Garage Conversion Cost Calculator

A garage conversion in the UK is priced per m² of floor area, and what the room is for sets the rate. A basic room such as a utility, playroom or storage space runs about £700 – £1,000 per m². A habitable room (a bedroom, home office or living room) sits around £1,000 – £1,400 per m². The biggest step comes with water and waste: a room with an en-suite or kitchenette costs £1,400 – £2,000 per m². Every one of those rates is a build cost before VAT.

Pick your garage type, floor area, room use and region, and the calculator turns the rate into a ballpark total. It adds the building control fee as its own line, because builders' quotes so often leave it out, and splits the build across the work so you can hold a quote up against it line by line.

Why the room's use sets the rate

Every garage conversion carries the same core jobs. The floor usually sits below the house floor, so it has to be raised, damp-proofed with a damp proof membrane and insulated. The big door opening comes out and gets a new front wall with a window in it. The walls and ceiling need insulating and boarding, and the room needs power and heat.

What changes between the three rates is how far past that core you go. A utility room or playroom sits at the bottom. A bedroom or office is a room someone lives in day to day, and it carries the full finish. Bringing in water and waste for a shower room or a kitchenette is the largest single jump, which is why the top rate is so far above the other two. If you are unsure whether you need that en-suite, run the calculator both ways before you ask anyone to quote.

Single, double and detached garages

A single garage is usually 15-18m² and a double 30-36m². A double costs more in total but less per m² than a single, because the new front wall, the services and the fees are shared across more floor. The calculator applies that discount when you choose a double, so enter the area you are actually converting rather than the whole footprint if you plan to keep part of it as a store.

Where the garage sits matters as much as its size. An integral or attached garage is the cheapest to convert: it already shares walls with the house, and power and heating are close at hand. A detached garage needs its own supply trenched across the garden, and a wet room needs water and waste run out to it as well. Budget about £1,000 – £4,000 for that services run, which the calculator adds when you choose a detached garage. Turning a detached garage into a separate dwelling, or an annex to let out, is a change of use and needs planning permission.

Region moves the figure too. The rates above are a Midlands baseline. London runs about a third higher, the South East about a fifth higher, and the North, Scotland, Wales and Northern Ireland about a tenth lower.

Planning, building control and what quotes leave out

Most garage conversions fall under permitted development, so you need building regulations approval rather than planning permission. The exceptions catch people out on newer estates, where a planning condition often removed permitted development rights when the houses were built. Listed houses and homes in a conservation area need planning too. If you want proof before you start, a lawful development certificate costs £274 in England and the council has 8 weeks to decide. Where you do need full planning, the householder fee in England is £548; tick the planning box in the calculator to include it. The planning checker walks you through the questions that decide which route you are on.

Building control is needed either way, and it typically costs £300 – £1,100. The spread is wide because a simple building notice sits at the bottom, while a full plans application with inspections, or a package that includes drawings, sits at the top.

Garage conversion quotes leave out more than they show. VAT is the first thing to check, since many builders quote ex VAT and a small firm that is not VAT-registered will not charge it at all. The building control fee is often missing. So are flooring and decoration. If the garage wall is shared with next door, a party wall agreement adds about £700 – £1,500 per neighbour, and it rarely appears on a builder's quote. The fees on this page are England's; Wales, Scotland and Northern Ireland run their own planning and building regulations systems.

A worked example

Take a single integral garage of the usual 15-18m², turned into a home office in the Midlands, with no planning needed. At the habitable-room rate of £1,000 – £1,400 per m², the build cost lands in the range the calculator shows above the breakdown, and the building control fee of £300 – £1,100 goes on top. Change the room use to include an en-suite and the rate moves up to £1,400 – £2,000 per m². Move the same office to London and the whole build rises by about a third.

Hold a contingency back as well. Garage floors are often lower than the house and uneven, and raising and tanking one can cost more than planned once the old slab is exposed. Enter your own garage type, size, room use and region in the calculator for a figure tuned to your project, then add VAT if your builder charges it.