Average Cost of a Home Extension in the UK (2026 Guide)

Planning a house extension is a major investment, and understanding the likely cost before work begins is essential. Whether you are adding a kitchen, extending your living room or building an extra storey, the price will depend on the size of the extension, the design, the materials and the condition of your existing property.

In 2026, a single-storey extension typically costs around £2,200 to £3,300 per square metre, according to Homebuilding & Renovating's extension cost guide. These figures exclude VAT and vary depending on the specification. Other published guides give lower starting figures, so it is important to understand what each estimate includes.

For example, a 20m² extension at these rates would cost approximately £44,000 to £66,000 before VAT. The final project budget may be higher once professional fees, kitchen fittings, external works and other costs are included.

This guide explains typical extension costs in the UK, what affects the price and how to protect your budget throughout the building process.

How much does a house extension cost in the UK?

The cost of an extension depends on more than its floor area. A straightforward rear extension with standard windows and finishes will generally cost less than a complex design with large glazed doors, structural alterations and premium materials.

The following estimates provide a starting point for budgeting.

Sources: Homebuilding & Renovating, 2026 and the Federation of Master Builders. Rates are indicative, exclude VAT and should not be treated as quotations. Conservatories are not directly comparable with fully integrated house extensions.

The Federation of Master Builders (FMB) publishes a broader estimate of £1,800 to £3,000 per square metre for a single-storey extension. The difference between this and the more detailed Homebuilding & Renovating figures reflects differences in pricing assumptions and specifications.

Neither range guarantees what a particular builder will charge. Use them to establish an initial budget, then obtain quotations based on your actual drawings and requirements.

How much does a single-storey extension cost?

Single-storey extensions are a popular way to create additional kitchen, dining or living space without moving house.

According to Homebuilding & Renovating, a relatively straightforward design can cost around £2,200 to £2,500 per square metre, excluding VAT. More complex designs and higher-quality materials can push the price towards £3,300 per square metre.

Using those figures, the approximate costs are:

These are calculations using the published per-square-metre rates, not market quotations for projects of those exact sizes. They exclude VAT and may not cover every professional fee, fitting or external work required for your project.

Smaller extensions can also cost more per square metre because fixed expenses, such as site setup and structural work, are spread across less floor area.

What affects the price of a rear extension?

A simple rectangular extension with standard windows will generally be more economical than one involving extensive glazing, roof lanterns, a complicated roof or major alterations to the existing house.

Kitchen extensions can be particularly expensive once the kitchen itself is included. Units, worktops, appliances, plumbing, electrical work and flooring can add considerably to the construction cost.

Before committing to a budget, decide whether you are pricing the building shell, a completed extension or the full project including all fixtures and fittings.

How much does a two-storey extension cost?

A two-storey extension can provide additional living space downstairs and bedrooms or bathrooms upstairs.

Although the total project cost will normally be higher than for a comparable single-storey extension, the cost per square metre can be lower. Foundations and roofing serve both floors, allowing some construction costs to be spread across a larger area.

Homebuilding & Renovating estimates approximately £2,000 to £3,200 per square metre, depending on specification.

For a two-storey extension providing 40m² of combined floor space, that equates to a broad indicative construction cost of £80,000 to £128,000 before VAT.

The calculation uses the total floor area of both storeys, not the ground-floor footprint alone.

Structural alterations, access, roof design and the condition of the existing property can all affect the final price. Adding a storey above an existing structure may also require additional engineering or strengthening work.

How much does a side-return or wraparound extension cost?

Side-return extensions are common on Victorian and Edwardian terraced houses, where extending into a narrow side passage can create a more practical kitchen and dining area.

They can cost more per square metre than a straightforward rear extension because access is restricted and the new structure must connect carefully to the existing building.

Homebuilding & Renovating gives an indicative range of £2,800 to £3,800 per square metre for side-return extensions, depending on the specification. It also notes that costs in London and the South East can be higher than its base rates.

A wraparound extension combines rear and side extensions to create a larger footprint. Its price depends on the total floor area, structural complexity, roof design and how extensively the existing house needs to be altered. Rather than relying on a separate generic rate, it is more useful to calculate a preliminary budget from the proposed area and then adjust it for the specific design.

