Short answer: do not compare the totals first. Put the quotes side by side and check that each one prices the same scope, then compare the preliminaries, provisional sums, prime cost sums, contingency, exclusions, payment terms and VAT. The cheapest total is often the one that has left the most out.
Two quotes for the same extension or refurbishment can be tens of thousands of pounds apart and both be honest. The gap is rarely in the bricks and plaster. It is in what each quote includes, what it leaves for later, and how much of the risk it has priced. This checklist is the one we would use if we were comparing builders ourselves.
Quote or estimate?
An estimate is a guide price. A quote is a price for a defined scope of work. Make sure every builder is giving you a quote based on the same drawings and specification, in writing. If one builder has priced from a conversation and another from your architect’s drawings, the numbers are not comparable.
The checklist
| Check | What good looks like | Warning sign |
|---|---|---|
| Itemisation | Every task listed with a quantity, unit and rate | One line such as “rear extension, as discussed” |
| Preliminaries | A separate line for running the site | No preliminaries at all |
| Provisional sums | Few, clearly labelled and realistic | Many small allowances for big items |
| Prime cost sums | Allowances for items you choose, with the amount stated | “Client to supply” with no figure |
| Contingency | Shown on its own line, and unspent money stays with you | Hidden in the rates, or not mentioned |
| Exclusions | A written list | Nothing excluded, which is never true |
| Payments | Stage payments against work completed | Large payments before work is done |
| Programme | A week-by-week plan | Only a start date |
| VAT | Clearly stated as included or excluded | Not mentioned |
Is the quote itemised?
An itemised schedule of works lists every task with a quantity, a unit and a rate: square metres of slab, metres of steel, numbers of doors to hang. On our own tenders we group the lines the same way every time, from preliminaries and demolition through structure, roof, windows and doors, plastering, joinery, finishes, electrics, mechanical and plumbing. When quotes are laid out like this, you can compare them line by line and see exactly where they differ.
Preliminaries: the cost of running the site
Preliminaries cover everything needed to run the job rather than build anything: site set-up, project management, welfare facilities, protection, insurance, skips and waste, and small plant. On our recent London tenders they came to between 9.5% and 14% of the total.
If a quote has no preliminaries line, those costs have not disappeared. They are either spread through the other rates, which makes the quote harder to compare, or they have been left out and will appear later as extras.
Provisional sums and prime cost sums
Provisional sum
An allowance for work that cannot be priced exactly yet, such as foundations that depend on what is found in the ground, or joinery waiting on a supplier’s price. It is replaced by the actual cost once known, and can go up or down. Provisional sums are normal, but they are also where a low quote can hide. Look at how many there are, and whether each allowance would really pay for the work.
Prime cost sum
An allowance for a specific item you will choose, such as tiles, sanitaryware or appliances. The builder includes a fixed amount and the labour to fit it. If you choose something more expensive, you pay the difference. Check the amounts against what you actually want to buy: a kitchen allowance of a few thousand pounds will not pay for most London kitchens.
Contingency: who owns it?
A contingency is money set aside for what cannot be seen until work starts, such as rotten joists behind plaster or old wiring. We show a 10% contingency as its own line. Ask every builder whether their quote includes one, how much it is, and whether anything unspent stays with you. A builder who hides contingency in the rates keeps it whether it is needed or not.
Exclusions
Every quote leaves something out, and the honest ones say so. Typical exclusions include planning application fees, party wall surveyors, Building Control fees, the architect and structural engineer, asbestos removal, furniture, and items you buy directly. A quote that looks cheap because it excludes the professional fees is not cheaper; it is less complete.
Payments and retention
Check what triggers each payment. On our projects, stage payments are made against valuations of work actually completed, recorded as a percentage against each line of the schedule, with 2.5% retention held back until the final valuation. Retention gives you leverage to get snagging finished. Large payments for work not yet done move the risk onto you.
The programme
A start date and a finish date are not a programme. Ask for a week-by-week plan that shows when each trade is on site, when inspections happen, and when you need to have chosen finishes. It shows whether the builder has thought the job through, and it gives you something to measure progress against.
Insurance, certificates and the contract
- Ask for certificates of public liability insurance and insurance for the works themselves.
- Electrical work in a home must meet Building Regulations and be certified, and gas work must be carried out by a Gas Safe registered engineer. Ask who will do this work and what certificates you will receive.
- Ask who applies for Building Control approval and arranges the inspections.
- Agree a written contract. JCT publishes building contracts written for homeowners.
VAT
Building work to an existing home is normally charged at 20% VAT. Some work qualifies for a reduced rate, and HMRC’s VAT Notice 708 sets out the rules. Make sure every quote you compare is either before VAT or including it, not a mix.
Questions to ask every builder
- Is the quote itemised, with quantities and units for each line?
- What do the preliminaries cover?
- Which figures are provisional sums, and what could make them change?
- What is excluded?
- Is there a contingency, and does it belong to me?
- How are stage payments valued, and is any retention held until the end?
- Can I see a week-by-week programme?
- Who arranges Building Control, party wall notices and the structural engineer?
Start with a fair benchmark
If you want a figure before any builder visits, our renovation cost calculator gives a London range with preliminaries, contingency, fees and VAT on separate lines, the same way we lay out a quote. You can see a real extract of one of our schedules on how we work, or read what a house extension costs in London.
Questions people ask
What should a builder’s quote include?
An itemised list of the work with quantities and rates, preliminaries, any provisional sums and prime cost sums, a contingency, a list of exclusions, payment terms, a programme, and a clear statement of whether VAT is included.
What is the difference between a provisional sum and a prime cost sum?
A provisional sum is an allowance for work that cannot yet be priced exactly, such as foundations, and is replaced by the actual cost. A prime cost sum is an allowance for a specific item you choose, such as tiles or appliances, and you pay the difference if you choose something more expensive.
Should I choose the cheapest builder’s quote?
Only once you have checked that it prices the same scope. The cheapest quote often has more exclusions, lower provisional sums or no contingency, and the missing costs appear later as extras.
What is retention in a building contract?
A small percentage of each payment held back until the end of the job, to make sure defects and snagging are dealt with. On our projects, 2.5% is held until the final valuation.
How much deposit should I pay a builder?
There is no fixed rule. Paying for work already completed is safer than paying in advance. A builder may reasonably ask for payment for specific items ordered for you, such as bespoke joinery or glazing, so ask exactly what any upfront payment covers.
Sources
- HMRC VAT Notice 708: buildings and construction
- GOV.UK: building regulations approval
- JCT: contracts for homeowners
Last reviewed 16 September 2026. General information for England, not legal, tax or financial advice.
