Why are extension quotes so different?

More than most people expect. The same footprint at a straightforward, good-quality or high-end finish is three different jobs: different floors, glazing, lighting, joinery and the hours of careful work that go with them. The table shows the all-in totals for a typical rear single and a typical rear dormer loft at each finish.

For the rear single, moving from a good-quality to a high-end finish adds about £6,000 to the likely figure, and dropping to a straightforward one saves about £14,000. For the rear dormer the same moves add about £7,000 or save about £14,500. If one quote assumes porcelain tiles and frameless glass and the other assumes laminate and a standard door, they will never agree.

Yes. The typical rear single here is 3.5 m deep and 5 m wide. Making it one metre deeper adds about £5,000 to the likely total and one metre shallower saves about £21,000. A metre is more foundation, more wall, more roof and more floor, and often a longer steel. If two builders measured the drawings differently, or one is pricing a sketch and the other the planning drawings, the size alone can explain the gap.

This is where the biggest single gaps hide. Adding a new kitchen to the rear single adds about £14,000 to the likely figure at a good-quality finish. Adding bifold or sliding doors adds about £500. One quote may include both, another neither, and a third may include the doors but leave the kitchen to a separate supplier.

Opening up the back of the house is another. Knocking through to join the extension to the existing rooms means a steel beam, temporary propping and a structural engineer's design. The report prices it when you say you want one; a quote may include it, price it separately, or leave it out until the engineer has done the calculations.

Check before anything else. Many builders quote without VAT, so a quote that says "plus VAT" will cost 20% more than the figure on the page. Add 20% to any "plus VAT" quote before comparing it with another quote or with the figures here, all of which include VAT at 20%.

A builder's quote is normally for the building work only. For the rear single at a good-quality finish that is £49,000 to £82,000. The professional and council fees are usually outside it: architect or designer £4,000 to £10,500; structural engineer £1,000 to £2,500; council prior approval fee £334; building control £1,000 to £2,000; party wall surveyors £0 to £6,000; Thames Water build-over agreement £0 to £1,000, coming to £6,500 to £22,500 in all. The party wall line is zero if the neighbours consent in writing.

A contingency is never in a builder's quote, because it is your money for what nobody can see until the ground is open or the walls are stripped. Allow 12% of the build at a good-quality finish, about £6,000 to £10,000 for the rear single. A quote that looks close to an all-in total may simply be missing these.

A provisional sum is an allowance for work that cannot be priced yet, such as drainage that has not been surveyed or foundations that depend on the soil. A prime cost or PC sum is an allowance for something you will choose later, such as the kitchen, tiles or sanitaryware. Both are placeholders, and the final bill moves with what they turn out to be.

A quote heavy with low provisional sums looks cheap and is not. Read each one, ask what it is based on, and set the same allowance across every quote so the figures can be compared.

Preliminaries are the costs of running the site rather than building anything: scaffolding, skips, welfare facilities, site supervision, insurance, protection of the house and parking. Overheads and profit keep the business going. Some builders list these separately; others spread them across every line. A quote without a preliminaries line has usually spread them across the other lines. Ask, because one that has genuinely left them out will find them later as extras.

A fixed price for a fully drawn scope puts more risk on the builder, who prices that risk in. A quote with many exclusions and provisional sums leaves the risk with you, so it looks lower on day one and grows as the job goes on. Neither is wrong, but they are not the same offer, and the lower number is often the one that carries more of your risk.

Give every builder the same written scope: the drawings, the finish level, what is in and what is out, and the allowances for any provisional or PC sums. Ask for the price with VAT shown, preliminaries on their own line, and a list of exclusions. Then the differences that remain are about price and approach, not about what each builder guessed you wanted.

A paid report's schedule of works lists every item and quantity of the job, so you can hand the same scope to each builder and compare the answers against the report's own ranges.

A higher quote can be the better buy. A firm that is busy can price to win only the jobs it wants; one that is careful prices the protection, the clearing up and the snagging; one with proper insurance, a warranty and a site manager carries real costs others skip. If the price is above the range for the job and finish, ask what it covers that the range does not.

A low quote is a warning when it is well below the bottom of the range for the job and finish, has few lines, leans on provisional sums, has no preliminaries anywhere, or asks for a large deposit up front. Ask what is missing before signing anything.

Should I just pick the middle quote?

Not by default. The middle number is only safe if all three quotes price the same scope. Line them up against the same written scope first, add VAT where it is missing, set the same provisional sums, and then judge them on what is left: price, programme, insurance and how they answered your questions.

Why is my quote so much lower than the figures here?

Usually because it is for the building work only and the figures here are all in. For a rear single at a good-quality finish, the building work is £49,000 to £82,000, the fees £6,500 to £22,500 and the contingency about £6,000 to £10,000. Check whether the quote is "plus VAT" and what it leaves out.

How many quotes should I get for an extension?

Three is the usual number: enough to see a pattern, few enough that each builder takes the job seriously. Give all three the same written scope, and allow a few weeks for them to visit, measure and price it properly. A quote returned in a day without a visit is a guess, not a price.