Adding a bedroom through an extension is one of the most financially rewarding home improvements you can make — it adds usable living space and typically adds more to a property’s value than it costs to build. In 2026, a modest single-storey rear extension runs from around £20,000 to £45,000, while a two-storey addition that delivers a bedroom above a ground-floor room costs £40,000 to £80,000 or more, depending on size, finish, and location.
The figures above are for a fairly standard specification in the Midlands or North of England. In London and the South East, add 25–40% to these baselines.
What Drives the Cost of a Bedroom Extension?
Several variables have an outsized effect on the final bill:
- Size — costs scale roughly with floor area. The industry norm is £1,800–£3,200 per m² for a finished, single-storey shell, including foundations, structure, roof, windows, insulation, and first-fix services. Fit-out (plastering, flooring, electrics, decoration) adds £300–£600/m² on top.
- Storeys — a two-storey extension is not simply double the cost of a single-storey: the structure is shared and economies of scale apply. Expect two-storey builds to cost around 1.4–1.6× the single-storey equivalent.
- Foundation type — strip foundations suit most domestic extensions (£3,000–£6,000); poor ground conditions may require piled or raft foundations (£8,000–£20,000+).
- Roof specification — a flat roof (warm deck, minimum 150 mm PIR) is the cheapest; a pitched tiled roof to match the existing house costs 20–35% more.
- Party walls — if the extension sits within 3 m of a neighbour’s foundations you will need a Party Wall Agreement (surveyor fees typically £700–£1,500 per adjoining owner).
- Structural engineer — required for any load-bearing alterations, typically £500–£1,500 for residential work.
Bedroom Extension Cost Breakdown (Worked Example)
The table below shows a realistic cost breakdown for a 4 m × 4 m (16 m²) single-storey rear bedroom extension in the Midlands, finished to a standard level with UPVC windows and a flat roof.
| Cost Element | Low | High | Notes |
|---|---|---|---|
| Design & drawings | £1,200 | £2,500 | Architect or architectural technician |
| Planning application (if req.) | £258 | £258 | England 2026 householder fee |
| Structural engineer | £500 | £1,200 | Calculations & details |
| Building Regs application | £300 | £600 | Full plans submission |
| Groundworks & foundations | £3,500 | £7,000 | Strip foundations assumed |
| External walls (blockwork) | £3,500 | £6,000 | Cavity wall, insulated |
| Roof structure & covering | £3,500 | £6,500 | Flat warm-deck roof |
| Windows & external door | £1,500 | £3,500 | UPVC, energy-rated |
| First-fix electrics & plumbing | £1,200 | £2,500 | New circuits, radiator |
| Insulation | £800 | £1,500 | Floor, wall, roof per Part L |
| Plastering & dry-lining | £1,500 | £2,500 | Skimmed finish |
| Second-fix & decoration | £1,500 | £3,000 | Sockets, flooring, paint |
| Total | £19,300 | £37,058 |
A higher-spec version — bi-fold doors, underfloor heating, hardwood floor — can push the same footprint to £45,000–£55,000.
Two-Storey Bedroom Extension Costs
If you extend upward to gain a bedroom on the first floor, the cost per m² of total new floor area actually falls because roof, drainage stack, and foundation costs are shared across two levels. A 4 m × 4 m two-storey extension (32 m² total) typically costs £48,000–£75,000 in the Midlands, and £65,000–£100,000 in London.
Planning Permission
Most single-storey rear extensions can be built under Permitted Development (PD) provided they meet the limits in the Town and Country Planning (General Permitted Development) (England) Order 2015, as amended:
- Single-storey: maximum 4 m rear projection from original rear wall (detached) or 3 m (semi/terrace); maximum 4 m ridge height.
- Two-storey: maximum 3 m rear projection; must be at least 7 m from the rear boundary.
In a conservation area, Article 4 Direction area, or if your property is listed, full planning permission is required. Always check with your Local Planning Authority before starting.
Building Regulations
All extensions require Building Regulations approval. Key requirements in 2026:
- Part L (Energy) — new walls must achieve U-value ≤0.18 W/m²K; roofs ≤0.13 W/m²K; floors ≤0.18 W/m²K.
- Part A (Structure) — foundations and structure must be independently assessed.
- Part F (Ventilation) — habitable rooms require background ventilation and purge ventilation.
- Part B (Fire safety) — interconnected mains smoke alarms required throughout the property.
The building inspector will make site visits at foundation, frame, and completion stages. Your builder should not progress past each stage without sign-off.
Day Rates for Key Trades (2026)
| Trade | Day Rate (Midlands) | Day Rate (London) |
|---|---|---|
| Groundworker | £220–£280 | £280–£380 |
| Bricklayer | £240–£320 | £320–£450 |
| Carpenter | £220–£280 | £300–£420 |
| Plasterer | £200–£260 | £280–£380 |
| Electrician | £230–£310 | £300–£450 |
| Plumber | £220–£300 | £300–£420 |
How to Keep Costs Under Control
Get at least three quotes. Labour and material pricing varies enormously between contractors. A quote 30% below the median is a red flag; so is one 30% above without a clear reason.
Fix the design before breaking ground. Changes made during construction are the single biggest source of cost overruns. Agree a schedule of works, a specification, and a programme before signing anything.
Consider a main contractor vs. self-managing trades. Managing individual trades yourself can save 10–15% but requires time and construction knowledge. If you have neither, a competent main contractor who warranties the whole job is better value.
Check VAT. Extensions on existing dwellings are generally subject to VAT at 20%. However, if any element qualifies as a conversion or a listed-building repair, relief may apply — worth checking with your contractor.
Is It Worth It?
Bedroom extensions consistently deliver a strong return. In 2026, estate agents typically value a four-bedroom house 15–25% above an equivalent three-bedroom in the same street. On a £450,000 home that equates to £67,500–£112,500 in added value — comfortably ahead of a mid-range single-storey bedroom extension. The payback is strongest in tight urban markets and weakest where values are already near their ceiling for the area.
