Understanding the cost per square metre is the quickest way to estimate what a house extension will cost — whether you are a homeowner budgeting for a project or a builder preparing a quote. But the headline numbers only tell part of the story. The actual cost depends on the type of extension, the specification level, the region, the ground conditions, and dozens of other factors that can push the price up or down significantly.
This guide covers the real cost per square metre for every type of extension in 2026, a detailed breakdown of where the money goes, how costs vary across the UK, what to include when quoting extension work, and the common mistakes that catch builders out.
Cost Per Square Metre by Extension Type
These are typical build costs per square metre in 2026 for different extension types at a standard specification (brick walls, tiled or flat roof, standard windows, plastered and decorated interior, basic kitchen or bathroom fitting not included).
| Extension Type | Cost Per m² | Typical Size | Total Cost Range |
|---|---|---|---|
| Single storey rear (flat roof) | £1,500 – £2,200 | 15 – 30 m² | £30,000 – £60,000 |
| Single storey rear (pitched roof) | £1,700 – £2,500 | 15 – 30 m² | £35,000 – £70,000 |
| Single storey side | £1,600 – £2,400 | 8 – 15 m² | £20,000 – £35,000 |
| Wrap-around (rear + side) | £1,600 – £2,300 | 20 – 40 m² | £40,000 – £80,000 |
| Double storey rear | £1,200 – £1,800 | 30 – 50 m² total | £45,000 – £90,000 |
| Over-garage / over-existing | £1,000 – £1,500 | 15 – 25 m² | £20,000 – £40,000 |
Double storey extensions have a lower cost per square metre than single storey because the most expensive elements — foundations, groundwork, and roofing — are shared across two floors. The additional cost of the second floor is primarily the first floor structure, additional walls, windows, and internal finishes. This makes a double storey extension significantly better value per square metre of usable space.
Cost Per Square Metre by Specification Level
The specification level — the quality of materials and finishes you choose — has a dramatic impact on the final cost. Here is how the cost per square metre changes across three specification levels for a single storey rear extension.
| Specification | Cost Per m² | What It Includes |
|---|---|---|
| Basic | £1,500 – £1,800 | Standard brick, flat roof, basic windows, plaster and paint, vinyl flooring |
| Standard | £1,800 – £2,500 | Matching brick, pitched or flat roof with lantern, aluminium bi-folds, underfloor heating, engineered wood or tiled floor |
| Premium | £2,500 – £3,500 | Bespoke brickwork or render, structural glazing, vaulted ceiling, premium flooring, high-end kitchen fitout |
Regional Cost Variations
Labour rates are the primary driver of regional cost differences. Material costs are broadly similar across the UK, but labour can vary by thirty per cent or more between the cheapest and most expensive regions.
| Region | Single Storey Per m² | Double Storey Per m² |
|---|---|---|
| London | £2,200 – £3,200 | £1,600 – £2,400 |
| South East | £1,900 – £2,800 | £1,400 – £2,100 |
| South West | £1,700 – £2,400 | £1,300 – £1,900 |
| Midlands | £1,500 – £2,200 | £1,200 – £1,700 |
| North West | £1,500 – £2,100 | £1,100 – £1,600 |
| North East | £1,400 – £2,000 | £1,100 – £1,500 |
| Wales | £1,400 – £2,000 | £1,100 – £1,500 |
| Scotland | £1,500 – £2,100 | £1,100 – £1,600 |
Detailed Cost Breakdown
Here is where the money goes on a typical twenty-square-metre single storey rear extension with a flat roof and bi-fold doors, at a standard specification.
| Element | Cost | % of Total |
|---|---|---|
| Foundations and groundwork | £5,000 – £8,000 | 12 – 15% |
| Brickwork and blockwork | £5,000 – £8,000 | 12 – 15% |
| Roof structure and covering | £3,500 – £6,000 | 8 – 12% |
| Steelwork (RSJs) | £2,000 – £5,000 | 5 – 10% |
| Windows and bi-fold doors | £4,000 – £8,000 | 10 – 15% |
| Plumbing and heating | £2,000 – £4,000 | 5 – 8% |
| Electrics | £1,500 – £3,000 | 4 – 6% |
| Insulation | £1,500 – £2,500 | 3 – 5% |
| Plastering | £2,000 – £3,500 | 5 – 7% |
| Flooring | £1,500 – £3,000 | 4 – 6% |
| Decoration | £800 – £1,500 | 2 – 3% |
| Drainage | £1,500 – £3,000 | 4 – 6% |
| External works (patio, landscaping) | £1,000 – £3,000 | 3 – 5% |
| Total | £31,300 – £58,500 | 100% |
Quote Extension Projects Professionally
QuoteSmith generates detailed, branded proposals for extension projects. Input your costs and let AI build the scope of work, timeline, and terms automatically.
Try QuoteSmith FreePlanning Permission and Building Regulations
Understanding the regulatory requirements is essential for pricing and programming extension projects correctly.
Permitted Development
Many single storey rear extensions fall under permitted development rights, meaning you do not need planning permission. The key limits are:
- Detached houses: rear extension up to eight metres (with prior approval) or four metres (without notification) from the original rear wall
- Semi-detached and terraced: up to six metres (prior approval) or three metres (without notification)
- Maximum height: four metres for single storey, three metres at the eaves if within two metres of a boundary
- Side extensions: must not exceed half the width of the original house
- Double storey: maximum seven metre projection, must be at least seven metres from the rear boundary
A Lawful Development Certificate costs eighty-five to two hundred and fifty pounds and confirms that the work is permitted development — valuable when the homeowner sells.
