Quick Answer
A single-storey extension costs £1,500–£2,000/m² for a budget build, £2,000–£2,500/m² for a mid-range finish, and £3,000+/m² for a premium specification. A typical 20m² rear extension costs £30,000–£60,000 all-in, including VAT and professional fees, in 2026.
A single-storey rear extension is one of the most popular home improvements in the UK. Whether you're creating an open-plan kitchen-diner, adding a utility room, or building a home office, an extension offers a significant and permanent increase in living space — and when done well, it can add considerably more value than it costs to build.
In 2026, build costs remain elevated compared to pre-pandemic levels due to ongoing material and labour cost pressures. However, competition among local builders means that getting three or four quotes can still reveal meaningful price differences. This guide gives you a realistic picture of what you should budget, what you'll get at each price point, and what drives the cost up or down.
All costs below are 2026 UK market rates inclusive of VAT at 20% unless stated otherwise. Ready to start comparing quotes? Post your extension project on GetQuickHelp for free.
In this guide
Cost Per m² by Specification Level
Extension costs are quoted per square metre of internal floor area. The per-m² rate varies significantly depending on build quality, glazing, internal fit-out, and your location:
| Specification | Cost per m² | What You Get |
|---|---|---|
| Budget | £1,500 – £2,000/m² | Blockwork walls, felt roof or basic tiles, standard double glazing, basic fit-out |
| Mid-range | £2,000 – £2,500/m² | Brick-faced walls, quality tiles or flat roof, bi-fold or slide-and-turn doors, plastered and decorated |
| Premium | £2,500 – £3,500/m² | Matching brickwork, structural glazing, oak flooring, underfloor heating, designer lighting |
| Luxury | £3,500 – £5,000+/m² | Architecturally designed, full glazing, premium finishes throughout |
Costs for Typical Extension Sizes
To give you a more tangible sense of total project costs, here are all-in prices (including professional fees, VAT, and contingency) for common extension sizes:
| Size | Budget | Mid-range | Premium |
|---|---|---|---|
| 10m² (small utility/office) | £15,000 – £20,000 | £20,000 – £25,000 | £25,000 – £35,000 |
| 16m² (single room) | £24,000 – £32,000 | £32,000 – £40,000 | £40,000 – £56,000 |
| 20m² (large kitchen-diner) | £30,000 – £40,000 | £40,000 – £50,000 | £50,000 – £70,000 |
| 30m² (wraparound/large) | £45,000 – £60,000 | £60,000 – £75,000 | £75,000 – £105,000 |
💡 Tip
Always add a 15–20% contingency to your budget. Extensions almost always uncover unexpected issues once groundworks begin — underground drainage, poor existing foundations, or asbestos in older properties. A contingency protects your project from stalling mid-build.
Stages of Work and Costs
| Stage | Typical Cost | Notes |
|---|---|---|
| Architect / designer drawings | £1,500 – £4,000 | Required for planning and Building Regs |
| Planning application fee | £258 – £578 | England 2026 householder fee |
| Building Regulations application | £600 – £1,500 | Full plans or building notice route |
| Structural engineer | £500 – £1,500 | Required for opening up walls and beam specifications |
| Party wall surveyor (if applicable) | £700 – £2,000 | Required if within 3m of neighbour's foundations |
| Groundworks and foundations | £4,000 – £12,000 | Highly variable — depth and soil conditions matter |
| Brickwork / structural shell | £8,000 – £20,000 | Walls, roof structure, weatherproofing |
| Glazing (bi-folds / roof lanterns) | £3,000 – £15,000 | Biggest variable — full glazing adds significant cost |
| First fix (electrics, plumbing) | £2,000 – £5,000 | Before plastering |
| Plastering and internal finishing | £2,000 – £5,000 | Boarding, skimming, decoration |
| Flooring | £1,500 – £5,000 | Screed, tiles, oak boards, LVT |
What Affects the Cost of a Single-Storey Extension
| Factor | Impact | Notes |
|---|---|---|
| Location | Very high | London costs 30–50% more than the national average |
| Glazing specification | Very high | A full-width roof lantern and bi-folds can add £10,000–£25,000 |
| Foundation depth | High | Deep strip or raft foundations needed on poor ground — adds cost |
| Knocking through to existing house | High | Structural steel RSJ beam, lintel, making good — £2,000–£5,000 extra |
| Party wall requirements | Medium-high | Surveyor fees and potential delay if neighbours don't agree |
| Roof type | Medium | Flat roof cheaper than pitched; pitched adds kerb appeal |
| Kitchen or bathroom included | High | Fitting a kitchen in the new space adds £5,000–£15,000 |
Permitted Development vs Planning Permission
Many single-storey rear extensions in England can be built under permitted development (PD) rights, saving you the cost and time of a planning application. Under the current rules, a single-storey rear extension can be built without planning permission if it:
- Does not extend more than 4m beyond the rear wall of a detached house (3m for a semi or terrace)
- Is not higher than 4m at the ridge or 3m at the eaves
- Does not cover more than 50% of the garden area
- Uses materials of similar appearance to the existing house
- The property is not in a conservation area, AONB, or National Park
Under the larger home extension scheme, you can extend up to 8m (detached) or 6m (semi/terrace) without planning permission, subject to a neighbour consultation scheme. Read our full guide on planning permission for single-storey extensions for the detail.
If your home is attached to another, you may also need a Party Wall Agreement — read our party wall agreement guide to understand when this applies and what it costs.
For finding the right builder for your extension project, read our comprehensive guide on how to hire a builder in the UK.
For a full overview of all permitted development rules, see our complete guide to permitted development rules in the UK.
Frequently Asked Questions
How long does a single-storey extension take to build?+
Most single-storey extensions take 10–20 weeks from start to completion. Groundworks take 2–4 weeks, the structural shell takes 3–6 weeks, and internal finishing takes 3–6 weeks. Planning and Building Regulations applications can add 8–12 weeks before work begins.
Do I need an architect for a single-storey extension?+
You don't legally need a qualified architect, but you do need detailed drawings for the planning application and Building Regulations. Many homeowners use architectural technicians or architectural designers for smaller projects — these can be cheaper than a fully chartered architect while providing equivalent drawings.
Can I build a single-storey extension without planning permission?+
Yes, in many cases. Most single-storey rear extensions in England fall under permitted development rights. The key limits are: 4m depth for detached houses, 3m for semi-detached and terraced. Always check with your local authority or use the Planning Portal's PD checker before starting work.
How much does a bi-fold door add to an extension cost?+
Bi-fold or slide-and-turn doors typically cost £3,000–£8,000 for a standard 3–4 panel set, including installation. Aluminium bi-folds are more durable and expensive; uPVC versions are cheaper. Full-width glazed doors (5–6 panels) can cost £8,000–£15,000.
Does a single-storey extension add value?+
Yes — typically 5–8% of property value. An extension that creates an open-plan kitchen-living space is particularly valued by buyers. The return is better in higher-value areas where floor space is at a premium. Over-specifying (e.g., spending £80,000 on a small extension in a £200,000 terrace) rarely returns full value.
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