How Much Does a House Extension Cost in 2026?

Introduction

If you are planning a house extension in 2026, one of the first questions you will ask is how much it is going to cost. The answer depends on several factors, from the size and type of extension to the specification of finishes and your location within the UK. Getting a clear picture of costs early on is essential for setting a realistic budget and avoiding unwelcome surprises once construction is underway.

As a general guide, house extension costs in 2026 typically fall within the range of £1,500 to £3,000+ per square metre, depending on the complexity and quality of the build. A straightforward rear extension with a standard specification will sit at the lower end of that range, while high-spec builds featuring bi-fold doors, underfloor heating, bespoke joinery and premium finishes will push costs towards the upper end and beyond.

In this guide, we break down what you can expect to pay for different types of extension, explain the key factors that influence pricing, and offer practical advice on how to get an accurate estimate before you commit to a builder.

Single-Storey Extension Costs

Single-storey extensions remain the most popular way to add space to a home. Whether you need a larger kitchen, an open-plan living area or a ground-floor bedroom, a single-storey extension is often the most cost-effective route. For a typical single-storey rear extension measuring between 15 and 25 square metres, you can expect to pay somewhere in the region of £30,000 to £60,000 in 2026.

The wide range in pricing reflects the significant variation in specification. A basic plastered shell with standard windows and a tiled roof will cost considerably less than an extension with structural glazing, vaulted ceilings and a high-end kitchen fit-out. It is also worth noting that smaller extensions do not always cost proportionally less, because many fixed costs such as foundations, drainage connections and building control fees apply regardless of size.

If you are considering a wrap-around or side-return extension, costs will increase because of the additional foundations, structural steelwork and roofing involved. Side-return infill extensions in terraced properties can be surprisingly cost-effective though, as they often make use of an existing party wall on one side. Expect to pay £35,000 to £55,000 for a well-specified side-return project.

Two-Storey Extension Costs

A two-storey extension offers significantly more additional floor space than a single-storey build, and while the overall cost is higher, the cost per square metre is often lower. This is because the foundations, roof structure and many of the external works are shared between two floors. As a general rule, a two-storey extension costs around 50 to 60 per cent more than an equivalent single-storey extension, not double.

For a typical two-storey extension providing between 30 and 50 square metres of additional space across both floors, you should budget in the range of £50,000 to £100,000. This wide range reflects differences in size, specification and location. A two-storey rear extension adding a kitchen-diner on the ground floor and a bedroom with en-suite above is one of the most common configurations we estimate.

One important consideration with two-storey extensions is the impact on the existing property. You may need to relocate radiators, re-route plumbing or alter the staircase to provide access to the new first-floor rooms. These internal alterations can add £5,000 to £15,000 to the overall project cost, so they should be factored into your budget from the outset.

Planning permission is also more likely to be required for a two-storey extension, particularly if it extends close to a boundary or overlooks a neighbouring property. We cover planning costs in more detail further down this guide.

What Affects the Price?

No two extension projects are the same, and costs can vary significantly even for builds that appear similar on paper. Understanding the factors that drive pricing will help you make informed decisions during the design stage and avoid budget overruns later. Here are the main elements that affect the cost of a house extension:

• Location: Build costs vary considerably across the UK. Labour rates in London and the South East are typically 20 to 40 per cent higher than in the North of England or Wales, and material delivery costs may also be higher in remote areas.
• Specification and finishes: The level of finish you choose has a major impact on cost. Standard emulsion paint and vinyl flooring will cost a fraction of polished concrete floors and bespoke cabinetry. Kitchens and bathrooms within an extension are often the most expensive elements per square metre.
• Access and site constraints: Limited access for deliveries, narrow side passages, sloping sites or the need to work over or around existing structures can all increase costs. If materials have to be carried by hand rather than delivered by crane or telehandler, labour costs rise accordingly.
• Ground conditions: Poor ground conditions, high water tables or the presence of tree roots near the foundations can require deeper or more complex foundation solutions such as piled foundations, which add significant cost.
• Existing services: If the extension route runs over drainage, gas mains or electrical cables, these may need to be diverted before construction can begin. Service diversions are often an unexpected cost that can add several thousand pounds to a project.
• Design complexity: Simple rectangular footprints are the most economical to build. Angled walls, curved features, split-level floors and complex roof geometries all increase both material and labour costs.

We always recommend having a detailed estimate prepared before accepting any builder quotations. This gives you an independent benchmark against which to compare prices and helps ensure that nothing has been missed or underpriced.

Regional Price Variations

Where you live in the UK has a significant bearing on how much your extension will cost. Labour rates, material availability and local market conditions all contribute to regional price differences. While it is impossible to give exact figures for every area, the following provides a useful overview of how costs compare across the country.

In London and the South East, extension costs tend to be at the top of the national range. Strong demand for builders, higher wages and elevated land values all contribute to premium pricing. It is not unusual for a standard single-storey extension in Greater London to cost 30 per cent more than an identical build in the Midlands.

The Midlands and East of England sit in the middle ground, with costs broadly in line with national averages. Cities such as Birmingham, Nottingham and Cambridge have active construction markets, and competitive tendering can help keep prices reasonable.

In the North of England, Scotland and Wales, build costs are generally lower owing to reduced labour rates and a less pressured market. However, projects in very rural or isolated locations may see increased costs for material delivery and specialist trade availability. It is important to obtain locally relevant pricing rather than relying on national averages, which is one of the key benefits of a professional estimating service.

Planning & Building Regs Costs

Beyond the construction costs themselves, there are a number of professional and statutory fees that should be included in your overall budget. These costs are often overlooked in early budgeting but can add up to several thousand pounds.

Planning permission is required for extensions that fall outside the scope of permitted development rights. In England, the standard householder planning application fee is in the region of £200 to £400. Your architect or designer will be able to advise whether your project needs planning permission and prepare the necessary drawings and documentation.

Even if planning permission is not required, all extensions must comply with the Building Regulations. Building control fees typically range from £500 to £1,200 depending on the size and complexity of the project. You can use either your local authority building control service or a private approved inspector.

Most extensions also require input from a structural engineer, particularly where new openings are formed in existing walls or where the extension connects to the original building. Structural engineering fees for a typical domestic extension range from £500 to £1,500. If your project involves a party wall, you will also need to serve notice under the Party Wall Act, and surveyor fees can add a further £700 to £1,500 per neighbour.

It is sensible to set aside a contingency of around 10 per cent of the total build cost to cover unexpected issues that arise during construction. This is standard practice in the industry and provides a financial cushion should you encounter unforeseen ground conditions, structural issues or specification changes.

How to Get an Accurate Estimate

One of the biggest mistakes homeowners make when planning an extension is relying on rough figures from the internet or verbal quotes from builders. While these can give you a ballpark idea, they rarely account for the specific details of your project, your site conditions or the current state of the local market.

A professional building estimate, prepared by a qualified Quantity Surveyor, provides a detailed breakdown of every element of your project: from foundations and structural steelwork through to roof finishes, drainage and decoration. This gives you a clear, itemised picture of where your money is going and helps you identify areas where you might be able to adjust the specification to fit your budget.

At First4Estimating, we prepare detailed cost estimates for house extensions of all sizes across the UK. Our estimates are based on current market rates and are tailored to your specific drawings and specification. Whether you are at the early planning stage or ready to go out to tender, having an independent estimate gives you the confidence to make informed decisions and negotiate effectively with builders.

If you are planning an extension and want to understand the true cost before committing, view our estimating services or get in touch with our team for a no-obligation quote. We typically turn around domestic extension estimates within five working days of receiving your drawings.