Single Storey Extension Costs UK: Full Price Breakdown

A single storey extension is one of the most popular ways UK homeowners add space — a bigger kitchen, a family room, a utility, or a knock-through that opens up the back of the house. The hard part is knowing what it actually costs before you commit. Builder quotes vary wildly, online "averages" hide huge swings, and the headline figure is rarely the final figure.

This guide breaks down a realistic single storey extension cost in the UK in plain English: foundations and shell, fit-out, design and approval fees, VAT, and the line items homeowners routinely forget. Use it alongside the free Builder Quote Checklist and the Renovation Budget Planner to put a proper number against your own project.

Typical single storey extension cost in the UK (2026)

As a working rule of thumb, a single storey rear extension in the UK currently lands in the region of £2,200–£3,200 per m² for a standard build, with London and the South East often £2,800–£3,800+ per m². A 20 m² extension therefore typically falls between £44,000 and £64,000 outside London, before kitchen, finishes and VAT in some cases.

Those numbers are deliberately wide because the same brief can be quoted very differently. The cost depends on access, ground conditions, how much glass you want, the kitchen spec, and how much of the existing house gets disturbed. Treat any single "average" you see online with caution.

1. Design and approval fees (before a brick is laid)

• Architect or architectural designer: £2,000–£6,000+
• Structural engineer (steel beams, foundations): £600–£1,500
• Planning application fee (if required): around £258
• Building control fees: £500–£1,200
• Party Wall surveyor (if neighbours involved): £900–£2,000+ per neighbour
• Drainage / CCTV survey if near a public sewer: £200–£600

These costs are real and they're almost never inside the builder's quote. Budget for them separately or you will be £5,000–£10,000 out before work starts.

2. Foundations and shell

The shell is everything that gets you from a hole in the ground to a watertight box. Expect:

• Strip or trench-fill foundations (deeper if there are trees nearby)
• Oversite, beam and block or insulated floor
• Cavity walls, lintels, steels, roof structure
• Roof covering — flat (EPDM / GRP) or pitched tile
• Rooflights or lantern (often a big cost line on its own)
• Bifold or sliding doors
• External finishes, gutters and downpipes

For a typical 20 m² rear extension the shell alone often sits between £28,000 and £40,000 before any internal finishes.

3. Knock-through and making good the existing house

A single storey extension almost always involves removing part of the existing rear wall and inserting a steel beam. This is where homeowners get caught out — the new extension is only half the project. You also need:

• Structural opening, padstones and steel beam
• Re-routing radiators, sockets and switches in the old room
• Re-plastering and re-decorating the existing room edges
• New flooring across old and new (so the join doesn't show)
• Possibly moving the boiler, consumer unit or soil pipe

"Making good" is one of the most commonly underquoted items in a UK single storey extension. Always ask explicitly what is included.

4. Fit-out: kitchen, bathroom, finishes

Because most single storey extensions become a kitchen-diner, the kitchen budget can rival the shell. Sensible UK ranges:

• Budget kitchen (Howdens / Wickes trade): £6,000–£12,000 supply
• Mid-range kitchen: £12,000–£25,000 supply
• Premium / bespoke kitchen: £25,000–£60,000+
• Worktops (quartz / stone): £2,000–£5,000
• Appliances (oven, hob, hood, fridge, dishwasher): £2,000–£8,000
• Tiling, flooring, decoration across old and new: £4,000–£10,000
• Electrics (lighting plan, sockets, extract): £3,500–£7,000
• Plumbing and underfloor heating: £3,000–£7,000

Builder quotes often list a "provisional sum" for kitchen, tiles or flooring. These are estimates, not fixed prices — see the provisional sums guide for how they tend to creep upward in practice.

5. VAT — and why it matters more than you think

Most domestic single storey extensions in the UK are standard rated at 20% VAT. If your builder is VAT registered, that 20% is on top unless their quote is explicitly inclusive. On a £55,000 extension, that is £11,000 — enough to derail a budget on its own. Always ask whether the price is inclusive or exclusive of VAT and whether the builder is VAT registered.

6. The items homeowners forget

• Skip hire and skip permits (£300–£900+)
• Scaffolding (£800–£2,500)
• Temporary kitchen / eating out while works run
• Storage of furniture and belongings
• Damage to garden, drive, lawn and reinstatement
• Curtains, blinds, furniture for the new room
• 10–15% contingency for the unknowns ground works always find

The Renovation Budget Planner includes a UK single-storey extension cost template with every line above so nothing gets quietly missed.

7. A realistic worked example (20 m² rear extension)

• Design, structural, planning, building control: £6,000
• Foundations, shell, roof, doors, glazing: £36,000
• Knock-through, steel, making good existing room: £6,500
• Kitchen supply (mid-range): £14,000
• Worktops and appliances: £5,500
• Electrics, plumbing, UFH: £8,000
• Flooring, tiling, decoration: £6,500
• Skip, scaffolding, sundries: £2,500
• VAT (where applicable, on builder work): variable
• Contingency (10%): £8,000

Total before VAT and finishing extras: roughly £93,000. That is a more honest number than the "£40k extension" you'll see on a forum thread, and it matches what UK homeowners actually spend when the project is finished, not when the shell is done.

How to use this on your own project

Before you accept any builder quote for a single storey extension, run it through the free Builder Quote Checklist so you can see what is included, what is excluded and what looks suspiciously cheap. Then use the Renovation Budget Planner to put your own numbers against every line in this guide — design fees, shell, fit-out, VAT, contingency — so you go in with a realistic total, not just the builder's headline figure. If you also want help comparing two or three quotes side by side, the Homeowner Quote Checker Pack gives you a scoring grid and a clarification email template.

The Builder's Brief UK provides practical homeowner guidance based on real trade experience. It is not legal, financial, architectural, structural engineering, planning or building-control advice, and is not a replacement for project-specific guidance from suitably qualified professionals.