Two Storey Extension Cost Calculator

A two storey extension in the UK costs around £2,050 – £2,900 per m² at a standard finish, £2,650 – £3,350 per m² at a good finish and £3,100 – £4,200 per m² for premium work. That rate is per m² of floor across both storeys, not per m² of footprint, so double your ground floor area before you multiply. The rates include VAT; fees and approvals come on top.

Why two floors cost less per m²

One set of foundations and one roof serve both floors. Those are two of the dearest parts of any extension, and on a single storey build they carry one room. Put a second floor on top and the same groundwork and roof are spread over double the area. That is why the two storey rate sits below the single storey build-only rate of around £2,650 – £3,000 per m². Both rates include VAT.

The total is still higher. You are building twice the floor, and the extra storey brings work of its own: deeper strip foundations to carry the heavier load, a first floor structure, a stair that has to land somewhere sensible in the existing house, and scaffolding for the second lift. Expect the on-site work to run about 1.5× the single storey programme.

What changes the price

Once the shell is up, finish and glazing move the rate more than anything else. Larger openings, better joinery and architectural glazing are what push a job into the good or premium band. Then come the ground (poor soil means more digging and more concrete), the steels where the extension opens into the house, and party wall work if you build on or near a shared boundary.

Region matters too. The shared planning model uses the Midlands as its baseline: London +20%, South East +10%, South West and East of England +5%, and the North, Scotland, Wales and Northern Ireland −10%. These are broad allowances, not measured regional averages; local quotes can differ.

The per-m² rate covers foundations, walls, the first floor structure, the roof, windows and doors, steels for the opening, electrics, plumbing and heating, plastering and finishes, with VAT included. It leaves out professional fees, planning and building control fees, a kitchen and any upstairs bathroom. The calculator adds the fees back as their own lines, plus a bathroom or en-suite at £4,500 – £9,000 if you tick the box. If you project manage the build, hold every quote against that list.

The house extension cost calculator gives a quick figure with one multiplier for the second storey. This one prices the floors, finish, fees and nation separately, so use it for a two storey budget.

Planning and approvals by nation

In England, a two storey extension is only permitted development if it is a rear extension no deeper than 3m beyond the original rear wall, at least 7m from the rear boundary, and built in matching materials. Upstairs side windows must be obscure-glazed with high-level opening only. A two storey side extension never qualifies. Most projects therefore need a householder planning application, at £548.

Wales, Scotland and Northern Ireland set their own householder fees: £585, £371 and £347. In Scotland the approval you need before work starts is a building warrant, priced on the value of the work, so the calculator's approval line is only an estimate there. Everywhere else it uses a full plans fee of £300 – £900 plus an inspection fee of £500 – £800.

A worked example

Take a rear extension 4m deep and 5m wide in the Midlands, at a standard finish. That is a 20m² footprint and 40m² of new floor, so the calculator prices 40m² at £2,050 – £2,900 per m² with no regional uplift. Fees add 8-15% of the build. Building control adds the full plans and inspection fees. At 4m deep it is past the permitted development depth, so the £548 householder fee applies.

Enter those inputs in the calculator above for the total and where the money goes. Then hold 15-20% contingency on top for ground conditions, drainage under the footprint and changes to the existing roof.