How Much Does a House Extension Cost in the UK (2026 Guide)

How Much Does a House Extension Cost in the UK in 2026?

Building a house extension remains one of the most popular ways to add space and value to a UK property. But costs vary enormously depending on the type of extension, your location, the specification of finishes, and your choice of builder. This guide breaks down realistic extension costs for 2026, so you can budget properly before starting your project.

Average Extension Costs Per Square Metre (2026)

Extension costs in the UK are typically quoted per square metre of floor area. Here are the current average ranges for 2026:

• Single storey rear extension: £1,500 to £2,200 per m² (basic to mid spec)
• Single storey rear extension (high spec): £2,200 to £3,000 per m²
• Double storey extension: £1,800 to £2,500 per m² (the upper floor is cheaper per m² since foundations and roof are shared)
• Side return / infill extension: £1,800 to £2,800 per m² (narrow access and structural work push costs up)
• Kitchen extension (single storey, 4x5m): £30,000 to £55,000 total including a mid-range kitchen fit-out
• Wrap-around extension: £2,000 to £2,800 per m² (combines rear and side)
• Loft conversion (for comparison): £1,200 to £2,000 per m² depending on dormer type

These figures include the building work, foundations, roofing, windows, plastering, electrics, plumbing, and basic decoration. They do not include kitchen or bathroom fit-outs, which are priced separately.

What Affects the Cost of an Extension?

1. Foundations

Standard strip foundations cost around £150 to £200 per linear metre. However, if your site has clay soil, trees nearby, or drains running beneath the footprint, you may need deeper or piled foundations. Piled foundations can add £5,000 to £15,000 depending on the number of piles required. A structural engineer's report (£500 to £1,000) will confirm what you need.

2. Roof Type

A flat roof is cheaper to build than a pitched roof. Budget around £80 to £120 per m² for a flat EPDM or fibreglass roof versus £120 to £180 per m² for a pitched tiled roof. However, a pitched roof often looks better and adds more value to the property, particularly on double storey extensions where planning officers may insist on matching the existing roofline.

3. Structural Steelwork

If you are removing a load-bearing wall to open up the kitchen-diner, you will need a steel beam (RSJ). Expect to pay £1,500 to £4,000 for supply and installation of a typical domestic steel, depending on the span. Larger openings or first-floor removals cost more and may need temporary propping.

4. Planning Permission and Permitted Development

Many single storey rear extensions of up to 6 metres (detached) or 3 metres (semi-detached and terraced) fall under Permitted Development rights and do not require a full planning application. If you do need planning permission, the current fee in England is £258 for a householder application. A Lawful Development Certificate (to confirm PD rights) costs £129. Always check with your local authority before starting work.

5. Glazing and Bi-Fold Doors

Large bi-fold or sliding doors are a popular feature in rear extensions but add significant cost. A 4-metre aluminium bi-fold door set costs £3,500 to £6,000 depending on the brand and finish. Roof lanterns or skylights add £1,500 to £4,000 each.

6. Specification Level

A basic extension uses standard materials: concrete block walls, plasterboard, vinyl flooring, and basic sanitaryware. A high-spec extension might include underfloor heating, engineered oak flooring, bespoke joinery, Porcelanosa tiles, and Quooker taps. The difference between basic and high spec can easily be £500 to £1,000 per m².

Regional Price Differences

Labour rates vary significantly across the UK. Here is a rough guide to regional cost multipliers compared to the national average:

• London and South East: 15% to 30% above national average. A single storey extension in London typically starts at £2,000/m² even for basic spec.
• South West and East Anglia: 5% to 10% above average. Strong demand from renovation projects in period properties.
• Midlands: Broadly in line with the national average.
• North West, Yorkshire, and North East: 5% to 15% below average. You can often get more extension for your budget in these regions.
• Scotland: Varies widely. Edinburgh and Glasgow are comparable to the Midlands, while rural areas can be cheaper but with limited builder availability.
• Wales: Generally 5% to 10% below the English average, though building regulations differ and you may need SAP calculations for energy compliance.

