How Much Does a House Extension Cost? A 2026 Surrey Guide

Extension costs are the question we are asked first and the one most builders answer last. It is a fair question, and it deserves a straight answer rather than an invitation to book a visit. What follows is how the money actually breaks down on the extensions we build across Surrey and South West London, and what separates a project at the bottom of the range from one at the top.

The Published Range

Our home extension projects typically fall between £35,000 and £90,000. That is a wide band, deliberately so, and the width is the honest part: a modest single-storey rear addition and a double-storey wrap-around with structural glazing are different undertakings that happen to share a word.

That figure covers the whole project rather than a shell price. Foundations, structure, roof, glazing, the internal fit-out of the new space, and making good where new meets old are all inside it. It does not include a kitchen specification, which is a substantial line in its own right where the extension is built to house one.

Where in the Range Your Project Lands

Five things move the figure more than anything else:

Footprint and storeys. Cost does not scale neatly with floor area. A second storey adds bedroom space at a lower cost per square metre than the first, because the expensive groundwork is already done. A wrap-around adds more external wall and roof per square metre gained than a straight rear addition.
Ground conditions. This is the line that surprises people. Trees, made ground, a high water table, drainage runs crossing the footprint or a shallow existing foundation can all mean deeper digs, engineered solutions or a build-over agreement with the water authority. It is invisible until someone digs, which is why any figure given without a site visit is a guess.
Glazing. Bi-fold and sliding systems, roof lanterns and structural glazing sit well above standard windows, and the structure supporting a large opening has to be designed for it. Glazing is often the single easiest place to move a budget in either direction late in the design.
Structural complexity. Removing the rear wall of a house to open it into the new space needs steel, engineer's calculations and building control sign-off. The wider the opening and the more load above it, the heavier the solution.
Specification of the fit-out. Underfloor heating, the floor covering, joinery, lighting and the standard of finish are where two extensions of identical size diverge sharply.

What a Complete Extension Should Include

When comparing proposals, check that each of these is priced rather than assumed:

Site set-up, protection and waste removal throughout
Groundworks, foundations and any drainage alterations or build-over agreement
Structural steelwork, with a chartered engineer's design and calculations
Superstructure, roof, and weatherproofing
Windows, doors and any rooflights or lanterns
First and second fix electrics and plumbing
Insulation, plastering, flooring and decoration
Making good the junction between the new space and the existing house
Building control inspections and the completion certificate

The last two are the ones most often left out of a cheap-looking figure. Making good is not a small job, and a proposal that ends at the point the extension is watertight has left a significant piece of work with you.

Comparing Proposals Fairly

A lower number is not automatically better value, and the difference between two proposals is usually scope rather than margin. Three questions expose most of it:

Is it itemised, or a single figure? A single figure cannot be interrogated, and cannot be adjusted without renegotiating the whole job.
What is excluded? Read the exclusions before the inclusions. Kitchens, landscaping, making good and building control fees are the common omissions.
What happens if the ground is not as expected? Every builder faces the same unknown. The question is whether the proposal says in advance how a variation is priced and agreed.

Planning and Building Regulations

Many extensions fall within Permitted Development, and many do not. Deeper single-storey rear extensions may be possible under the Larger Home Extension scheme, which requires Prior Approval rather than a full application. Conservation areas, listed buildings and properties subject to an Article 4 direction are treated differently, and flats are excluded from Permitted Development entirely. Building Regulations approval is required regardless of the planning route. We confirm the position for your specific property as part of every Cost Proposal, and our guide to planning permission for home extensions sets out the routes in more detail.

If the Extension Is About the Kitchen

A large share of the extensions we build exist to create a kitchen-diner. If that is your intention, the kitchen is a major line rather than a detail, and it is worth reading our kitchen extension versus new kitchen comparison before settling on the route. Some households who assume they need an extension are better served by a structural opening and a properly designed kitchen within the existing footprint, at a fraction of the cost and disruption.

How We Price

We design and build under one roof, which means the person pricing the work is the person who will carry it out. After a free site visit we produce a fully itemised Cost Proposal, line by line, with the exclusions stated plainly. Our weekly billing model means you pay for completed, signed-off work each week rather than handing over a large deposit in advance, so the cost stays visible as the project runs rather than arriving as a surprise at the end.

If you would like a figure for your own property rather than a range, our extensions service page explains how we work, and a free Cost Proposal is the next step.

How Much Does a House Extension Cost? A 2026 Surrey Guide