Garage Conversions – The Ultimate Guide to Design, Planning & Costs

Do You Need Planning Permission for a Garage Conversion & How Much Does this Cost?

With many garage conversions – particularly when dealing with integral garages – most of the garage renovation work will be internal. The main external changes are like to be upgrading the exterior cladding and adding a window or two – think about which materials may complement your home best, such as slim-profile brick slips or timber cladding for a design statement.

The large double doors on this London garage conversion help to maintain a n authentic garage-style look from the street

A straightforward garage renovation like this is often considered permitted development (PD), so won’t usually need formal planning consent. In some cases, though, such as in conservation areas, PD rights for a change of use from garage to residential may have been removed. So, consult the local authority before you begin. For peace of mind, you could apply for a lawful development certificate.

In addition, some houses built in more recent times are subject to restrictive covenants, which may require the garage to be retained as parking rather than converting it completely.

Before you can go ahead with a garage conversion, any such covenant would need to be discharged (usually for a small fee). Check the title deeds to find out if this applies. In this scenario, you’re also more likely to need formal permission to change the use of a detached garage.

Part of a London mews house, this garage conversion has been opened up to the rest of the ground floor – providing space for a stunning kitchen by Cue & Co leading through to a family room and courtyard garden

You may still be able to convert a garage into a room even if PD rights have been removed – but you’ll need to put together suitable drawings and apply for householder planning consent. As of December 2023, householder planning applications cost £258, plus any design fees.

A full planning application will also be necessary if you want to significantly alter the garage’s external appearance, such as making big changes to the windows, using new materials or adding an extension. Among the other permissions you might need to secure are listed building consent (if you live in a listed building) and party wall agreements with any adjoining neighbours. Full planning applications cost £578.

Does a Garage Conversion Require a Building Regulations Approval?

As this type of project involves a change of use, a garage conversion will always be subject to the Building Regulations.

For straightforward garage conversions, the building notice route may be sufficient. This is where you or the contractor simply informs the local authority of your intent to start work 48 hours prior to commencing on site. With a building notice, you don’t have to submit structural drawings for scrutiny.

With more complex garage conversions, you may prefer to have full structural plans drafted. This gives you peace of mind that building control has inspected the drawings and confirmed that – if it’s constructed as per the approved schematics – your conversion will conform to the Regs.

In addition to structural safety, key areas your building control officer or approved inspector will look at are damp proofing, ventilation, insulation and energy efficiency, fire safety (including escape routes), electrics and plumbing. These are all essential for a good-quality garage conversion that will be safe and feel comfortable.

What Are the Key Building Stages of a Garage Conversion Project? 

Before a garage conversion project can start in earnest, the walls and roof must be made sound and watertight. Thereafter, most of the work will take place inside the existing garage.

The first job will be stripping out the main structure, at which point you’ll get the clearest view yet of what’s in store – including where you’re most likely to encounter unexpected problems (such as patchy foundations or hidden issues in the walls) that could add to costs.

Here are the key considerations for a garage conversion

Foundations & floor slabs

If you want to convert a garage to living space, a key first step is to investigate whether the existing garage’s foundations need upgrading to take the new loads post-conversion. This will be particularly important if you’re adding new doors or windows as part of the garage conversion.

This is usually done by digging trial pits. If the foundations are at least 200mm deep, they should be able to cope with the loads of a converted garage. If not, or your structural engineer says otherwise, they may need additional support (for instance by underpinning).

An existing concrete garage floor might well be strong enough to cope with general domestic use. However, it may need to be levelled (consider a self-levelling liquid screed), damp-proofed with a suitable membrane (lapping into the walls’ DPC) and insulated to achieve adequate thermal performance.

Garage floors tend to be lower than those in the main house, so it may be possible to incorporate all of this and still achieve a step-free threshold between the two zones.

Here is a garage conversion project getting underway, with alterations made to the original structure as a the old space is converted into a liveable zone

Infilling or replacing the garage door

The most common route is to replace the main garage door with conventional walling matching the rest of the building. In most cases this would be a masonry infill, which should be fully toothed and bonded into the existing brickwork.

Planning allowing, you could add windows or a glazed doors and introduce more daylight into your new space, which may also help reduce the loads imposed. If your budget will stretch, you could even have a little fun with the design – perhaps by going for a fully glazed wall.

Some choose to keep the garage’s original doors to retain the structure’s character. Just check these are in suitable condition.

Vine Architecture Studio has restored the charm of this unloved late Victorian coach house for a young family in Camberwell, London. The building had already been converted into a residential dwelling many years ago, but remained untouched and still featured the original stable doors and windows. The studio combined these defining details with a bright, airy open-plan renovation to maximise the floorplan and elevate the space for modern living. Photo: Nicolas Worley

Insulating garage walls

Integrated garages are often built to the same standard as the main house, so the walls may already be insulated. If not, attached or detached garages of single-skin construction can be insulated internally. This is usually done by erecting studwork on the inside of the garage walls, using timbers deep enough to accept sufficient insulation (plus an air gap), and then plasterboarding over the top.

