Garage conversion cost: what decides the price

The short answer

Garage conversion cost depends mainly on what state the garage is in and what the room will be used for. A sound, attached single garage turned into a simple study or playroom needs less work than a detached garage with a weak floor, a leaking flat roof and plans for a shower room. The building work follows the same pattern every time: the floor, the walls, the roof, the door opening, insulation, windows, services and finishes. Our cost calculator gives a starting range per square metre, and we give a free estimate after a site visit.

Single or double garage

A double garage conversion costs more in total because there is more floor, wall and roof to insulate and finish, but it is often cheaper per square metre. Fixed items, such as filling the door opening, the Building Regulations application and the connection into the house, are spread over a larger room. Some people convert half of a double garage and keep the other half for storage, which splits the cost and keeps somewhere for bikes and tools.

The floor

Garage floors are usually lower than the house and have no insulation or damp-proof membrane. Most conversions need the floor raised to meet the house level, with a membrane and insulation laid over the existing slab and a new screed or timber floor on top. If the slab is cracked, sloping a long way or sitting on poor ground, it may need breaking out and relaying, which adds both labour and materials.

Walls and the door opening

Garage walls are often a single skin of brick or block, which does not meet modern insulation standards on its own. They are usually lined on the inside with insulation and plasterboard, or built up with an inner leaf of blockwork. The old garage door opening is filled with brickwork on a new footing, normally with a window, and the brick needs matching to the house so the front elevation looks finished. That brickwork is one of the most visible parts of the job.

The roof

A pitched garage roof sharing the main house roof is usually in good order and just needs insulation. Flat roofs on garages are often at the end of their life and may need replacing, which is a good time to add insulation above the deck and a rooflight. Turning a flat garage roof into a pitched one adds height and looks better, but it is a bigger job.

Building Regulations and planning

Almost every garage conversion needs Building Regulations approval, covering the foundations under the new wall, insulation, ventilation, fire safety and escape. The application fee is part of the budget. Many conversions do not need planning permission, but some estates have conditions requiring the garage to be kept for parking, and conservation areas and Article 4 directions change the rules, so check first.

Services and finishes

Electrics, heating and lighting are needed in every conversion. A shower room or utility adds plumbing and drainage, and the cost depends on how far the room is from existing drains. Finishes such as flooring, doors, skirting and decoration are where the price varies most from one job to another, because they depend entirely on what you choose.

Cost per m2 and what makes it misleading

Garage conversion cost per m2 is a useful way to compare quotes, but small rooms always look expensive per square metre because the fixed costs are spread over less floor. A detached garage that needs a new floor and roof can cost far more per square metre than an attached garage that only needs lining. Use the figure as a starting point, not a final number.

About us

D & A Brickwork & Building is a family-run firm based in Bidwell, Dunstable, with more than 26 years of building experience. Garage conversions are part of our conversions work, and our case studies include a garage extension for a separate music-practice room away from the house.

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