Double Storey Extension Cost Guide 2025

Double storey extensions are an increasingly popular space-saving solution for homeowners looking to extend their property and transform the space. Understanding the costs involved is the first step to making that dream a reality; in this cost guide, the experienced team at BASE bring you this detailed cost guide to provide insights, advice and budgeting tips for planning your very own double storey extension.

Explore our architectural services

What is a double storey extension?

A double storey extension is a structural addition to an existing property that expands both the ground floor and the first floor. With an innovative design that builds up, instead of out, they’re a popular design solution for increasing square footage while preserving as much outdoor space as possible.

What’s the average cost of a double storey extension?

The average cost of a double storey extension can vary from anything between In 2024, the £1200/m² to £2200/m². Factors affecting the final costs include the size and design of the build, the quality of the build, and the internal features. A professional and experienced architect can help you design your extension project, providing guidance and expertise from the very start, helping your build stay on budget and timeline.

For a basic build, or small double storey extension (approximately 30 square metres)

£1500/m2 new build areas
£800/m2 refurbished areas
£40,000 – £60,000

For a standard high-quality build, or medium double storey extension ( approximately 50 square metres)

£1650/m2 new build areas
£875/m2 refurbished areas
£60,000 – £90,000

Premium quality build, or large double storey extension ( approximately 70 square metres)

£1800/m2 new build areas
£950/m2 refurbished areas
£90,000 – £120,000

Types of double storey rear extensions

Double storey rear extension

Arguably the most popular type of build, a double storey rear extension extends at the back of the house. This type of extension is best suited to terraced houses with limited space, as well as semi-detached properties if space is limited.

Double storey side extension

If your property has the space for a side extension, consider a side extension- it may be the perfect solution, although planning restrictions may be in place depending on the external finish.

Double storey wraparound extension

For homeowners with the space, time and budget, a double storey wraparound extension enables you to radically alter the layout of your home, reconfiguring the space to suit your needs. A blend of traditional stone work can blend with a sleek, contemporary steel frame, resulting in a completely unique build that transforms your home.

Feeling inspired? Check out some of our latest architectural projects for more extension ideas.

Calculating the cost of a double storey extension

Material costs

Material costs will be one of your highest outgoings – consider everything you need for the entirety of the project. This includes any necessary structural material, bricks and mortar, glazing, roof tiles and insulation, and any timber or steelwork required to bring your vision to life. Finishing touches such as a kitchen fittings, bathroom fittings and fixtures, and window fittings will also need to be costed up before the project begins.

Labour costs

Your labour costs will also count for a lot of your project budget. Tradespeople and specialists will be needed from the very start of your project, and will usually charge at an hourly or daily rate. The larger/ more complex your extension is, and the longer it takes, the more money you will spend, which is why having a specialist on board to help you plan the project will keep costs from spiralling. Specific trade services that will add to the extension cost will include builders, electricians, plumbers, window fitters, kitchen fitters and any other specialists that you need to complete the build. Always aim to work with accredited contractors for quality craftsmanship, efficiency and cost effectiveness,

Construction costs

This includes groundwork, foundation work, and structural elements such as adding additional walls, floors and roofing. New roofing will require a mix of construction materials as well as roof tiles, the cost of which will depend on the roof design. Think about groundwork and any site clearance, planning and preparation that will be needed before the work can go ahead. Demolition costs include removing any existing structures and debris, which’ll fall into the overall budget.

Planning permission fees and obtaining legal documentation

Before you start construction, you’ll need to obtain the necessary permits and planning approval from your local council. This may include obtaining planning permission, although some builds will not require it. The amount this will cost entirely depends on where you live, and the size and scope of your extension.

You will not be required to obtain planning permission if:

The eaves and pitch heights are no higher than your existing house
The roof pitch matches the existing house as much as possible
Your extension is more than seven metres from the rear boundary
The extension does not exceed more than 50% of the total land you own within the property
Your two storey extension extends the back of the property. Front and side extensions do not fall within permitted development rules.
No verandas or balconies added, and windows are obscured and non-opening if positioned within 1.7 metres from the floor.

Interior decor

The interior design of your extension is as important as the build; think carefully about how you plan to use the space, and bring in stylish fittings, furnishings, lighting and accessories that will truly make the house a home. If you are fitting a new kitchen or bathroom, this’ll add a considerable cost to the project.

Size of the extension

The overall square footage of your extension will greatly impact the price. Working with an architect can help forecast financially, as well as provide you with accurate site plans, measurements and drawings, so you can complete your extension within a budget and timeframe that suits you.

Complexity of design

Unique designs, rooflines, and architectural features can add to the cost of your double storey extension. Aim to work with an architect to design the extension so that you can make the right creative decisions to keep costs down while still being happy with the final build. The materials and finishes you choose will also have an impact on the price.

Location

Costs can vary depending on the location within the UK due to regional price variations. For example, if you live in a conservation area, you will need to take the appropriate steps as per local planning regulations. Similarly rural areas will have an impact on the cost depending on available access and site safety. Building a two storey extension in central London may also incur higher fees. This is due to the higher labour costs, expensive material costs, and the difficulties that come with working in such a compact site.

With over 20 years of experience, BASE architects can help bring your vision to life. If you’re feeling inspired for your own double storey extension project, speak to our team.