Worked example: 20m² rear extension · UK average · contemporary finish
Quick budgeting answer: Our UK-average model places this example at £70,500, with lower and higher construction-rate scenarios of £52,500–£88,000. These are planning allowances, not a narrow predicted contractor quote.
Illustrative model results for the precise example shown, not current quotes from builders. Budgets include an assumed 20% VAT on construction, professional/approval allowances and 10% contingency. A survey or different specifications could change them.
Construction price and full project budget are different
The Federation of Master Builders gives £1,800–£3,000 per m² as a broad starting range for single-storey extension construction. At 20m², that multiplication produces £36,000–£60,000 before considering the full list of project-specific costs. Our example above uses a central construction rate of £2,400/m², then adds modelled professional costs, standard-rated construction VAT and a contingency.
Which parts of a single-storey extension cost most?
The soil, drains, access and existing building position influence excavation and foundation design. A survey can change the budget.
Wall materials, insulation and any supporting steels to open up the original rear wall can be significant items.
Rooflights, roof form, sliding doors and bespoke glazing change the price as well as how much daylight enters.
Electrics and basic finishes are different from a fully fitted kitchen, utility room or premium flooring.
What should a builder’s quote include?
Ask every tendering builder to price the same drawings and a written specification. Compare whether demolition, spoil removal, drainage, electrical changes, plastering, flooring, waste removal and making good the original house are included. The cheapest number is not the best value if essential work is excluded.
| Budget heading | What to verify |
|---|---|
| Construction | Foundations, walls, roof, windows, doors and ordinary internal finish |
| Professional and approvals | Surveyor, architect or designer, engineer and building control fees as applicable |
| Tax and contingency | VAT position and a realistic allowance for unexpected site conditions |
Start with the right dimensions
A 5m × 4m extension has 20m² of new floor area. A 5m × 3m extension has 15m², but the smaller job still needs a roof, foundations and building services. Cost does not fall in exact proportion to area. Read the 20m² example or use our calculator for a different footprint.
Some house extensions in England can fall under permitted development, subject to restrictions, while most still require building regulations approval. See our planning permission and building regulations explainers.
Calculate your own extension budget
The examples above describe specific assumptions. Change the dimensions, region, finish and extras to explore your project.
Calculate my extension price →Common questions
What is the average single-storey extension price per m²?
The Federation of Master Builders publishes a broad £1,800–£3,000 per m² single-storey construction benchmark. It is not necessarily an all-in homeowner budget, and project conditions can push a quote outside that range.
Does the calculator include VAT and fees?
Our planning model assumes VAT on construction and adds indicative professional/approval fees and a contingency. Check every contractor proposal for what is actually included.
Can I calculate the cost for a 3m-deep extension?
Yes. Enter your approximate width and depth in the calculator to see how changing floor area affects the illustrative budget.
Pricing context: Federation of Master Builders construction benchmark; the full-budget numbers are generated by our published illustrative model, which makes additional assumptions. For permissions in England see Planning Portal. Other UK nations have different rules. Always obtain site-specific professional advice and comparable written builder quotes.