How Much Does a Single Storey Extension Cost in the UK in 2026?

Single storey extension costs in 2026

Builders price extensions per square metre, and in 2026 the market splits into three clear bands. A basic build — simple rectangle, flat roof, modest glazing — runs £1,700–£2,000 per m². A standard specification, which is what most families end up choosing, runs £1,900–£2,600 per m². Premium and glass-heavy designs reach £2,800–£3,500 per m².

• Basic (£1,700–£2,000 per m²): blockwork, warm flat roof, standard windows and painted plaster
• Standard (£1,900–£2,600 per m²): pitched or upgraded flat roof, a rooflight, decent glazed doors and underfloor heating in the new room
• Premium (£2,800–£3,500 per m²): structural glazing, large-span doors and high-end finishes throughout

Add 20–30% in London and the South East. Builder quotes to homeowners normally include VAT, but always confirm it in writing. For the whole-house picture see our house extension cost guide, or dig into the cost per m² breakdown to see where every pound goes.

What drives the price up or down

Two extensions of identical size can be £20,000 apart, and the gap is almost always explained by a handful of decisions and site conditions:

• Glazing: bifold or sliding doors add £3,000–£8,000, and a glass-heavy design pushes the whole project into the premium band
• Kitchen: fitting a new kitchen in the extension adds £10,000–£25,000 on top of the build cost
• Bathroom: a new bathroom or WC adds £4,000–£8,000
• Ground conditions: clay, tree roots or poor bearing soil mean deeper foundations, and drains under the footprint may need diverting or a build-over agreement
• Structural openings: knocking through the back wall needs a steel beam, engineer calculations and temporary works
• Access: a tight terrace where everything is barrowed through the house costs more than an open driveway

Single storey extension cost by size (2026)

Totals are for a standard specification outside London and cover the finished build, but not a new kitchen, which adds £10,000–£25,000. Add 20–30% in London and the South East.

Planning permission and permitted development

Most single storey extensions in England are built under permitted development rather than a full planning application. The rules that matter:

• A rear extension can project up to 3m beyond the original rear wall on a semi or terrace, or 4m on a detached house
• Under the prior approval (larger home extension) route, 6m and 8m are possible respectively — the council consults your neighbours first
• Maximum height is 4m
• Permitted development does not apply in conservation areas or to listed buildings

Check your address on the Planning Portal before you design anything, because rights can be removed by local conditions. Building regulations always apply to a habitable extension whatever the planning route — expect inspections covering foundations, structure, insulation, drainage and electrics.

Professional fees, surveys and contingency

The build price is not the project price. Professional fees — architect, structural engineer and surveys — typically add 10–15% on top. A structural engineer alone costs £600–£1,400 for a typical single storey design. If you are excavating within 3m of a shared structure, the Party Wall Act applies and a formal agreement costs £700–£1,500 per affected neighbour.

Then hold a 10–15% contingency. That is not pessimism; it is what unknown ground, hidden drains and mid-build changes reliably cost. On a £40,000 build it means having £4,000–£6,000 in reserve that you hope never to spend.

How long it takes and how to get the right price

Most single storey extensions take 8–12 weeks on site once the foundations start, with design and approvals adding two to four months before that. The weather-sensitive stages sit at the start, so spring starts tend to run smoothest.

Get at least three itemised quotes and make sure each covers the same specification, the same glazing and the same assumptions about ground and drains. Our single storey extension cost calculator gives you a benchmark figure to test quotes against, and you can browse all extension services to find vetted builders for your project.

FAQs: How Much Does a Single Storey Extension Cost in the UK