How do I compare builders' quotes for an extension?

The short answer

You cannot compare three quotations that price three different things. The single most useful step before tendering is producing one set of drawings and one specification and asking every contractor to price exactly that.

Without it, the lowest number almost always reflects the most that has been left out. That is not dishonesty on the contractor's part. It is what happens when three experienced people each make their own assumptions about an incomplete brief.

Quotation or estimate

These are different documents and the distinction matters.

A quotation is a fixed offer to carry out defined work for a stated sum. An estimate is an opinion of likely cost, carrying no commitment. If a document does not say which it is, assume it is an estimate, because that is how it will behave when the final account lands.

Look for what the sum covers, what it excludes, how long the price holds, and what happens when something changes.

What a comparable quotation contains

Preliminaries. Scaffolding, skips and waste removal, welfare facilities, site protection, temporary works, insurance, and the contractor's management time. These are real costs on every project. A quotation that omits them is not cheaper, it is incomplete.

A breakdown by element. Substructure, superstructure, roof, external envelope, windows and doors, services, finishes. One figure for the whole job tells you nothing and makes variations impossible to price fairly later.

Named products, or a stated allowance. Rooflights, glazed doors, sanitaryware and ironmongery vary enormously in price. A quotation that says glazed doors without naming a system is not a price, it is a placeholder.

Programme. A start date, a duration, and what the contractor expects from you and when.

The contract. What form is proposed, what the payment terms are, and what retention is held.

Provisional sums, and how they behave

A provisional sum is an allowance for work that cannot be priced accurately yet, typically because it is hidden. Drainage alterations, unknown foundations, or making good after opening up.

They are legitimate and often unavoidable on an existing building. What matters is how many there are and how large they are. A quotation carrying several substantial provisional sums is closer to an estimate wearing a quotation's clothes, and the final figure will settle above it.

Ask what each provisional sum covers, what would make it larger, and when it will be resolved into a firm price.

What to check beyond the number

Insurance. Public liability as a minimum, and contractors' all risks for the works themselves. Ask for the certificate rather than the assurance.

Recent, relevant work. Two projects of comparable scale, in comparable properties, completed in the last two years, with owners willing to speak. A Victorian terrace in London is not the same job as a new build in the suburbs.

Who is actually doing the work. On a domestic extension, the person who quotes is not always the person managing the site. Ask who runs it day to day, and how often they will be there.

Payment structure. Staged payments against measured progress are normal. A large deposit before anything arrives on site is not.

The cheapest quotation

If one price is significantly below the others on the same information, something is different, and it is worth finding out what before you decide.

Sometimes it is a genuine gap in the programme the contractor wants to fill. More often it is a thinner specification, an omitted element, an optimistic allowance, or a contractor who intends to recover the difference through variations once work has started.

The point is not that cheap is bad. It is that a price you do not understand is a risk you have not priced.

Why the drawings decide the price

Every ambiguity in a tender package becomes either a contractor's risk allowance or a variation later. Both cost you money, and the second costs more, because you have lost the competitive tension once someone is on site.

Spending properly on drawings and a specification before tendering is the cheapest money in the project. It is what turns three quotations into a comparison rather than a guess.