Compare Extension Builder Quotes: Get the Best in 2026 (UK)
The best extension quote is rarely the cheapest one — it is the one that is complete, itemised and comparable. For the same job, builder quotes commonly spread 15–30%, usually because the scope and spec differ, not just the profit. This guide shows you how to gather like-for-like quotes, what every quote should itemise, the red flags in a suspiciously low price, and the questions to ask before you sign.
What makes an extension quote the best one
Homeowners often rank quotes by the bottom-line figure alone. That is how projects go wrong. The best quote is the one you can trust to hold: it lists every element of the job, prices them separately, states what is excluded, and sets out when money changes hands. A slightly dearer quote that itemises the steels, the drainage and the kitchen is worth more than a cheaper lump sum that hides them, because the hidden items reappear later as variations you cannot easily challenge.
Judge a quote on three things: completeness (does it cover the whole scope?), clarity (is each line priced and explained?) and comparability (is it built on the same drawings and spec as your other quotes?). Only once those match can price be the deciding factor.
Why three quotes for the same job differ — 2026
Illustrative quotes for the same 30m² extension. The cheapest is not always the best — check what each one includes before you judge on price.
Illustrative figures · a 15–30% spread is typical on the same job
How to gather quotes you can actually compare
The single most valuable thing you can do is give every builder the same information. If one prices from a sketch and another from full drawings, their numbers mean nothing side by side. Before you invite quotes:
- Have drawings and a written spec ready. Even outline architect drawings put everyone on the same footing — see our architect cost guide.
- Write a short scope document listing exactly what is in and out: kitchen, flooring, decoration, landscaping.
- Ask for itemised quotes, not lump sums. Request the same breakdown from each builder so the lines align.
- Get three. Three is enough to reveal an outlier without stalling the job — more on that in how many quotes to get.
- Set the same start-date question so you can compare availability, not just price.
What every extension quote should itemise
Lay your quotes side by side and check each line is present in all of them. A missing line is usually the reason one quote looks cheaper.
| Line item | Why it matters |
|---|---|
| Design and professional fees | Architect, structural engineer and planning — often 10–15% and easy to omit. |
| Groundworks and foundations | Highly site-specific; a vague allowance here hides risk. |
| Structure and steels | Corner openings and knock-throughs; a common source of later variations. |
| Roof build-up | Warm roof, covering and rooflights should be named, not assumed. |
| Glazing and doors | Bi-folds and lanterns swing the price by thousands. |
| Kitchen / bathroom fit-out | Keep it a separate line so builder prices stay comparable. |
| VAT and stage payments | Confirm VAT is included and never pay more than 10–15% up front. |
Red flags in a suspiciously cheap quote
- A lump sum with no breakdown — you cannot see what has been left out.
- Scope gaps: no line for steels, drainage, making good or waste removal.
- A large cash deposit or a VAT-free offer — you lose protection if it goes wrong.
- Pressure to sign quickly, or a price that expires in days.
- No written programme, and no willingness to name recent projects you can see.
- A figure far below the other two on the same drawings — something is missing, not generous.
Questions to ask before you accept
- Can I see two recent extensions like mine, and speak to those owners?
- Who signs off Building Regulations, and is that cost in your quote?
- What is the stage-payment schedule, and what triggers each payment?
- How are variations priced if the scope changes mid-build?
- What is the start date and programme, and what could delay it?
- Can I see your public liability insurance? Check the cover level yourself, and ask what it excludes.
Treat trade-body membership and insurance as things you verify — ask to see the certificates and check them on the relevant register rather than taking a logo at face value.
Choosing between the builders themselves
Comparing the quotes is half the job; comparing the builders is the other half. Our main guide walks through vetting, references and how to weigh price against fit.
Your quote-comparison checklist
- Same drawings and written spec sent to every builder.
- Three itemised quotes, each with the lines above present.
- Exclusions written down on each quote, not left to assumption.
- VAT confirmed and a stage-payment schedule agreed in writing.
- Two recent, comparable projects seen or referenced for each builder.
- Insurance seen and the cover level checked by you.
- The winning quote judged on completeness first, price second.
FAQs: comparing extension builder quotes
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