How Much Does a House Extension Cost in 2026?
A house extension is a major investment that can transform your living space and add significant value. In 2026, costs range from £25,000 for a small single-storey extension to £100,000+ for a large double-storey project.
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15–20 sqm rear extension. Open-plan kitchen-diner is the most popular use.
25–40 sqm including structural glazing, bi-fold doors, and high-spec finish.
Two floors of additional space. Requires more structural work but better cost per sqm.
Factors That Affect the Cost
- Extension size in square metres
- Single or double storey
- Foundation and ground conditions
- Roof type (flat, pitched, glazed)
- Internal fit-out and specification
- Planning permission and building regulations
Regional Price Variations
Extension costs per sqm in London average £2,000–£3,000 compared to £1,200–£1,800 in the Midlands and North. These differences are driven primarily by labour costs.
Frequently Asked Questions
Do I need planning permission for an extension?
Small single-storey rear extensions often fall under Permitted Development. Larger, side, or front extensions typically need planning permission.
How long does a house extension take?
A single-storey extension takes 3–4 months. Double-storey extensions typically take 4–6 months from groundworks to completion.
How much value does an extension add?
A well-designed extension can add 10–20% to your property value, often exceeding the build cost in high-demand areas.
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