is on site daily. Ask what is excluded in one clear list. Exclusions such as "any work found necessary after opening up" without a variation procedure are a warning.

Vetting before you pick the middle quote

• Match the company on Companies House to the quote and the bank details.
• Confirm TrustMark or trade-body claims on the scheme's own register.
• Check public liability and employer's liability certificates.
• Take references for similar London jobs and, if possible, visit a site mid-build.
• Prefer a written contract (for example a JCT Home Owner form) with staged payments. Citizens Advice advises deposits no higher than 25 per cent and as low as possible.

Under the Consumer Rights Act 2015, services must be performed with reasonable care and skill. A cheap quote does not reduce that duty - but it does reduce your practical leverage if the firm runs out of money.

Reading the outliers

The highest quote is not automatically padded; it may be the only one that included Party Wall attendance, temporary works and a realistic contingency. The lowest quote is not automatically a bargain; it may omit VAT, scaffolding or making good. Ask the outliers to explain variances against your comparison table in writing. If they cannot, discard them. Paying a mid-range firm that answered every row is usually cheaper than funding variations on a hopeful low bid.

Red flags in the quoting process

• Refusal to itemise or to put exclusions in writing.
• Pressure to accept "today only" pricing.
• A deposit demand above 25 per cent before a contract is signed.
• Quotes that expire in days without a materials reason.
• Different company names on the quote, the website and the bank account.
• No interest in visiting the property before pricing a complex job.

Like-for-like comparison is tedious for an afternoon and cheap compared with a disputed final account. Do the table before you fall in love with a number.

Insurance, health and safety, and who is the client

Ask each quoting firm for current public liability and, where they employ people, employer's liability certificates, and match the insured name to the contracting company. On larger domestic projects you may have client duties under the Construction (Design and Management) Regulations 2015 - GOV.UK explains when domestic clients' duties pass to the contractor or principal contractor. Clarifying who plans, who manages and who is principal contractor avoids three firms each assuming someone else owns the risk. If a quote is silent on welfare, waste and scaffolding licences, price those lines yourself before you declare a winner. A tidy comparison table that includes insurance and CDM roles is worth more than a verbal assurance that "we handle all that".

Finally, sleep on the decision. The contractor who pressures you to sign the same evening you opened the quotes is selling urgency, not competence. Take references, revisit your table, and only then place the deposit - at the Citizens Advice level of as low as possible and not more than 25 per cent - into the verified company account or an agreed protection arrangement.

What to do if things go wrong

Keep a single folder - digital is fine - with the contract, every quote, every variation, photographs with dates, and every payment reference. Write to the trader clearly setting out what is wrong and what you want fixed, and give a reasonable deadline. Under the Consumer Rights Act 2015 you can ask for a repeat performance of a service that was not carried out with reasonable care and skill, and in some cases a price reduction. If the firm is TrustMark registered, use the scheme's complaints route. For suspected fraud or intimidation, contact the Citizens Advice consumer service on 0808 223 1133 or Report Fraud; call 999 if you are at immediate risk. Card payments should be raised with the issuer promptly while evidence is fresh. Do not keep paying "to keep them sweet" without a written plan for completion.

Sources checked on 28 October 2026: Checkatrade, House extension cost (June 2026); HomeOwners Alliance, House extension costs; Citizens Advice, Before you get work done on your home; Consumer Rights Act 2015, section 49; TrustMark. Ranges are published guide figures, not quotations. Nothing here is legal advice.