Worked example: 5m × 4m footprint · two floors / 40m² · UK average
Quick budgeting answer: Our UK-average model places this example at £111,500, with lower and higher construction-rate scenarios of £73,500–£146,500. 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.
Calculate total floor area correctly
Our example is a 5m-wide by 4m-deep footprint. That is 20m² on each level and 40m² altogether. A price per square metre must be applied consistently to the total new floor area, not only the ground footprint. The model uses a lower construction-rate schedule for two storeys than for a one-storey build, but the total budget is higher because there is more usable floor space.
Where the economies can occur
The groundworks support two storeys, although a structural engineer may specify stronger foundations.
A shared roof can cover both levels, avoiding duplication of certain roof costs.
A two-storey design can provide rooms upstairs without consuming a larger garden footprint.
Upper-floor openings, matching roof lines, structural ties and temporary support can offset some savings.
Cost questions to resolve before drawings
- Check whether the existing house and proposed foundations can accommodate a two-storey structure.
- Agree how the new first floor joins the original: corridors, door positions and the roof junction matter.
- Ask how builders will access the upper work, protect the property and phase weatherproofing.
- List all first- and second-fix items, including new heating circuits, ventilation, bedrooms or bathrooms.
Planning and building control deserve their own allowance
Height, neighbour impact, boundary positions and the dwelling’s history affect planning considerations. In England, the permitted-development criteria for two-storey additions are more restrictive than for many single-storey rear extensions. Buildings in Scotland, Wales and Northern Ireland have their own regimes. Most extension projects also require building control approval, regardless of planning status.
A nominal 40m² addition may lose some usable space to walls, access, stairs or awkward connections. Ask your designer for measured room sizes, then compare the all-in cost with what those rooms will actually deliver.
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
Is a double-storey extension twice the cost of a single-storey one?
Not necessarily. Foundations and roof can be shared across the two floors, but structural complexity, stairs and interior works can add substantial costs. Compare like-for-like plans.
Is 40m² the ground-floor size?
Not in the example. The footprint is 20m²; with two levels the model counts 40m² of total new floor area.
Does a two-storey extension need planning permission?
The rules depend on the property, location and proposal. Two-storey extensions have specific limits in England and may require a planning application. Obtain advice before assuming permitted development applies.
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.