Getting and Comparing Builder Quotes

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.

Quote vs Estimate: The Distinction That Costs People Thousands

A quote is a fixed price. Once you accept it, the builder is legally bound to deliver the specified work for that amount. The Consumer Rights Act 2015 (Section 50) is clear: anything said or written to you about the service becomes part of the contract. A builder who quotes £48,000 and then invoices £56,000 has breached the contract, unless you authorised the additional work in writing.

An estimate is a rough guess. It carries no binding commitment. The final price just has to be "reasonable," and courts have accepted 10-25% above the original estimate as reasonable in construction disputes.

That gap matters enormously. A builder who "estimates" £38,000 and finishes at £52,000 hasn't done anything illegal. They gave you a non-binding number and the final cost, while painful, may well be deemed reasonable. But a builder who "quotes" £38,000 and invoices £52,000 without agreed variations owes you the difference.

Some builders blur the line deliberately. They'll say "quote" in conversation but write "estimate" on the paperwork. Read the document. The word at the top of the page is the one that matters in court.

The Specification Document: How to Make Quotes Comparable

The single most important thing you can do before requesting quotes is write a specification document.

Builders quoting from architect's drawings alone will each make different assumptions. One assumes you want a basic finish. Another assumes premium. One includes skirting boards and door furniture. Another assumes you'll sort that yourself. The drawings show walls, windows, and a roof. They don't show the level of finish in each room, the number of sockets, the type of flooring, whether you want downlights or pendant lights, or whether the garden gets reinstated afterwards.

A specification document fills that gap. You write it once, send it to every builder alongside the drawings, and suddenly they're all pricing the same job.

What to include

Go room by room. For each space in the extension, specify:

• Walls: plastered and painted, or tiled? If tiled, to what height?
• Floor: what finish? Tiles, engineered wood, vinyl? Who supplies and fits?
• Ceiling: plasterboard and skim, or will you want a vaulted ceiling with exposed beams?
• Electrics: how many double sockets, single sockets, light switches, and light fittings per room? Any outdoor sockets or lighting?
• Plumbing: sink location, dishwasher connection, boiling water tap, outside tap?
• Heating: radiators or underfloor heating? If UFH, what type of screed?
• Doors: internal door style, handles, ironmongery. Bifold or sliding doors: brand, size, colour
• Windows: style, material (uPVC, aluminium, timber), colour, opening mechanism
• External works: patio, path reinstatement, fencing, drainage, landscaping

Then add a section for general items: scaffolding, skip hire, waste removal, site welfare, protection of existing property during works, and locating and avoiding underground services before excavation. That last one (the CAT and Genny scan and trial holes) is a legal duty under HSG47 and is normally built into the groundwork price rather than a separate line, but make the builder confirm it is included rather than assumed, so it never reappears as an extra.

Click to enlarge

This takes an afternoon to write properly. It saves weeks of confusion and thousands of pounds in mismatched expectations.

Our template is a spreadsheet you can edit as decisions firm up and keep as the master record for the build. It's laid out by the eight site phases, in the order work actually happens, and a second tab shows a completed worked example for a single-storey rear kitchen extension so you can see what a thorough, builder-ready specification looks like before writing your own.

Spreadsheet

Builder Specification Spreadsheet

Fill-in specification structured by the eight site phases, with a completed worked example. Send identical copies to every builder to guarantee comparable quotes.