What a garage conversion costs, and what drives the price

National averages for garage conversions disagree with each other, and they are not much use anyway: two garages of the same size can need very different amounts of work, mostly in things you cannot see from the drive.

We do not publish prices, for the same reason. What is useful is knowing where the money goes, so you can look at your own garage and work out whether it is a simple job or not.

Single, double or integral

An integral garage (inside the footprint of the house, with a door to the hall) is usually the most straightforward. It already shares walls with heated rooms, it often has a room above it, and the new room joins the house where a door already exists.

An attached garage may share only one wall with the house and have no internal door. Forming an opening through a house wall means a lintel or a steel, and possibly a structural engineer.

A double garage is more floor area, so more insulation, more plasterboard, more floor. It may also have a wide door opening or a central pier. If the plan is to remove a pier or change how the roof is carried, that is a beam, an engineer’s design and padstones, a separate cost in its own right.

The floor

This is often the biggest variable inside the garage.

Garage floors usually sit below the house floor, and often slope towards the door. They rarely have a damp-proof membrane or any insulation. The new floor has to include both, and should ideally finish level with the hall, so there is no step into the room.

How much that costs depends on the height difference. A garage that sits well below the house leaves room for a full insulated floor build-up. One that sits only slightly lower leaves little depth for insulation, which can push you towards thinner, more expensive insulation boards, or a compromise on level. Any damp already coming through the slab or the walls has to be dealt with first.

The door infill and its foundation

The new wall where the garage door was needs a foundation, because the existing slab was not designed to carry a wall. Usually this is a short length of trench and concrete. It costs more where the soil is clay and there are trees nearby, where a drain runs close by, or where the ground turns out to be made-up fill: the same ground risks as any extension, on a smaller scale.

What goes in the infill matters too: a window, a window and a door, or a wall built to match the house brickwork. Matching an older brick, or a rendered finish, takes more care than plain new work.

Insulation and windows

The existing walls are often a single skin of brick or block and need insulating and dry-lining. The roof needs insulating, unless there is already a heated room above. New windows and doors must meet current energy efficiency limits. None of this is optional (it is what building regulations require when an unheated space becomes part of the house), but the specification still varies.

Heating

The cheapest route is a radiator run from the existing system. Whether that works depends on the boiler having spare capacity and how far the pipework has to travel. If it does not, the alternatives (electric heating, or underfloor heating) change both the build cost and the running cost.

An en-suite or utility room

Plumbing is where a garage conversion moves from modest to substantial. A bathroom or utility needs water supply, a waste connection and mechanical extract ventilation. The question that decides the cost is where the nearest drain is, and whether waste can fall to it by gravity. If it cannot, a pumped waste unit is an alternative, but it adds cost and has to be maintained.

Electrics

New circuits, sockets, lighting, and often a new consumer unit if the existing one has no spare ways. A new circuit is notifiable work under Part P, so it needs either a registered electrician’s certificate or building control inspection.

Fire safety

If the new room does not open directly onto a hall leading to an exit, it generally needs an emergency escape window and mains-powered smoke alarms, which can decide what goes in the infill. The detail is in garage conversion planning and building regulations.

The fixed costs

Some costs barely move with the size of the job:

• Building control. Needed for any conversion into a room. In Horsham District, applications go to Sussex Building Control, which publishes a fee schedule with a set charge for converting a garage to habitable rooms. Check the current schedule rather than relying on a figure from elsewhere. Fees differ between councils.
• Planning. Usually only if a planning condition removes the right to convert, or the house is listed or in a conservation area. A householder application is currently £548. A lawful development certificate to confirm you do not need permission costs half the fee for a full application.
• A structural engineer, if anything structural is being altered.
• Drawings for the building regulations application.
• Skips and waste disposal.

Does it add value?

It depends where you live, and anyone who gives you a flat percentage is guessing.

A converted garage adds a room. But it also removes covered parking, and on streets where parking is tight, buyers notice. In some places a garage can be worth more as a garage than as a small extra room. And a good conversion (warm, level with the house, properly signed off) is a very different thing to sell from a garage that has been carpeted and given a radiator. A conversion with no completion certificate can actively count against the house when you sell.

If resale is the reason, ask a local estate agent before you start. If it is the space you want, that is a reason on its own.

How long it takes

Usually shorter than an extension, because the shell already exists, but it depends on the job. A straightforward single garage with no plumbing is a much shorter programme than a double with an en-suite and a beam. Building control inspects at stages, and the work cannot move past each one until it is inspected. Ask for a written programme with your quotation.

Getting a number you can rely on

It takes somebody looking at the garage. Give every builder the same information (the size, what the room is for, whether it needs a bathroom) and ask for an itemised quotation with exclusions written down. How to compare builders’ quotes fairly, and what should be in a quotation.

Send us photos of your garage and what you want it to become, and we will come back to you.