A 20m² (5m by 4m) single storey rear extension at standard spec costs £54,000 to £62,000 including VAT and professional fees at West Midlands rates, about £2,870 per m². The same extension is £49,500 to £59,500 in the North East and £71,000 to £88,500 in inner London. Here is what sits inside that figure, what changes it, and the things people forget to budget for.
- Single storey rear, 20m²: £54,000 to £62,000
- Single storey side return, 15m²: £50,000 to £57,500
- Wraparound (rear 20m² plus side 15m²): £100,500 to £115,500
Figures from Quoteio’s free calculator. For your own area, see prices by region.
What drives the price of a single storey extension?
A “20m² extension” isn’t a fixed product. The same footprint can cost very different amounts depending on five things:
- What goes inside it. An empty room is the cheapest. A kitchen, bifold doors and a roof lantern add the most.
- Spec level. Blockwork and a flat roof cost less than face brick and a pitched slate roof.
- Size. Bigger costs more in total but less per m².
- Region. London and the South East cost more; the North East, Wales and Northern Ireland less.
- The site. Access, drains, soft ground, trees and party walls, none of which can be priced without a visit.
What does a 20m² single storey extension cost, trade by trade?
Here is how the engine builds up a 5m by 4m single storey rear extension, ex VAT, at standard and mid spec. These are the same component rates the calculator uses, taken from the costing sheets of completed Living Design & Build jobs.
| Trade or item | Standard | Mid |
|---|---|---|
| Foundations & slab | £4,300 | £4,300 |
| Screed | £1,700 | £1,700 |
| External walls | £4,406 | £4,590 |
| Roof | £3,100 | £3,100 |
| Fascias & guttering | £1,980 | £1,980 |
| Plastering | £1,800 | £1,800 |
| Skirting & architrave | £630 | £630 |
| Scaffold | £819 | £819 |
| Drainage | £575 | £575 |
| Electrics | £4,488 | £4,488 |
| Plumbing | £4,713 | £4,713 |
| Labour | £16,251 | £16,356 |
| Structure ex VAT | £44,762 | £45,051 |
Structure only: no kitchen, no special glazing. Labour is about 36% of the structure cost. Professional fees and VAT are added below.
What do you have to add on top of the build?
Professional fees and compliance
Every price in this guide already includes these. On the standard 20m² extension, ex VAT:
- Architectural drawings: £1,200
- Structural engineer: £800
- Building Regulations: £500
- Householder planning application: £528 in England. Many single storey rear extensions are permitted development and don’t need one, but we include it to be safe.
Party wall agreements
If you build on or near a shared boundary in England or Wales, the Party Wall etc. Act 1996 usually requires you to serve notice on your neighbours. If a neighbour doesn’t consent, surveyors are appointed and you normally pay their fees. Our side extension prices include one party wall surveyor at £700 ex VAT.
VAT at 20%
Extensions are standard-rated for VAT. On our 20m² example the VAT alone is £9,558. Some builders quote ex VAT and some inc VAT, so always ask which.
How much does spec change the price?
| Spec level, 5m by 4m | Typical price inc VAT and fees |
|---|---|
| Standard Blockwork walls, flat GRP roof. About £2,870 per m² | £54,000 to £62,000 |
| Mid Cavity walls, flat GRP roof, fuller drawings. About £2,950 per m² | £55,500 to £64,000 |
| High Face brick, pitched slate roof. About £3,380 per m² | £63,500 to £73,000 |
How much does size change the price?
Bigger extensions cost less per square metre, because drainage connections, scaffold set-up and fees hardly change with size.
| Size | Typical price | Per m² |
|---|---|---|
| 3m by 3m (9m²) | £38,500 to £44,500 | £4,560 |
| 4m by 3m (12m²) | £43,000 to £49,500 | £3,830 |
| 5m by 4m (20m²) | £54,000 to £62,000 | £2,870 |
| 6m by 4m (24m²) | £59,500 to £68,000 | £2,630 |
| 8m by 4m (32m²) | £70,000 to £80,500 | £2,330 |
Standard spec, VAT and fees included, West Midlands rates.
