A house extension in the UK typically costs tens of thousands of pounds, with larger or more complex projects reaching six figures. The price depends on its size, your location, ground conditions, access and the standard of finish, so a useful estimate needs more than floor area alone.
Before comparing prices, decide what you want the extra space to do. A simple living room extension generally involves fewer services than a kitchen or bathroom, where plumbing, drainage, electrics and fittings all add to the budget.
Start with size and shape
A straightforward rectangular extension with a simple roof is usually easier to build than one with several corners, changes in level or complicated roof connections. Bigger extensions cost more overall, although some fixed costs can be spread across a larger floor area.
Two-storey extensions can make better use of foundations and roof work, but they still require extra structure, floors, stairs or circulation space. They also need careful consideration of neighbours, daylight and the existing layout.
Look below ground
Foundations are a major source of uncertainty. Soil conditions, nearby trees, sloping land and neighbouring buildings can affect the design. Some sites need deeper foundations or specialist engineering, which increases labour, materials and excavation costs.
Existing drains can also affect the layout and budget. Building near or over a public sewer may need agreement from the sewerage provider. Early surveys and investigations can help identify these issues before you commit to a design.
Allow for structural alterations
The extension itself is only part of the job. Opening up the back of a house may require structural steelwork, temporary supports and alterations to existing walls. A structural engineer should assess what is needed.
Connecting old and new spaces can also mean replacing flooring, moving radiators, adjusting electrics or making good ceilings and plasterwork. Ask whether these works are included in the estimate.
Choose finishes early
Large glazed doors, rooflights, bespoke joinery and premium flooring can raise costs quickly. Kitchens and bathrooms vary especially widely, so agree realistic allowances for units, appliances, sanitaryware and installation.
Think about heating, insulation and ventilation at the design stage. Your existing heating system or electrical installation may need changes to serve the extra space safely and comfortably.
Check access and location
Labour rates and material delivery costs vary across the UK. A house with narrow side access, limited parking or nowhere to store materials may take more time to build than an otherwise similar property with easy access.
Waste removal, scaffolding and protecting occupied rooms also need budgeting for. If you plan to stay at home, discuss how the builder will manage dust, security and interruptions to water or power.
Include fees, approvals and VAT
Allow for drawings, structural calculations, surveys and Building Regulations approval where required. Planning permission depends on the proposal and property; some extensions qualify as permitted development. Check with your local council, particularly for flats, listed buildings or homes in designated areas.
Ask whether VAT is included in every quote. Party wall procedures may also apply to work near neighbouring structures or boundaries.
Compare the same scope
Give builders the same drawings and specification, then compare inclusions, exclusions and provisional allowances. Check responsibility for decorating, waste removal, fittings and making good the garden. Keep a separate contingency for unforeseen work and agree changes in writing before they proceed.
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