For either project, obtain professional advice on the design and ground conditions before settling on a final budget.

What about conservatories and orangeries?

Conservatories can be a less expensive way to add space, depending on the design and how the structure connects to the house.

Homebuilding & Renovating's indicative rates range from £1,800 to £2,400 per square metre for a basic uPVC lean-to conservatory, rising to £3,500 to £4,200 per square metre for an oak-frame structure.

However, a conservatory should not automatically be treated as a cheaper equivalent to a fully integrated extension. The construction, thermal performance, glazing and intended use may differ.

Check the requirements for your particular design, including whether planning permission or Building Regulations approval applies. The Planning Portal's guidance on extensions explains how permitted development rights work in England and when formal planning permission may be needed.

What drives the cost of a home extension?

Two extensions with the same floor area can have very different prices. The following factors are often responsible for the difference.

Location

Labour rates, contractor availability and local demand vary across the UK. London and the South East can be more expensive than other regions, so national figures may not reflect the quotations available in your area.

Use published figures as a starting point, then compare prices from contractors working in your local market.

Ground conditions and drainage

Foundations are designed around the ground conditions and proposed structure. Unexpected drainage, tree roots, poor soil or the need for deeper foundations can add cost.

Existing drains and underground services may also need to be moved or altered.

Structural work

Removing load-bearing walls, installing steel beams or creating large openings between the existing house and the extension can require additional engineering and labour.

Projects involving neighbouring structures or complex connections to an older property may need further professional input.

Windows, doors and roof design

Large sliding doors, aluminium bifolds, roof lanterns and bespoke glazing can increase the cost considerably compared with standard windows and a straightforward roof.

These choices can have a significant effect on both material costs and installation.

Internal finishes

Flooring, lighting, kitchen units, worktops, heating systems and decoration all affect the finished price.

A quotation that covers the structure but excludes the kitchen and decoration will look very different from one that includes a completed room.

How the project is managed

Employing a main contractor can cost more than arranging individual trades yourself, but it also changes who is responsible for coordinating the work.

Self-managing may reduce certain costs, but requires time, organisation and an understanding of how the trades fit together. Delays or poorly coordinated work can offset the initial savings.

What other costs should you include in your budget?

The construction price is only one part of the total amount you may need to spend. Depending on the project and what the quotation includes, additional costs can include the following.

Most building work on existing houses is subject to the standard VAT rate of 20%, although exceptions apply. Check HMRC's guidance on VAT for builders and confirm the treatment of your specific project with your contractor.

Most extensions also require Building Regulations approval, even where planning permission is not necessary. Requirements vary depending on the proposal and property. The Planning Portal's Building Regulations guidance explains the position in England.

For more detail on costs that homeowners frequently overlook, read our guide to the hidden costs of home renovation.

A worked example: budgeting for a £50,000 extension

Suppose a builder quotes £50,000 excluding VAT for the agreed construction work. Assuming the standard 20% VAT rate applies, the calculation would look like this:

This example demonstrates how quickly the budget increases beyond the original quotation.

The £6,000 contingency is an illustrative allowance, not money that must necessarily be spent. Homebuilding & Renovating recommends allowing at least 10% for unexpected issues and suggests considering a further allowance for possible inflation.

Importantly, this example does not include professional fees, approvals, kitchen fittings, landscaping or other items excluded from the builder's quotation. These should be budgeted separately where necessary.

Your actual budget will depend on the scope, specification and risks associated with your property.

How to protect your extension budget

Choosing a contractor is important, but the way you define, agree and manage the work also affects your financial control.

1. Agree a detailed scope of work

Ensure the drawings, specification and schedule of works make clear what the contractor is expected to provide.

Confirm which materials, finishes, fittings and associated tasks are included, and identify any exclusions before accepting the quote.

2. Compare quotations fairly

Obtain multiple written quotations where practical, using the same scope of work.

Check whether VAT is included, whether allowances are provisional and whether any items will need to be purchased or arranged separately. The cheapest quote is not necessarily the cheapest completed project.

