How much does a house extension cost, and how do you set a realistic budget?

Most UK homeowners start with a number in their head and no idea whether it is realistic. This guide gives you the cost ranges the industry actually works to, then shows you how to turn them into a budget you can defend.

What a house extension costs in the UK

Costs are usually quoted per square metre of new floor area, excluding VAT and fees. As broad 2026 planning figures:

• Single-storey rear extension: roughly £2,200-£3,000 per m2 outside London and the South East, £2,800-£4,000 per m2 in London.
• Two-storey extension: often 15-25% cheaper per m2 than single-storey, because you get more floor area under one roof and one set of foundations.
• Loft conversion: roughly £1,600-£2,600 per m2 for a straightforward dormer, more for a hip-to-gable with a new staircase.
• Garage conversion: usually the cheapest per m2, because the shell already exists.

A 20 m2 single-storey rear extension therefore lands somewhere between £45,000 and £80,000 for the build alone, before kitchens, bathrooms, landscaping or fees.

The costs people forget

The build contract is typically 70-80% of what you actually spend. Budget separately for:

• Professional fees: architect or architectural designer, structural engineer, and possibly a party wall surveyor. Allow 10-15% of build cost combined.
• Statutory fees: planning application, building control, and lawful development certificate if you are going down the permitted development route.
• Surveys: measured survey, drainage/CCTV survey, asbestos survey in anything built before 2000.
• Fit-out: kitchens, sanitaryware, flooring, decoration, blinds and furniture are usually excluded from a builder's price unless you asked for them explicitly.
• Living costs: storage, temporary kitchen, or renting elsewhere if you move out.
• VAT: most domestic building work is standard-rated at 20%. Check whether quotes include it.

How to build the budget

1. Set your total ceiling first — the number you will not go past, including everything above.
2. Work backwards. Take off 10-15% contingency, then fees and fit-out. What remains is your build budget.
3. Convert that to a per-m2 figure and sense-check it against the ranges above. If your figure is well below the range, the design is too ambitious for the money — change the design, not the budget.
4. Decide in advance which items are fixed and which are flexible. Structure and insulation are fixed. Worktops, rooflights and ironmongery are flexible.

Why contingency is not optional

10% is the minimum on a new-build extension. 15-20% is realistic on anything involving an older property, underpinning, unknown drainage, or work to a structure you cannot see until it is opened up. Contingency is not slush money for a nicer kitchen — hold it until the structural work is complete.

What moves the price most

• Ground conditions and foundation depth, especially near trees or on clay.
• Steel: large open spans and bifold openings need beams, padstones and sometimes temporary propping.
• Glazing: rooflights and sliding doors are frequently the single biggest line after the shell.
• Drainage diversions and moving a soil stack.
• Access: a terraced house with no side access means everything goes through the hallway, and that costs labour time.
• Specification level: the same 20 m2 can cost £45,000 or £110,000 depending on finishes.

Getting prices you can compare

Ask at least three contractors to price the same drawings and the same written specification. A price against a sketch is a guess. If you have not fixed the specification, you have not fixed the price — see the guide on comparing builders beyond the headline price.

Next step

In BuildWise Workspace you can capture your budget, track decisions that affect it, generate a realistic programme, and send one consistent enquiry pack to matched professionals near you, so the quotes you get back are genuinely comparable.