How much does a single storey extension cost in my region?
| Region | Single storey rear, 20m² |
|---|---|
| Inner London | £71,000 to £88,500 |
| Outer London | £64,000 to £79,500 |
| South East | £59,500 to £71,000 |
| East of England | £57,000 to £68,000 |
| South West | £55,500 to £66,000 |
| East Midlands | £52,000 to £62,500 |
| West Midlands | £54,000 to £62,000 |
| North West | £52,000 to £62,500 |
| Yorkshire and the Humber | £50,500 to £60,500 |
| North East | £49,500 to £59,500 |
| Wales | £50,500 to £60,500 |
| Scotland | £52,500 to £63,000 |
| Northern Ireland | £47,500 to £56,500 |
Our West Midlands prices come straight from completed local jobs. Other regions use a cost index; see London or Scotland for the local detail, or read how we price.
Is a double storey extension worth the extra?
A double storey rear extension on the same 5m by 4m footprint costs £86,500 to £99,500 for 40m² of new floor, about £2,310 per m², against £2,870 per m² for single storey. The foundations, roof and drainage are paid for once and shared by both floors. The catch is planning: double storey extensions usually need a full householder application. Our full extension cost guide compares every type.
What quietly inflates the bill?
On the same 20m² standard spec extension, the calculator adds:
| Addition | Adds about, inc VAT |
|---|---|
| French doors | £2,160 |
| Bifold doors (3-pane) | £2,160 |
| Roof lantern | £2,040 |
| Velux rooflight | £816 |
| Kitchen units allowance | £3,840 |
Bifolds and a lantern together are typical on kitchen extensions, priced in full in our kitchen extension guide.
Some costs only show up once a builder has seen the site, so ask for them to be priced separately:
- Knocking through to the existing house: steel beams, padstones and making good.
- Underfloor heating, and a boiler upgrade if the existing one can’t cope.
- Drainage diversion if a soil pipe or public sewer runs under the footprint.
- Deeper foundations on clay or near trees.
How long does a single storey extension take?
Allow time for design before anyone digs: drawings and structural calculations, then either a lawful development certificate or a planning application (councils aim to decide householder applications within 8 weeks), then Building Regulations approval. The build itself depends on size, what goes inside and the weather; ask each builder for a written programme with their quote.
Should you extend or move?
Moving brings stamp duty, agent and legal fees that you never get back, while an extension leaves you with the space. Whether the extension adds more value than it costs depends on your street, so ask a local estate agent. We run the numbers side by side in loft conversion vs moving house; the same logic applies to extensions.
The short version
A standard 20m² single storey rear extension costs about £54,000 to £62,000 at West Midlands rates, with structure, fees and VAT included and no kitchen. Price your own with the free calculator, get three written quotes from the same drawings, and keep a contingency for what nobody can see until the ground is open.
Common questions
Do I need planning permission for a single storey rear extension?
Often not. In England many single storey rear extensions fall under permitted development if they stay within the depth and height limits (for example 3m deep on an attached house and 4m on a detached one, and no more than 4m high), do not extend beyond the front of the house and use similar materials. Larger rear extensions may be possible through prior approval. Homes in conservation areas, listed buildings, flats and houses whose permitted development rights have been removed usually need planning. Check with your council, and you still need Building Regulations approval.
Is it cheaper to use a main contractor or manage the trades yourself?
Managing the trades yourself can save the main contractor’s margin, but you take on the scheduling, the ordering, the risk of trades not turning up and the job of making sure each stage passes Building Control. For most homeowners with day jobs a main contractor is worth it. Either way, get at least three written quotes from the same drawings.
What is the cheapest way to extend?
Keep it small and simple. A 3m by 3m single storey rear extension at standard spec costs £38,500 to £44,500 at West Midlands rates, and a 6m by 2.5m side return costs £50,000 to £57,500. The cost per square metre falls as the room gets bigger, though, so a slightly larger room is often better value.
Why is VAT charged on extensions but not new builds?
Building a brand new dwelling is zero-rated for VAT. Extending an existing home is standard-rated at 20%, so VAT applies to the building work on an extension. Every Quoteio extension price includes it. Check whether each builder’s quote is shown with or without VAT.