House Extension Cost Per M2 UK: Complete Pricing Guide

Average house extension cost per m2 in the UK

If you are at the early planning stage, calculating the house extension cost per m2 is one of the quickest ways to test whether your ideas fit your budget. However, there is no universal rate that every builder follows.

As a broad 2026 budgeting guide, many homeowners should allow around £2,000–£3,300 per m² for a conventional extension. A straightforward build in a lower-cost area may sit towards the bottom of that range, while complicated structural work, restricted access, expensive glazing or a higher specification can push the price considerably higher.

For a broader explanation of the costs that sit around the building work itself, see our guide to budgeting for an extension.

How to calculate your house extension cost per m2

The basic calculation is simple:

That £75,000 should not automatically be treated as your complete project budget. Instead, it gives you a useful starting point before the design is developed and builders can price from proper information.

Why small extensions can cost more per m2

A smaller extension is not always cheaper on a proportional basis. Foundations, structural openings, drainage alterations, site setup and roof junctions still need to be completed even when the new floor area is modest.

Consequently, a compact 10–15 m² kitchen extension may have a higher cost per square metre than a simple 30–40 m² rectangular extension.

Single storey vs double storey house extension cost per m2

A double storey extension does not normally cost twice as much as an equivalent single storey extension. Both versions require major items such as foundations, site establishment and a roof, so adding a second floor can spread some of those costs across more new floor area.

Single storey extension

Single storey rear and side extensions are extremely common because they can create a larger kitchen, dining area, utility room or accessible living space. However, kitchens and large areas of glazing can make the finished project relatively expensive.

Our separate single-storey budgeting guide looks at the additional costs homeowners should consider.

Double storey extension

A two-storey scheme usually costs more in total, but the average cost per square metre can be more efficient because the floor area increases without duplicating every element of the foundations and roof.

If you are comparing both options, see our two-storey project breakdown.

What affects the cost of a house extension per square metre?

Two extensions of exactly the same floor area can have very different prices. Therefore, square metre rates are most useful when you understand what changes them.

1. Location and labour rates

Labour costs, contractor demand and site overheads vary around the country. London and parts of the South East commonly attract a premium, while other regions may be less expensive.

2. Ground conditions and foundations

Poor ground, nearby trees, drains or unusually deep foundations can add thousands of pounds before the walls have even started.

3. Structural alterations

Removing a large section of the original rear wall may require steelwork, padstones and temporary support. These structural elements can materially affect the final price.

You can read more about when an engineer may become involved in our structural design guide.

4. Kitchens and bathrooms

A basic square-metre build figure can be misleading for kitchen extensions. Units, worktops, appliances, plumbing, extraction, lighting and electrical upgrades can add a substantial amount to the shell cost.

5. Glazing and doors

Large aluminium sliders, roof lanterns, structural glazing and bespoke rooflights cost much more than standard windows and doors. They may also change structural and energy calculations.

6. Shape and design complexity

A straightforward rectangular extension is generally easier to build than one with multiple corners, unusual roof shapes or complicated junctions with the existing house.

What may not be included in an extension cost per square metre?

This is where early budgets often go wrong. Before relying on any average cost, check whether the following items have been allowed for:

• VAT where applicable;
• architectural or technical drawings;
• planning application fees;
• Building Control fees;
• structural engineer calculations;
• party wall surveyor costs where required;
• drainage alterations or build-over agreements;
• kitchen units and appliances;
• bathroom sanitaryware;
• floor finishes and decorating;
• landscaping and making good outside;
• temporary accommodation or storage; and
• a contingency for unforeseen work.

Good drawings also make pricing clearer. Our guide to extension drawings explains what should normally be prepared before construction.

House extension cost per m2 in London and other UK regions

Search results often ask for separate figures for London, Manchester, Birmingham, Glasgow and other locations. Regional differences are real, but postcode alone does not determine the cost.

A restricted London terrace with no rear access can cost substantially more than an equivalent extension where machinery and materials can reach the garden easily. Conversely, a complicated rural site can also be expensive because of travel, deliveries and ground conditions.

How to build a realistic extension budget step by step

For the wider planning process, our step-by-step permissions guide explains what homeowners should check before work begins.

You should also understand the separate technical approval process. Our Building Control guide for extension projects covers the main compliance areas.

Cost per m2 vs total extension cost: which figure matters most?

The cost per square metre is most useful during the feasibility stage because it lets you compare different extension sizes quickly. Once drawings have been prepared, however, the total itemised project cost becomes much more useful.

For example, reducing an extension from 30 m² to 25 m² does not necessarily cut the whole project cost by one-sixth. The same boiler alterations, steel beam, kitchen, planning work and site setup may still be required.

Therefore, use £/m² to answer, “Is this idea roughly affordable?” and detailed quotations to answer, “What will my actual project cost?”

FAQs: house extension cost per m2

Next steps & useful guides

If you are moving from rough costs into actual project planning, these PME guides are useful next steps:

• Read our complete homeowner guide to extending a house
• Understand the wider project budget and common extras
• Explore the budget factors for a ground-floor addition
• Compare the economics of adding a second floor
• See what drawings are normally required before building
• Understand what Building Control will check
• Work through the permission process from the beginning

Official and independent sources

Costs and regulations change, so it is sensible to check current information before committing to a project:

• Planning Portal guidance on permissions for home extensions
• GOV.UK technical guidance on householder permitted development rights
• RICS Building Cost Information Service for independent construction cost data
• Office for National Statistics construction output price indices

Using cost per m2 without getting caught out

A square-metre calculation is a useful way to test an extension idea before spending heavily on design work. However, it works best as a range rather than a promise.

Start with the approximate floor area, apply a sensible low and high £/m² allowance, then add the items that are specific to your project. Kitchens, structural steel, glazing, drainage, professional fees and difficult access can all have a much bigger effect than a simple online calculator suggests.

Once your design develops, replace the generic figures with quotations based on consistent drawings and specifications. That gives you a far stronger basis for deciding whether to proceed, reduce the scope or change the design before construction begins.