How Much Does it Cost to Build a House Extension?

House extensions are typically very expensive to build but with the current financial climate, stamp duty and other house moving fees, many homeowners are choosing to extend rather than relocate.

The government has even relaxed many of the planning permission rules and you may be able to build your dream extension via the “Permitted Planning” route.

This page is all about house extensions and how much you can expect them to cost.

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Where Your Money Goes

The chart below shows how your money is spent:

Single Storey House Extension Cost – Two Examples

As with any build, the size, location and quality of the build are deciding cost factors.

The cost per square metre for a house extension cost is higher for a single storey project as you still need to pay for the foundations and a roof but you have less floor space than a double extension.

The price below assumes the work is carried out by a main contractor to a good standard. Many homeowners save money by project managing at least some of the build themselves.

Be sure to read our section about what we’ve excluded from this price:

Project:Price:Per Sq Mtr:
Single Extension - total area of 15 sq mtrs in SURREY£26,250£1750 psm
Single Extension - total area of 15 sq mtrs in DONCASTER£20,250£1350 psm
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Double Storey House Extension Cost – Two Examples

As with a single storey extension, the location of the project. its size and the quality of the finish are deciding factors. Below is our price guide for a two storey rear extension with a total floor space of 30 square metres.

The house extension price below assumes the work is carried out by a main contractor to a good standard. You could save money by project managing a least some of the work yourself, more about that later.

Be sure to read our section about what we’ve excluded from this price:

Project:Price:Per Sq Mtr:
Double Extension - total area of 30 sq mtrs in SURREY£46,500£1550 psm
Double Extension - total area of 30 sq mtrs in DONCASTER£37,500£1250 psm
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Loft Extension Price Guide

Windows in loft converted space

Why build out when you can build up?

Loft extensions are one of the most popular ways to get extra living space. They’re also cheaper than a typical extension and you won’t lose any of your garden to the build.

Added bonus – loft extensions are one of the least disruptive projects, most of the work is done from the outside via scaffolding. The only major work done inside your existing home is the staircase installation, piping and wiring.

A  simple “bare bones” one-room house extension can cost upwards of £27,000 but a fully fitted one with two rooms and an en-suite shower/bath will probably cost you closer to £43,000.

Our guide to loft conversions is a great read.

Basement Extensions

Basements cost more than loft, single storey and double storey extensions, this is because of the groundwork.

Most basement extensions cost in the region of £2500 per square metre for the basic construction and fitting out fees from £1000 per square metre.

Our recently updated basement price guide has more information about costs.

What’s Excluded From These House Extension Prices?

Architect fees can be anything from 6-12% of the build price, we have excluded this from our estimate.

We have excluded VAT from our guide price as some builders are trading below the VAT threshold, thus you’ll only pay VAT on the materials, not the labour.

Our prices are based on a standard extension using regular materials such as bricks and mortar. Glass or unusual materials will add considerable cost to the build.

We’ve also assumed the build is constructed to a good standard. Some homeowners prefer to have the project half-built, they then finish off the work themselves. Others prefer a higher quality finish with expensive fixtures and fittings.

Our house extension price guide doesn’t include any significant remodelling of kitchens or bathrooms.

Very small or very large projects are priced differently but our guide is relevant for anything from 15 – 45 square metres.

Planning Permission – Do You Need It?

The government relaxed planning permission rules several years ago and your project may now fall under “Permitted Development” regulations.

There are specific requirements you’ll need to adhere to, such as the location, aspect and size of your extension but many projects are now constructed without going down the traditional planning route.

The most common “permitted development” currently projects include:

  • Loft extensions where the dormer faces the rear garden.
  • Rear extensions that aren’t close to the boundary and are within size limits.
  • Porches that are no greater than 3 square metres.
  • Internal garage conversions where you’re not increasing the size of your home’s footprint but are changing an internal garage to a living space.

As the rules regarding permitted developments can change at any time, we suggest exploring the government’s Planning Portal for the most up to date advice.

Our guide titled 6 major home improvements you can do without planning permission is a great read.

How to Keep Costs to a Minimum

Many homeowners choose a builder to oversee the work, this person acts as a project manager and while he’ll undertake some of the work, much of it will be subcontracted to specialists.  This is the easiest option as the homeowner doesn’t need to manage the build day-to-day. The project manager will find suitable tradespeople, arrange for materials to be delivered on time, liaise with the architect and will handle all aspects of building control regulations.

The cost of using a builder/project manager adds around 15-20% to the cost of a typical build.

Some homeowners choose to save money by project managing the build themselves.

Key aspects of project managing include:

  • Working with the designer/architect.
  • Handling all aspects of building control, including arranging inspections.
  • Locating and hiring tradespeople and ensuring they start work at the correct time.
  • Ordering materials and ensuring they are delivered to the site at the correct time.
  • Quality control – it will be your responsibility to ensure that the project is completed to a high standard. Mistakes early in the build often have a knock-on effect that could cost you more in the long run.
  • Waste management, including the hiring of skips and any permits if you need to place them on a public road.

The project will almost certainly take longer and at least some experience of the building trade is preferable but savings can be made. Project managing is very difficult if you already have a full-time job but is possible if you work part-time, aren’t in work or work from home.

VAT Savings

VAT is a tax of 20% that is applied to most of the items used in a typical house extension. It’s also added to the labour a builder or company charge.

You can legally avoid VAT by:

  • Choosing tradespeople that are operating below the VAT threshold, they won’t charge you VAT on the labour element of the build.
  • Consider using second-hand materials as most private sellers are below the threshold for VAT registration. Ebay is full of homeowners selling leftover materials from building projects. You’ll find tiles, patio slabs, rolls of felt and timber all on sale without VAT.

We’ve created a great guide to legally avoiding VAT here.

Keep It Simple

Square or rectangular designs made with off-the-shelf materials are usually the cheapest builds.

Irregular shapes and custom made-to-order materials can add considerable cost to the project.


The cost of your extension will depend upon:

  • Location.
  • Size.
  • Quality of finish.
  • How much work you want to do yourself if any.

As a very general guide, for locations near London, you’ll pay between £1500 and £1800 per square metre for a side/rear extension.

For areas in the north of the country, the price drops to between £1100 and £1350 per square metre for a side/rear extension.

It’s very difficult to provide a price guide for basement extensions as there are many things that could significantly impact the cost. A ballpark figure of £3000 per square metre is a good starting point.

Loft extensions typically cost between £27,000 and £45,000 depending on where you live, how many rooms and how many en suites.

How Long Does it Take to Build an Extension?

On our blog, we’ve published a nifty guide that contains a week-by-week guide to building an extension.

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This guide was written by and was last updated in May 2021.

Don’t forget; we have price guides for hundreds of home improvement projects.

Explore our full list of detailed price guides here.

Author - Danny Woodley
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