Hidden Costs You Need to Budget For

Many homeowners focus on the build cost and forget about the professional fees and statutory charges that can add 15% to 20% on top. Here are the most common ones:

• Architectural drawings: £1,500 to £4,000 for planning and building regs drawings
• Structural engineer: £500 to £1,500 for calculations and beam specifications
• Planning application fee: £258 (England, 2026)
• Building regulations fee: £400 to £900 depending on project size and your local authority
• Party Wall Act: If your extension is built on or near the boundary with a neighbour, you must serve a Party Wall notice. If your neighbour appoints their own surveyor, costs can reach £1,500 to £3,000 per neighbour.
• Thames Water build-over agreement: Free but can take 4 to 8 weeks if you are building over a public sewer (common in London and the South East).
• VAT: Most extension work by VAT-registered builders is charged at the standard 20% rate. Make sure quotes state whether VAT is included.
• Temporary accommodation: If the build is disruptive (major kitchen works, for example), you may need to eat out or stay elsewhere for 2 to 4 weeks. Budget £500 to £2,000.
• Landscaping and making good: Skips, scaffolding removal, garden reinstatement and redecorating disturbed rooms can add £2,000 to £5,000.

How Long Does an Extension Take to Build?

Timescales depend on the size and complexity of the extension:

• Single storey rear (15 to 25 m²): 10 to 14 weeks on site
• Double storey (30 to 50 m²): 14 to 20 weeks on site
• Side return / infill: 8 to 12 weeks on site
• Wrap-around: 14 to 18 weeks on site

Add 8 to 16 weeks before that for design, planning, structural engineering, Building Control approval, and contractor procurement. Most extension projects take 6 to 9 months from initial design to completion.

How to Get an Accurate Extension Quote

The best way to budget for an extension is to get itemised quotes from at least three builders. A lump-sum price with no breakdown makes it impossible to compare like for like. Look for quotes that break costs down by trade: groundworks, brickwork, roofing, carpentry, plumbing, electrics, plastering, and decoration.

If you are a builder or trades professional pricing extension work for clients, tools like MyQS can generate detailed, itemised costings from floor plans or project descriptions in minutes. This is especially useful for producing professional quotes quickly when competing for work.

When comparing quotes, check that each builder has included the same scope of work. Common items that get missed include: scaffolding, skip hire, building regs fees, drainage connections, and making good to existing finishes.

Is a House Extension Worth It in 2026?

In most cases, yes. A well-designed extension typically adds more value to a property than it costs to build. According to recent estate agent estimates, a single storey rear extension can add 5% to 10% to a property's value, while a double storey extension can add 10% to 20%. However, the return on investment depends on the local property market and whether the extension is designed to a high standard. Over-extending a small property on a modest street may not deliver the same return as extending a family home in a sought-after area.

Frequently Asked Questions

Do I need planning permission for a house extension?

Not always. Many single storey rear extensions fall under Permitted Development rights in England, meaning you can build without a full planning application. The limits are 6 metres for detached houses and 3 metres for semi-detached and terraced properties (or 8m and 6m respectively under the Prior Approval / Larger Home Extension scheme). However, if your property is in a Conservation Area, is a listed building, or you want to extend to the front or side with certain dimensions, you will need planning permission. Always check with your local planning authority or apply for a Lawful Development Certificate to confirm.

How much does a 4x5 metre kitchen extension cost?

A 20 m² single storey kitchen extension typically costs between £30,000 and £55,000 in 2026 including the build and a mid-range kitchen. At the lower end, expect basic units, laminate worktops, and standard appliances. At the higher end, you might get handleless units, quartz worktops, integrated appliances, underfloor heating, and bi-fold doors to the garden. In London and the South East, add 20% to 30% to these figures. The build itself (excluding the kitchen fit-out) typically costs £30,000 to £44,000 for 20 m² at standard specification.

Can I build an extension myself to save money?

You can manage the project yourself and hire individual trades rather than a main contractor, which can save 15% to 20% on the total cost. However, this requires significant time, knowledge of the build sequence, and confidence in coordinating multiple trades. Key areas like foundations, steelwork, electrics (Part P), and gas work must be carried out by qualified professionals. For most homeowners, hiring an experienced builder or project manager is worth the additional cost to avoid delays and costly mistakes.

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