Get a realistic cost estimate for your house extension based on size, type, and location.
A house extension is one of the most effective ways to add space and value to your home without the hassle and cost of moving. But getting an accurate cost estimate before approaching builders is essential — it helps you set a realistic budget, compare quotes, and avoid being overcharged.
Extension costs in the UK vary enormously depending on the type, size, specification, and location. This guide breaks down the key factors so you know what to expect.
| Extension type | Typical cost per m² | Example: 20m² (mid-spec, South) |
|---|---|---|
| Single storey rear | £1,500–£2,500/m² | £30,000–£50,000 |
| Single storey side | £1,400–£2,400/m² | £28,000–£48,000 |
| Double storey | £1,200–£2,000/m² | £48,000–£80,000 (40m²) |
| Wrap-around | £1,600–£2,800/m² | £40,000–£70,000 |
| Loft conversion (dormer) | £1,300–£2,200/m² | £26,000–£44,000 |
| Basement conversion | £2,500–£4,000/m² | £50,000–£80,000 |
All costs include labour, materials, and basic fit-out. Excludes kitchen fitting, bathrooms, and furniture unless stated. London adds 25–35% to base costs.
A typical extension quote from a reputable builder will include:
Watch out for these common exclusions that can significantly increase your final bill:
In most cases, yes. A well-designed extension in the right location can add more than it costs — particularly in areas with high property values. Rules of thumb vary, but a quality kitchen extension adding 20–25m² in a £400,000 home might add £40,000–£60,000 in value against a build cost of £35,000–£55,000.
That said, over-extending a property beyond the "ceiling" for its street or area can be a poor investment. Always check what comparable extended properties are selling for in your road before committing to a large project.
Many single-storey extensions qualify as Permitted Development (PD), meaning they don't need a full planning application. The rules allow rear extensions of up to 4 metres (detached) or 3 metres (semi/terrace) under PD, extendable to 8m/6m under the Larger Home Extension scheme with neighbour consultation. Side extensions are more restricted. You'll always need a Certificate of Lawfulness or planning permission if your property is in a conservation area, is listed, or if the extension exceeds PD limits.
A standard single-storey rear extension typically takes 8–16 weeks from groundwork to completion, depending on size and complexity. Allow extra time for planning permission (8–12 weeks), architect/designer planning, finding a builder (4–8 weeks lead time for good local contractors), and material lead times. Total project duration from decision to move-in is often 6–12 months.
Flat roofs are generally cheaper upfront — saving £5,000–£15,000 compared to a pitched roof on a typical 20m² extension. However, flat roofs have a shorter lifespan (15–25 years for EPDM, longer for GRP or zinc) and may be more prone to issues if not installed correctly. A pitched roof to match the existing house typically looks better aesthetically and adds more value, but costs more.