How Many Quotes Should I Get for a House Extension in London?

Every piece of consumer advice about hiring tradespeople says to get three quotes. For a London house extension, three is still the right minimum — but the process of getting and comparing them requires more care than most homeowners expect. Done properly, the quote process gives you a realistic price expectation, a window into how different builders work, and the confidence to make a genuinely informed decision. Done badly, you end up with three numbers that you can't meaningfully compare.

This guide explains how many quotes to get, what you need before you start asking, how to make quotes comparable, and how to interpret what you receive.

The Case for Three Quotes — and Why Two Isn't Enough

With two quotes, you have no reference point. If one comes in at £65,000 and the other at £80,000, you don't know whether £65,000 is reasonable or dangerously low, whether £80,000 is fair or overpriced, or whether both are missing something important. You're guessing which is better.

Three quotes gives you a spread. If two quotes cluster in a similar range and one is significantly outside it — in either direction — that tells you something useful. It's not proof that the outlier is wrong, but it's a prompt to understand why. Has the cheap builder left something out? Has the expensive one included something the others haven't? Three gives you enough to identify a credible market rate.

Four or five quotes can be appropriate for very large projects (over £150,000) where the cost differential between contractors might be substantial and the stakes are high. Beyond five, you're creating noise rather than signal — and you're using up the time of builders who invest real effort in preparing detailed estimates.

What You Need Before You Invite Anyone to Quote

This is where most homeowners go wrong. Inviting builders to quote before you have drawings leads to verbal estimates that bear little resemblance to the final price. It wastes everyone's time and sets expectations that are almost impossible to manage.

Before you ask anyone to quote, you should have at minimum:

• Architectural drawings — floor plans and elevations showing the proposed extension. These don't need to be planning drawings if the project is PD, but they need to show what you want built.
• Structural engineer's calculations — if you're removing a load-bearing wall or spanning a significant opening, the builder needs to know what beam they're installing. Without this, steelwork is priced provisionally and may change.
• A clear specification — what type of roof, what glazing, what external finish, what floor level do you want? The more detail in the brief, the more comparable the quotes will be.

If you're not sure what's involved in getting from initial idea to builder's quote, our guide on how to choose a builder for a London extension walks through the full process, including what to ask at each stage.

How to Make Quotes Comparable

Give every builder exactly the same drawings and specification. Invite them to a site visit at a similar point in the design process. Ask them all for the same format of response: a breakdown of labour, materials, groundworks, steelwork, and a list of exclusions. Ask them to flag anything they've priced as provisional and to give their best estimate of the contingency risk.

The exclusions list is as important as what's included. Typical items that builders exclude — and that homeowners are surprised to find they need to budget for separately — include:

• Kitchen units, worktops, and appliances
• Floor finishes (tiles, wood, carpet)
• Decoration (painting and decorating after plastering)
• Skip hire and waste disposal (sometimes included, often not)
• Structural engineer's fees (if not already appointed)
• Party wall surveyor costs (if adjacent to a neighbour)
• Building regulations fees

When you add these costs back to each quote, the gap between the "cheapest" and "most expensive" builder often shrinks — or inverts. A quote that includes everything is often better value than a lower headline number that hides a long exclusions list.

Understanding Why Quotes Vary

A 10–20% spread between quotes for the same project is completely normal. Each builder has different labour costs, different supplier relationships, different overhead structures, and different views on the project risks. None of this means one of them is wrong.

A spread wider than 30–40% usually signals one of the following:

• Different scopes — one builder has included something the others haven't, or excluded something they've included
• Different quality assumptions — one builder has priced a higher-specification finish than the brief specified
• One builder is significantly under-resourced or underpriced and may be struggling for work
• One builder has identified a significant structural risk that the others have ignored

If you have a big outlier, go back to that builder and ask. Don't assume they're wrong — ask them what's driving their number. Sometimes the most expensive quote comes from the builder who has most carefully thought about the project.

The Cheapest Quote Is Rarely the Safest Choice

London homeowners lose significant sums every year to builders who win work on a low quote and then recover their margin through variation orders, delays, and poor workmanship that needs correcting. The cheapest quote feels like a win until the extras start arriving.

A quote that is meaningfully below market rate for a London extension in 2026 should prompt specific questions. What has been left out? How experienced is the builder in comparable projects? How full is their order book? The answers to these questions matter more than the number itself.

The opposite is also true. The most expensive quote isn't automatically the best quality — it may reflect a builder's high overhead, a busy schedule that requires a premium to displace other work, or simply an aggressive profit margin. Pay for what you understand, not for what you assume.

Negotiating Once You Have Quotes

Once you have your quotes, it's reasonable to go back to your preferred builder and have a candid conversation. If a specific line item seems higher than the others — materials, steelwork, a particular subcontract trade — ask them about it. Is there flexibility? Is there a specification change that could reduce cost without compromising quality?

What you should not do is ask a builder to simply match a competitor's number without any substantive discussion. If they agree without question, they either haven't thought about why their number was different, or they'll recover the margin elsewhere. Any agreed change in price should be reflected in a revised written quote that you both sign before work starts.

For background on the broader cost picture — what drives extension prices in London in 2026 and how material costs have moved — our article on kitchen extension costs in London gives a detailed breakdown that helps you calibrate what you're seeing in your quotes.

Using My Local London Builder to Get Started

One of the challenges in London is finding builders worth inviting to quote in the first place. The market is large, and identifying three genuinely experienced, reputable extension builders in your area takes time and carries risk if done without a reference point.

My Local London Builder connects homeowners with pre-vetted local builders who have a proven track record in London extension projects. When you submit your project details, we match you with suitable builders based on location, project type, and availability. We don't set prices — quotes come from the builders after they've seen your drawings and visited the site. What we do is reduce the risk in the selection process so that the builders you're comparing are genuinely comparable.

The quote process works best when you go into it with your drawings ready, your spec clear, and a realistic sense of what comparable projects cost. With those three things in place, three good quotes from vetted builders is all you need to make a confident decision.