House Extension Cost UK 2026: Single Storey vs Double Storey

House extension cost UK 2026 guide: compare single and double storey extension prices, key cost factors, and budgeting tips before you build.

A house extension is one of the most significant home improvement projects you can undertake — and one of the most rewarding when it goes well. Costs vary considerably depending on the type, size, specification, and location. This guide gives you realistic figures for both single and double storey extensions in 2026.

Single Storey Extension Costs

A single storey rear or side extension typically costs £1,500 to £2,500 per square metre for a standard specification. For a 20 sqm extension:

• Budget specification: £30,000 to £40,000
• Mid-range: £40,000 to £55,000
• Premium (with bi-folds, lantern roof, high-spec finish): £55,000 to £80,000+

Double Storey Extension Costs

Adding a second storey costs less per square metre than a single storey because foundations, groundworks, and roofing are shared between floors. A double storey extension of 20 sqm per floor typically costs:

• Budget specification: £50,000 to £70,000
• Mid-range: £70,000 to £100,000
• Premium: £100,000 to £150,000+

What Drives the Cost?

• Size and height of the extension
• Roof type — pitched, flat, glass lantern, or sedum all have different costs
• Glazing specification — large bi-fold or sliding door sets add £5,000 to £15,000
• Ground conditions and proximity to trees
• Internal fit-out — flooring, plastering, kitchen or bathroom fitting if applicable
• Location — London and South East add 30 to 50 per cent

Professional Fees and Planning

An architect or architectural designer is needed for most extensions — allow £2,000 to £6,000 for drawings, planning, and building regulations support. A structural engineer adds £500 to £2,000. Householder planning applications cost £206 as of 2025. Many single storey rear extensions meet Permitted Development criteria and don't need planning permission, though building regulations are always required.

Getting the Right Builder

Use an architect to tender the project where possible — they know local contractors and can evaluate quotes properly. Otherwise, the Federation of Master Builders and TrustMark are reliable sources. Get at least three quotes from builders working from the same drawings and specification.

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