Garage Conversion Cost Calculator

Why a garage conversion is the cheapest room you can add

The structure already exists. Walls, roof and foundations are built and paid for, so a conversion is a fit-out rather than a build, which is why it costs a fraction of an extension for a comparable amount of space.

In the example below, converting a 15 m² integral garage to a standard specification comes to £17,950, or around £1,197 a square metre. A single-storey extension of the same size typically costs two to three times that, and takes considerably longer.

The trade-off is that you are not adding floor area to the property, only converting it. That matters for resale in a way the cost comparison hides, and it is the single most important thing to think about before starting.

Does it add value, or lose it?

Usually it adds value, because habitable space is worth more per square metre than storage. A converted garage that becomes a proper bedroom, office or annexe generally returns more than it costs.

But not always, and the exception is predictable. In areas where off-street parking is scarce and valuable, removing the only parking space can cost more than the room adds. The same applies to larger family homes where buyers expect a garage, and to properties where the garage is the only practical storage for bikes, tools and bins.

The other consideration is whether the conversion reads as deliberate or improvised. A converted garage with a standard window and matched brickwork looks like part of the house. One with an infilled garage door opening that is obviously an infilled garage door opening does not, and buyers price that.

Ask a local estate agent before committing. They will know whether buyers in your specific street want the room or the parking, and that answer varies street by street rather than region by region.

Planning permission and building regulations

Converting a garage within the existing footprint is normally permitted development and does not need planning permission, provided the work is internal and does not enlarge the building. Changing the external appearance, such as replacing the garage door with a window and wall, is generally also covered.

There are important exceptions. Listed buildings, conservation areas, some new-build estates and flats are treated differently, and many modern developments carry a planning condition or restrictive covenant requiring the garage to remain available for parking. That condition survives regardless of permitted development rights, so check the deeds and the original planning consent rather than assuming.

Building regulations approval is always required, without exception. A garage is not built to habitable standards, so the work has to bring it up to them: insulation in floor, walls and roof, damp proofing, ventilation, fire safety, structural support over the old opening, and electrical work certified to Part P.

That approval is what makes the room count as habitable when you sell. A conversion done without it is a liability at conveyancing, and retrospective regularisation is more expensive and more intrusive than doing it properly first time. The building control fee is a few hundred pounds; the example below includes £500.

Where the cost actually goes

The floor is usually the biggest hidden item. Garage floors are typically laid lower than the house floor and without insulation or a damp-proof membrane, so the floor commonly has to be built up, insulated and tanked. That is disruptive, and it is why the flooring and damp-proofing lines are substantial rather than incidental.

Damp is the related risk. Garage walls are often single-skin brick, and converting them means either building an insulated inner leaf or applying an insulated lining system, both of which lose internal space. A garage that has been damp is a garage that needs investigating before any quote is trusted.

Heating is usually extended from the existing system, which is straightforward if the boiler has capacity and expensive if it does not. Adding an ensuite or kitchenette pushes the cost up sharply because it brings drainage into the job, £3,000 in this calculator.

Detached garages cost considerably more than integral ones, £22,450 against £17,950 on the same 15 m² in the example, because services have to be run to a separate building and the whole envelope needs insulating rather than just the exposed faces.

Practical things worth deciding early

Where will the things in the garage go? This is the question that derails conversions, because the answer is usually a new shed, which is a cost nobody budgeted for and which uses garden space.

Keep the garage door opening structurally honest. Removing it entirely needs a lintel and proper support; infilling it needs matched brick and a window proportioned like the rest of the house. Cutting corners here is what makes a conversion look like a conversion.

Think about the room's purpose before the specification. A home office needs power, data and daylight. A bedroom needs a window that meets escape requirements and will be assessed for it. A gym needs a floor that can take the load and neighbours who cannot hear it.

Get at least three quotes and check each is for the same specification, particularly on insulation, floor build-up and whether building control fees and drawings are included. Quotes for garage conversions vary more than for almost any other job, precisely because what is included varies so much.