Extensions & Conversions Costs
More space: extensions, lofts, garages and garden rooms
Quotes for adding space are hard to compare: some include VAT, design, building control and making good, and some don't. Every page here gives our estimates for the build and the fees separately, so you can see the true total.
Many extensions, loft conversions and outbuildings don't need planning permission on a house in England: a single-storey rear extension up to 3m deep on a semi or terrace is permitted development, for example. Almost all of them still need building regulations approval. Each page sets out the limits from the legislation itself, and our planning checker tests an extension against them.
| Job | Typical cost | For |
|---|---|---|
| House extension | £2,400 to £3,200 per m² | single-storey extension, standard finish, including VAT |
| Single-storey extension | £47,300 to £69,400 | 3m x 6m rear extension, standard finish, with fees |
| Double-storey extension | £94,300 to £127,100 | two-storey, 20m² a floor, standard finish, with fees |
| Kitchen extension | £60,500 to £94,400 | 18m² extension, opened up, mid-range kitchen |
| Side return extension | £30,100 to £53,800 | 10m² side return, with fees |
| Porch | £3,000 to £7,500 | uPVC porch, built on a new base |
| Conservatory | £10,000 to £16,500 | uPVC Edwardian, about 12.25m², built |
| Orangery | £30,400 to £48,000 | aluminium orangery, 16m², built |
| Conservatory roof replacement | £7,800 to £12,000 | solid insulated roof, about 12m² |
| Loft conversion | £45,000 to £68,000 | rear dormer conversion, about 30m² |
| Hip-to-gable loft conversion | £50,000 to £78,000 | hip-to-gable with rear dormer, about 30m² |
| Garage conversion | £12,000 to £24,000 | single integral or attached garage, about 16m² |
| Basement / cellar conversion | £36,000 to £79,200 | existing cellar, about 30m², converted |
| Loft boarding | £1,200 to £2,150 | 20m² raised storage floor, fitted |
| Loft ladder / hatch | £300 to £550 | folding ladder in an existing hatch, fitted |
| Garden room | £20,000 to £36,000 | insulated garden room, 12 to 15m², installed |
| Garden office | £12,500 to £20,000 | small insulated garden office, installed |
| New garage | £21,000 to £40,000 | detached single brick or block garage |
| House renovation | £36,900 to £73,800 | 3-bed: rewire, heating, kitchen, bathroom, plaster, decorate |
| Knocking through a wall | £2,800 to £5,000 | load-bearing, 2m opening, with engineer, regs and making good |
| RSJ / steel beam | £800 to £2,500 | domestic steel beam, supplied and installed |
| Underpinning | £6,000 to £21,000 | one wall, mass concrete |
| Groundworks | £4,500 to £6,500 | rear extension foundations, about 24m of trench |
| Concrete base | £800 to £1,000 | 3m x 4m shed base, laid |
| Demolition | £1,000 to £3,300 | single garage demolished and cleared |
| Architect fees | £3,600 to £8,400 | single-storey extension, planning and building regs drawings |
Extensions
Single and two-storey extensions, kitchen extensions, side returns and porches, priced per m² with the fees most quotes leave out.
House extension£2,400 to £3,200 per m² · single-storey extension, standard finish, including VAT
Single-storey extension£47,300 to £69,400 · 3m x 6m rear extension, standard finish, with fees
Double-storey extension£94,300 to £127,100 · two-storey, 20m² a floor, standard finish, with fees
Kitchen extension£60,500 to £94,400 · 18m² extension, opened up, mid-range kitchen
Side return extension£30,100 to £53,800 · 10m² side return, with fees
Porch£3,000 to £7,500 · uPVC porch, built on a new baseConservatories
Conservatories, orangeries and replacement roofs. Keep the doors to the house and they're usually exempt from building regulations.
Conversions
Lofts, garages and cellars turned into rooms, plus loft boarding and ladders for storage. Usually cheaper per m² than building new.
Loft conversion£45,000 to £68,000 · rear dormer conversion, about 30m²
Hip-to-gable loft conversion£50,000 to £78,000 · hip-to-gable with rear dormer, about 30m²
Garage conversion£12,000 to £24,000 · single integral or attached garage, about 16m²
Basement / cellar conversion£36,000 to £79,200 · existing cellar, about 30m², converted
Loft boarding£1,200 to £2,150 · 20m² raised storage floor, fitted
Loft ladder / hatch£300 to £550 · folding ladder in an existing hatch, fittedOutbuildings
Garden rooms, garden offices and garages. Keep them low near the boundary and most don't need planning permission.
Renovation & structural work
Whole-house renovations, knocking through, steel beams, underpinning, foundations, concrete bases and demolition.
House renovation£36,900 to £73,800 · 3-bed: rewire, heating, kitchen, bathroom, plaster, decorate
Knocking through a wall£2,800 to £5,000 · load-bearing, 2m opening, with engineer, regs and making good
RSJ / steel beam£800 to £2,500 · domestic steel beam, supplied and installed
Underpinning£6,000 to £21,000 · one wall, mass concrete
Groundworks£4,500 to £6,500 · rear extension foundations, about 24m of trench
Concrete base£800 to £1,000 · 3m x 4m shed base, laid
Demolition£1,000 to £3,300 · single garage demolished and clearedDesign & planning
What architects and designers charge, and the three ways they price.
Questions people ask
How much does an extension cost?
On our estimates, about £2,400 to £3,200 per m² for a single-storey extension to a standard finish, including VAT, plus fees for drawings, an engineer and building control.
How much does a loft conversion cost?
On our estimates, about £45,000 to £68,000 for a rear dormer conversion of about 30m².
Do I need planning permission for an extension?
Not always. On a house in England, a single-storey rear extension up to 3m deep (4m detached) and 4m high is permitted development, among other limits. Designated areas and flats are different.
How much does a garage conversion cost?
On our estimates, about £12,000 to £24,000 for a single integral or attached garage, including VAT.
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