How Much Does it Cost to Build a House Extension?

On this page we’ll cover extensions and how much they cost per square metre. The prices displayed below were updated in April 2022 and were verified by a selection of building contractors and architects.

Do bear in mind that the cost of a home extension is very different in the south of the UK compared to somewhere in the north. London also seems to have its own pricing structure, but we have a few examples of prices from finished extensions that should help you get an idea of how much they cost.

The cost of extensions and other major home improvements has also gone up considerably over the last 18 months due to a number of reasons and the cost of some building materials is still volatile.

An extension is a great way to get extra space in the home and with the UK’s “Permitted Developments” rules, you may be able to build one without planning permission too.

In a Hurry?

Here’s an overview of how much a home extension costs in the UK as of 2022.

We’ve included prices per square metre for the most common types of extension and all the prices include materials, labour, finishing off to a good standard and VAT:

Single Storey Extension (London)£2500 - £2750 per square metre
Single Storey Extension (South)£2000 - £2250 per square metre
Midlands and North£1750 - £2000 per square metre
Double Storey ExtensionAdd 50-60% to the cost of a single storey extension
Loft Conversion (Velux style, one room)£30,000 - £35,000
Loft Conversion (Velux windows to front and dormer to rear, one large room + ensuite)£45,000 - £50,000
Basement Extension£3750 - £4500 per square metre
Garage Conversion£800 £1200 psm (internal garage)
£1150 - £1450 psm (attached garage)
£1400 - £1600 psm (detached garage)
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Single Storey House Extension Cost: Two Examples

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

In early 2021 we advised a young couple with regards to a single storey extension they were considering.

By April 2022 the project was complete and they got in touch to let us know how much the final cost was.

We also worked on an extension in Portsmouth in late 2021, the prices for that project are also shown below:

Project:Details:Per Sq Mtr:
Single Storey Extension - SurreySide extension for a dining room, patio doors to rear garden£2000 psm
Single Extension - PortsmouthRear extension to extend the kitchen and create a kitchen diner area. Roof light and patio doors to rear garden£2300 psm
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Double Storey House Extension Cost – Two Examples

As a general rule of thumb, double-storey extensions are priced at plus 40 – 50%.

Any extension has essentially three parts:

  • Foundation.
  • A storey.
  • Roof.

Adding an extra storey doesn’t mean the cost doubles as the foundations and roof have already been budgeted for.

Assuming we go with the higher figure of +50%, the cost of the two previous extensions would now be:

Project:Details:Per Sq Mtr:
Dingle Storey Extension - SurreySide extension for a dining room, patio doors to rear garden. Room above£3000 psm (footprint)
or £1500 psm (total floorspace)
Single Extension - PortsmouthRear extension to extend the kitchen and create a kitchen diner area. Roof light and patio doors to rear garden, room above.£3450 psm (footprint) or £1700 psm (total floor space)
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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 a larger room and an en-suite shower/bath will probably cost you closer to £47,000. Most other types of loft extension will probably fall between these two prices, although London prices may be higher.

Read: Our guide to loft conversions

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 £3500 per square metre for the basic construction and fitting out fees from £750+ 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 5-10% of the build price, we have excluded this from our estimate.

We’ve included VAT in these prices.

Our prices are based on a standard extension using regular materials such as bricks and mortar. Glass or metal finishes or unusual materials can 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 which can easily add many thousands of pounds to the project.

Very small or very large projects are priced differently but our guide is relevant for projects with a footprint of 15 – 25 square metres.

Party wall notices and agreements can either cost very little or a small fortune, it depends on whether the neighbour consents to the work. This is how much a party wall notice and agreement costs.

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 in 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 choose to do yourself, if any.

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 April 2022.

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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