House Extension Cost UK 2026

How much does a house extension cost? Single storey, double storey, rear, side, and wrap-around extension prices with a full breakdown of foundations, walls, roof, and finishes.

Last updated: April 2026

A house extension remains the most popular way to add living space in the UK. With the average cost of moving home now exceeding £12,000 in fees alone — before you factor in stamp duty — extending your current property is often the more practical and cost-effective option. A well-designed extension can add 10–20% to your property value while giving you the exact space you need.

This guide covers realistic extension costs across the UK in 2026, including single storey rear extensions, double storey builds, side returns, wrap-arounds, and over-garage conversions. Whether you are a homeowner budgeting a project or a builder preparing a quotation, these figures provide a reliable benchmark based on current material and labour rates.

Summary Cost Table

All figures include VAT and assume a mid-range specification. London and the South East typically sit 15–25% above these averages. Rural and northern areas may come in 10–15% below.

Detailed Cost Breakdown

Understanding where the money goes helps you budget accurately and identify where savings are possible. The following breakdown is based on a typical 20 m² single storey rear extension.

Foundations and Groundwork

Strip foundations for a standard single storey extension typically cost £3,000–£6,000. Trench fill foundations — required where trees are nearby or ground conditions are poor — push this to £5,000–£9,000. Groundwork also includes excavation, hardcore, oversite concrete, DPM (damp-proof membrane), and connecting to existing drainage. On clay soils or sites with mature trees, a structural engineer may specify deeper foundations or even piled foundations, which can add £8,000–£15,000.

Walls and Superstructure

Most extensions use cavity wall construction: an outer leaf of facing brick, a 100 mm cavity filled with insulation, and an inner leaf of 100 mm dense blockwork. Wall costs for a 20 m² extension typically run £5,000–£10,000 including brickwork, blockwork, cavity insulation, wall ties, and DPC (damp-proof course). Steel beams (RSJs) to create open-plan layouts through the existing rear wall add £1,200–£3,500 depending on span and specification.

Roof Structure

A flat roof with a GRP fibreglass or EPDM rubber finish is the most common choice for single storey rear extensions, costing £2,500–£5,000 for a 20 m² footprint. A pitched roof with concrete or clay tiles costs £4,000–£8,000 but offers a more traditional appearance and longer lifespan. Rooflights set into a flat roof add £800–£2,000 each depending on size and whether they are fixed or opening.

Windows and Doors

Bifold doors are the most popular choice for rear extensions, typically £2,500–£6,000 for a 3–4 panel aluminium set. Sliding patio doors offer a slightly cheaper alternative at £1,800–£4,000. Velux rooflights cost £600–£1,500 each fitted, while larger lantern rooflights start from £2,000. Standard double-glazed windows for side walls run £400–£800 each supplied and fitted.

Electrics and Plumbing

Electrical first fix (running cables, consumer unit upgrades, back boxes) costs £1,000–£2,500. Second fix (fitting sockets, switches, lights, and testing) adds £800–£1,500. If the extension includes a kitchen or utility room, plumbing first fix (hot and cold supplies, waste connections, heating pipework) typically costs £1,500–£3,000 and second fix £800–£1,500. A new radiator circuit extension runs £400–£800 per radiator, or £2,000–£4,000 for underfloor heating in a 20 m² space.

Plastering and Internal Finish

Plastering a 20 m² extension (walls and ceiling) costs £1,500–£3,000. This includes dot-and-dab plasterboard to blockwork walls, skim coat finish, and any coving or shadow gap detailing. Painting and decorating adds £800–£1,500. Floor finishes vary widely: engineered wood flooring costs £40–£80 per m² fitted, porcelain tiles £50–£90 per m², and polished concrete £80–£120 per m².

Kitchen or Bathroom Fit-Out

If the extension houses a new kitchen, budget £5,000–£15,000 for a mid-range fit-out including units, worktops, splashback, sink, and appliance connections. A high-end kitchen can easily exceed £25,000. A bathroom or shower room within an extension adds £3,000–£8,000 for sanitaryware, tiling, and installation.

External Works

External works are often underestimated. Drainage connections and soakaway installation cost £1,500–£3,500. Making good the existing garden — patios, fencing, turfing — adds £1,000–£4,000. Rendering or cladding the exterior walls costs £2,000–£5,000. Skip hire over the project duration typically totals £500–£1,200.

Factors That Affect Cost

• Size — the single biggest variable. A 15 m² extension costs significantly less than a 40 m² one, though the per-square-metre rate decreases as size increases because fixed costs (foundations, steelwork, drainage) are spread across more floor area.
• Number of storeys — a double storey extension does not cost twice as much as a single storey. The foundations, drainage, and groundwork are shared, so a second storey typically adds 40–60% to the cost of a single storey build.
• Ground conditions — clay soils, high water tables, tree roots, and existing drains all increase foundation costs. A site investigation (£500–£1,000) can prevent costly surprises.
• Access — restricted site access (terraced houses, narrow side passages) increases labour time and may require smaller plant, pushing costs up by 10–20%.
• Location — labour rates in London and the South East are 15–25% higher than the national average. Material prices are broadly consistent across the UK, but delivery charges vary.
• Specification level — the difference between a basic and high-end finish can double the total cost. Bifold doors, underfloor heating, engineered timber flooring, and lantern rooflights all add significant cost but also add value.
• Planning and party wall — if your extension requires full planning permission (rather than falling under permitted development), budget £250–£500 for the application. Party wall agreements with neighbours cost £700–£1,500 per neighbour if a surveyor is required.

How Long Does an Extension Take?

These timescales cover construction only and do not include the design, planning, and building regulations approval stages, which can add 8–16 weeks before any building work starts.

How to Save Money on Your Extension

• Stay within permitted development — avoiding the need for full planning permission saves both money (£250–£500 application fee) and time (8–12 weeks). Check the permitted development limits for your property type before designing the extension.
• Choose a flat roof — a flat roof with GRP or EPDM costs significantly less than a pitched tiled roof and allows a greater internal ceiling height. Modern flat roof systems carry 25–30 year guarantees.
• Keep the layout simple — rectangular footprints are cheaper to build than L-shaped or angular designs. Every internal corner adds cost to foundations, brickwork, and roof detailing.
• Fix the specification early — changing your mind mid-build is expensive. Finalise door choices, floor finishes, and kitchen layouts before construction starts to avoid costly variations.
• Get three detailed quotes — compare like for like. A quote that includes provisional sums for steelwork and drainage is more honest than one that excludes them. Use TailoredQuote to generate professional, itemised quotations in minutes.
• Build in autumn or winter — demand for builders drops from October to February. Many contractors offer better rates during this period to maintain workflow. Ground conditions permitting, this is the most cost-effective time to start.

Common Questions