3. Agree changes before the work proceeds

Variations can increase the cost of a project, especially when they affect work already completed or require other trades to change their plans.

Ask the contractor to document the proposed change, revised cost and any effect on the programme. Agree the change before proceeding wherever practical.

4. Link payments to project milestones

Avoid paying large sums without a clear agreement about what the payment covers.

A milestone-based payment schedule sets out the stages of work, the payment associated with each stage and what must be completed before the payment is due. The stages and amounts should reflect the particular project and be agreed by both parties.

Read our guide to milestone payments for homeowners to understand how a structured payment arrangement can improve clarity.

5. Keep project records organised

Store quotations, drawings, invoices, photographs, messages and approved changes together. A clear record makes it easier to check what was agreed and resolve questions about costs as the work progresses.

Our UK builder vetting checklist also explains what to check before appointing a contractor.

How Fort helps homeowners manage extension projects

A realistic budget is a good starting point, but keeping the project organised matters just as much once construction begins.

Fort helps homeowners bring their Contractors, project information, agreed work and milestone payments together in one place. You can post a project and connect with Contractors on the platform, or invite a contractor you already know to use Fort.

Throughout the project, Fort helps you keep relevant documents, communications and agreed changes organised. This gives both parties a clearer record of the scope of work and any variations that affect the project.

Fortified milestone-based payments allow homeowners and contractors to agree defined stages of work, with funds secured before each milestone begins and released when the agreed stage is completed. This helps align payments with project progress.

If a disagreement arises, Fort provides a structured dispute-resolution process, with an independent chartered surveyor involved where appropriate to assess work against the agreed project information.

Fort cannot guarantee that a project will remain within its original budget or eliminate unexpected construction problems. It is designed to help homeowners and Contractors manage project information, changes and payments more clearly.

Whether you are planning a small rear extension or a more substantial renovation, find out how Fort helps homeowners manage construction projects.

Frequently asked questions

What is the average cost of a home extension in the UK?

Homebuilding & Renovating's 2026 guide estimates a single-storey extension at approximately £2,200 to £3,300 per square metre, excluding VAT, depending on specification. Other industry guides publish broader or lower ranges, so the final price should be based on the actual design and scope of work.

Is it cheaper to build a two-storey extension?

It can be cheaper per square metre because foundations and roofing serve both floors. However, the total cost is usually higher because more floor space is being built. Structural changes, access and specification still affect the price.

Does the extension cost per square metre include VAT?

The published Homebuilding & Renovating figures used in this guide exclude VAT. Always confirm whether a builder's quotation includes VAT, as well as which work and materials are covered.

How much contingency should I allow?

A 10% contingency is a useful starting point for many extension projects. Older properties, complex structural work or uncertain ground conditions may justify a larger allowance. The appropriate amount depends on the risks identified before work begins.

How can I prevent my extension going over budget?

Start with a detailed scope, compare like-for-like quotations, agree variations in writing and link payments to defined stages of completed work. Keep project records organised and maintain contingency funds for unexpected problems.

Final thoughts

The cost of a home extension depends on more than the size of the new space. Location, structural requirements, materials, fittings and the condition of your property all influence the final price.

Published cost guides provide a useful starting point, but only a detailed specification and written quotation can establish what your own project is likely to cost. Make sure you understand what is included, budget for additional expenses and set aside money for unexpected issues.

Once work begins, clear agreements and well-managed payments help you stay informed as the project develops.

Plan the work, understand the costs and keep your project organised with Fort.

Find out more about Fort for homeowners.

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Sources and further reading

• Homebuilding & Renovating: How much does an extension cost in 2026?
• Federation of Master Builders: Single-storey extension costs
• HomeOwners Alliance: House extension costs by size and type
• HMRC: VAT for builders
• Planning Portal: Planning permission for extensions
• Planning Portal: Building Regulations for extensions

Cost figures are indicative and based on published 2026 industry guidance. They are not fixed prices or quotations for an individual property. Actual costs vary by location, design, specification, site conditions and the work included in the quotation. Planning guidance linked here relates to England; rules differ across the UK.