The rates
Single-storey rear or side extension: £1,900 to £2,600 per m² plus VAT. Double-storey: £1,700 to £2,300 per m² of total new floor area. Wraparound: £2,000 to £2,700 per m². Loft conversion with a rear dormer: £42,000 to £68,000 as a job. Garage conversion: £13,000 to £24,000.
These are build prices for a finished, decorated room with electrics, heating and flooring, in Cheshire, from a contractor who runs the job properly. The kitchen units and appliances that usually go into a rear extension are on top.
Three worked examples
A 4 m × 5 m single-storey rear extension on a Northwich semi (20 m²): £38,000 to £52,000 plus VAT. Add £12,000 to £25,000 for the kitchen.
A 6 m × 4 m double-storey rear extension (48 m² over two floors) on a Winsford detached: £82,000 to £110,000 plus VAT, giving a kitchen-diner below and a bedroom with en-suite above.
A wraparound on a 1930s Wilmslow semi, 32 m²: £64,000 to £86,000 plus VAT, usually with steels to open the whole back of the house.
Fees outside the build price
Architectural drawings £1,500 to £3,500. Structural engineer £600 to £1,500. Planning application fee (if needed) £528 for a householder application in England; a lawful development certificate is £264. Building regulations fees £800 to £1,500. Party wall surveyors, if your neighbour appoints one, £1,000 to £2,000.
What moves the price
Glazing is the big one: a 4 m run of bifolds is £5,000 to £9,000. Steels and opening up the existing house add cost. Underfloor heating, a flat roof with lanterns versus a pitched roof, and the level of kitchen all move the figure. Ground conditions in parts of Northwich and Winsford can mean deeper foundations.
Use our estimate tool for a ballpark on your own numbers, then book a visit for a fixed price.