Full Planning Permission
If the extension does not qualify as permitted development, a full planning application is required. The application fee in England is two hundred and fifty-eight pounds for householder applications in 2026. Architect's fees for preparing and submitting a planning application typically cost one thousand to three thousand pounds. The decision takes eight weeks for most householder applications.
Building Regulations
All extensions require building regulations approval, regardless of planning permission status. Key areas include structural calculations, foundations design, thermal insulation, fire safety, drainage, ventilation, and electrical safety. Building regulations fees range from five hundred to one thousand five hundred pounds. A structural engineer's calculations cost four hundred to one thousand two hundred pounds depending on complexity.
Professional Fees Summary
| Fee | Cost |
|---|---|
| Architect / architectural designer | £2,000 – £6,000 |
| Structural engineer | £400 – £1,200 |
| Planning application fee | £258 |
| Building regulations fee | £500 – £1,500 |
| Party wall surveyor (if needed) | £1,000 – £3,000 |
| Lawful Development Certificate | £85 – £250 |
How to Quote an Extension Project
Quoting an extension accurately requires a thorough survey, detailed measurement, and careful pricing of every trade and material. Here is a practical approach.
Study the drawings. Get the architect's drawings and structural engineer's calculations before quoting. These tell you the size, specification, steel requirements, foundation design, and materials. If the customer does not have drawings, they need them before you can price properly — do not guess.
Visit the site. Check access for deliveries and machinery, ground conditions, proximity to boundaries and neighbours, the condition of the existing house where the extension will connect, and any trees, drains, or services that might affect the build. For more on site surveys, see our guide on how to write a scope of work.
Price each element separately. Break the project into phases — groundwork, brickwork, roofing, steelwork, carpentry, plumbing, electrics, plastering, flooring, decoration, drainage, and external works. Get subcontractor prices for any trades you are not handling yourself. See our guide on how to calculate labour costs.
Add contingency. Extensions regularly encounter unforeseen issues — unexpected ground conditions, hidden drainage, structural problems when opening up the existing house, or weather delays. Add ten to fifteen per cent contingency to your costs. This is not profit — it is a realistic allowance for the unknowns.
Present professionally. An extension is a major investment — typically thirty thousand to eighty thousand pounds. Customers expect a detailed, professional proposal that breaks down costs clearly, includes a realistic programme, specifies exclusions, and sets out payment terms. QuoteSmith generates these automatically. Learn about why professional proposals win more work.
Common Mistakes When Quoting Extensions
Underestimating groundwork. Foundations are one of the biggest variables in extension costs. If the ground conditions are poor — clay, fill, high water table, or tree roots nearby — deeper or wider foundations are needed, sometimes including piled foundations costing five thousand to ten thousand pounds more than standard strip foundations.
Forgetting drainage. Moving or extending drainage runs is expensive and often overlooked. If the extension covers an existing manhole or drain run, it must be diverted or rebuilt. Build-over agreements with the water company add time and cost.
Not accounting for the existing house. The point where the extension meets the existing house always requires work — removing the existing external wall, making good internally, matching floor levels, and connecting heating and electrics. This interface work is often more complex and time-consuming than builders anticipate.
Ignoring Party Wall requirements. If the extension is on or near a boundary, the Party Wall Act may apply. Serving notice and appointing surveyors can cost one thousand to three thousand pounds and adds four to six weeks to the programme before you can start on site.
Frequently Asked Questions
How much does an extension cost per square metre in the UK in 2026?
A single storey extension costs one thousand five hundred to two thousand five hundred pounds per square metre at a standard specification. A double storey extension costs one thousand two hundred to one thousand eight hundred pounds per square metre. Premium specifications can reach three thousand five hundred pounds per square metre. Costs are highest in London and lowest in Wales and the North East.
How much does a single storey rear extension cost?
A single storey rear extension costs thirty thousand to sixty thousand pounds in 2026 for a typical fifteen to twenty-five square metre build. A small extension costs thirty thousand to forty thousand pounds. A large extension with premium finishes can exceed seventy-five thousand pounds. The final cost depends on size, specification, and location.
Do I need planning permission for a house extension?
Many single storey rear extensions qualify as permitted development. Detached houses can extend up to eight metres with prior approval. Semi-detached and terraced houses can extend up to six metres. Side extensions and double storey extensions have stricter limits. All extensions need building regulations approval regardless of planning permission status.
How long does it take to build an extension?
A single storey extension takes ten to sixteen weeks on site. A double storey extension takes fourteen to twenty weeks. Allow additional time for design, planning, and building regulations before construction begins. The total timeline from concept to completion is typically six to nine months.
What is the cheapest type of extension to build?
A single storey rear extension with a flat roof is the cheapest full extension. It avoids the cost of a pitched roof and is quicker to construct. An over-garage or over-existing extension is also cost-effective if the existing structure can support it. A loft conversion is often cheaper than an extension if you need additional space without extending the footprint.
Does an extension add value to a house?
A well-designed extension typically adds five to fifteen per cent to the value of a property. In higher-value areas, the return often exceeds the cost. However, over-extending beyond the ceiling price for your street can mean you spend more than you gain. A large open-plan kitchen-diner is consistently the most valuable type of extension for resale.