Insulated plasterboard may also be an option for improving the performance of single-skin garages. Garages built with cavity walls can have insulation blown into the gap, thus preserving the internal floor space.

If you’re going for a partial garage conversion to retain a parking space, you’ll need to erect a fully-insulated internal dividing wall that is designed to provide 30-minute fire protection. This can be done in blockwork, or switch to timber studwork lined with pink fireline plasterboard on the garage side.

Winner of the 2023 Build It Award for Best Insulation, Isolair Multi is a wood fibre insulation board from Pavatex, available from Unity Lime. It is supplied in an easy-to-install tongue-and-groove profile and can be used in retrofit applications, without the need for additional breather membranes

Roof insulation

The simplest way to insulate a garage roof is at loft level. With a pitched covering, 270mm of mineral wool should be sufficient – 100mm between the joists, and the rest beneath them.

Warm roof setups, which are insulated at rafter level, are also possible and can enable the use of rooflights to bring in more natural brightness.

Garage conversions with flat roofs will need to be fitted with rigid insulation between and under the ceiling joists, with a ventilation gap above to prevent condensation. If you want to preserve floor-to-ceiling height, go for slim multifoil or PIR (polyisocyanurate) products.

Adding new doors & windows when converting a garage to living space

The windows and doors you choose for your conversion will of course need to suit the style of your home or garage. For instance, a timber stable door might look great on a detached oak-style structure.

In terms of performance, Building Regulations set a minimum U-value of 1.4 W/m2K for new external doors and windows in existing dwellings. Your garage conversion’s fenestration will also need to need to match your security expectations, provide adequate ventilation (you’ll probably need trickle vents for air quality management) and suit the style of your home.

If you’re keen to keep your the garage conversion cost down, aim to work to standard-sized units for both any new or replacement garage doors and windows. Casements, sashes and doorsets can be incorporated by punching a suitable hole in the wall and adding lintels as required. The same is true of openings between the main house and garage.

For larger spans enabling a more open-plan feel, a reinforcing steel beam may be needed. This kind of work may require calculations from a structural engineer. If you’re creating a habitable room that doesn’t offer a direct protected route to an external door, or its own door leading to outside, then you’ll need to provide an escape window.

This must have a width and height of no less than 450mm, a clear openable area of at least 0.33m² and should be sited so the bottom of the openable area is no more than 1,100mm from finished floor level.

Heating & electrics for a garage conversion

The most straightforward way to get this infrastructure in place is to engage professionals qualified to self-certify their work under Part P of the Building Regulations.

You’ll almost certainly need new electrical circuits and heating loops when converting a garage. This will put additional loads on your consumer unit and boiler. If these systems need upgrading, this could easily add upwards of £2,000 to your overall project budget.

Efficient LED downlights are a good choice for lighting in garages, as they can be easily integrated into the new ceiling structure. Heating-wise, plumbing in a suitably-sized radiator will be the cheapest solution.

This garage has been converted a stylish, retro-inspired kitchen with family breakfast area. Photo: David Giles

Slimline underfloor heating is a sleek alternative that can maximise the floorplan and free up wall space. This could be a retrofit central heating UFH system, or electric UFH matting. If you’re planning a kitchen or bathroom in your garage conversion, you’ll need to account for hot and cold water supplies as well as drainage.

Ventilation is another key issue for garage conversions. Openable windows, fitted with trickle vents, will be sufficient in most cases – but if you’re incorporating a bathroom or kitchen, you’ll need a powerful enough extractor fan to manage moisture build-up.

How Much Does a Garage Conversion Cost?

How much does a garage conversion cost? Your budget will depend on a few different factors such as the condition of the structure, any replacement windows and insulation upgrades.

Provided the structure is in reasonable condition, converting a garage to a habitable room should be more cost-effective than adding an extension or carrying out a loft conversion. A 15m² integrated garage in good condition could be converted for as little as £8,000 (£533 per m²), ready for your choice of internal fit-out.

It’ll cost significantly more to convert a detached garage, partly because it’s trickier to bring in services – expect to pay from around £20,000 (£1,333 per m²) to renovate a standalone 15m² single garage. Even this should prove cheaper than most single-storey extensions, which will typically start from around £1,900 per m².

Ultimately, the cost of a garage conversion will depend on the kind of space you’re creating and the quality of finish you want to achieve. Kitting your conversion out with a kitchen, bathroom or utility could add another £2,000-£3,000 in plumbing and electrical work – plus whatever you choose to spend on furniture and fittings.

These following bullet points are key to note when calculating costs:

• Planning applications will add to your budget – scroll up to find out how much it costs to make a planning application.
• Garage conversion design fees – for example, are you hiring an architect to design the conversion for you, or will you be going down the design and build route?
• You’ll need to budget for the services of a structural engineer when undertaking a garage conversion project.
• Whether the garage’s existing foundations need reinforcing.
• If the garage’s existing structure is in poor condition and needs reinforcing.
• If the garage conversion involves raising the ceiling height. You’ll need between 2.2-2.4 metres of headroom inside the garage conversion to comply with UK Building Regulations.