How many builder quotes should I get for an extension?

Aim for at least three written quotes so you have enough to spot an outlier and judge what a fair price looks like. Fewer than three leaves you with no reliable reference point.

What is the difference between a quote and an estimate?

A quote is a fixed price that legally binds the builder to deliver the specified work for that amount, while an estimate is only a rough non-binding guess. The word at the top of the document is what counts, so always ask for a fixed written quote.

How do I compare builder quotes like for like?

Every builder should be pricing the same architect-issued specification, then you strip out provisional sums, check VAT treatment, and list every exclusion side by side before looking at the headline price. Quotes for different scopes of work are not comparable, however similar the numbers look.

Why are builder quotes for the same job so different?

Quotes vary because each builder interprets the work differently: one may include scaffolding or be VAT-registered while another is not, and exclusions like decoration or external works can swing the total. The gap usually reflects different scope, not a better deal.

What is a provisional sum in a builder's quote?

A provisional sum is a placeholder figure for work that has not been fully specified yet, such as screed or finishes. It can rise sharply once the real cost is known, so two quotes are only comparable once provisional sums are identified and set aside.

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Phase 2 · Pre-Construction · Task 02 of 12

Getting and Comparing Builder Quotes

How to get like-for-like quotes from builders, what each line item means, where the hidden costs are, and a systematic approach to choosing the right builder at the right price. UK-specific for extensions.

17 min readUpdated 2026Premium

Builder quote comparison infographic showing three quotes side by side with highlighted differences in scope, exclusions, and VAT treatment

Prerequisites

Complete these first

Three quotes come back for the same extension, drawn by the same architect, from the same set of plans. One says £42,000. Another says £54,000. The third says £48,500. You're looking at a £12,000 spread and thinking the cheapest builder is the obvious choice.

They're not. And picking on price alone is one of the most expensive mistakes you can make on a building project.

Those three numbers don't represent three prices for the same work. They represent three different interpretations of what "the work" actually is. One includes scaffolding. One doesn't. One is VAT-registered and the other two aren't. One has a provisional sum of £1,300 for screed that will actually cost £2,600. One excludes decoration, flooring, and external works entirely. Until you strip all of that away and compare like with like, the headline number is meaningless.

This guide shows you how to do that.

Do this first

You should have already shortlisted builders using our finding a builder guide, and you need a clear picture of your total project budget from budgeting and contingency. Without both of those, you'll struggle to evaluate what's in front of you.

What this guide covers

01Quote vs Estimate: The Distinction That Costs People Thousands
02The Specification Document: How to Make Quotes Comparable
03What a Proper Quote Should Contain
04The Exclusions List Matters More Than the Total
05Provisional Sums and Prime Cost Sums: Where Quotes Hide
06The VAT Trap
07How to Compare Quotes Systematically
08When Quotes Are Wildly Different
09Fixed Price vs Cost Plus
10Protecting Your Deposit
11Quote Validity and Timing
12When to Use a Quantity Surveyor
13Making the Decision
14What Happens Next

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