A side extension is one of the most effective ways to add space to a semi-detached or detached home — a wider kitchen, a utility and WC, a home office, or with a second storey, an extra bedroom and bathroom. The first question most homeowners ask is simple: how much will it cost? This guide gives you realistic 2026 figures and shows you how to get an indicative cost for your own property.
How much does a side extension cost per m²?
For 2026 we use an indicative build cost of £2,000–£2,500 per m² (plus VAT) for extension work. This is a shell-and-core figure: the structure, roof, walls, windows and external doors, and the building ready for finishing. Kitchens, bathrooms and decorative finishes are budgeted separately, because they vary so much from one home to the next.
Worked examples
Single-storey side extension, around 15m²: approximately £30,000–£37,500 + VAT.
Two-storey side extension, around 15m² on each floor (30m² in total): approximately £60,000–£75,000 + VAT.
A two-storey side extension often gives the best value per square metre of new space, because the foundations and roof are shared across two floors of accommodation.
What affects the cost of a side extension?
Size and number of storeys — the biggest driver. Access and site conditions — narrow side passages, drains and boundary positions all shape how a side extension is built. Structural work — opening up the side wall of the house usually needs steel beams designed by a structural engineer. Roof design — a pitched roof matched to the main house or a flat roof with rooflights. Glazing — large sliding or bi-fold doors add light and cost. Specification — the quality of windows, doors and finishes you choose.
Do you need planning permission for a side extension?
Some single-storey side extensions can be built under permitted development, while two-storey side extensions normally need a householder planning application. The right route depends on your house type, its location and what has been built before. Our guide to permitted development or planning permission explains the options, and the free Feasibility Check tells you which route is likely for your address.
Don't forget design and approval costs
Before any building work starts you'll need a measured survey, planning drawings, building regulations drawings and structural calculations. Hampshire Build covers all of these in one fixed fee, with our Pre-Build Package from £1,950 + VAT — so you know your design costs from day one.
Get a figure for your own side extension
The quickest way to see what your side extension could cost is our free Feasibility Check. Answer a few questions about your home and your plans and you'll receive an indicative build cost and a view on whether you're likely to need planning permission.
Frequently asked questions
Using an indicative £2,000–£2,500 per m² + VAT, a two-storey side extension with around 15m² on each floor (30m² in total) would cost roughly £60,000–£75,000 + VAT for the shell and core.
Often, yes. The foundations and roof serve two floors of new space, so the cost per square metre of accommodation is usually more efficient.
Some single-storey side extensions can be built under permitted development; two-storey side extensions normally need planning permission. Our free Feasibility Check tells you the likely route for your home.