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

Considering a garage conversion? If it's a possibility for you, garage conversions can be an easy and affordable route to transforming an underused space into a new, multi-functional zone – Chris Batesmith takes a look at how you can complete a successful garage conversion that’ll add value to your home
Chris Bates, Editor of Build It magazine
by Chris Batesmith
27th March 2024

Garage conversions are one of the speediest and most affordable routes to adding flexible floor space to your existing home. The process will significantly enhance your home’s value and a garage conversion will offer a versatile new space that can be used as a home office, bedroom or home gyms or even a small guest room complete with bathroom and kitchenette.

The ways that garage conversions can be used are endless, but ensuring you’re creating a successful design from the beginning will be key to the overall look, comfortability and longevity of the newly converted space. This will require you to address any insulation upgrades and fit proper ventilation to make sure the new zone can be enjoyed and used all year round. You may also need to incorporate new electrical outlets, windows or accessibility features as part of the redesign.

Before setting out on a garage conversion project, you should consult your local planning authority. While most garage conversions won’t require formal planning permission – as the work will mostly affect the interior spaces – your home’s location will influence the changes you can make. And, as a conversion involves a change of use from garage to residential space, you’ll need to ensure your project is meeting Building Regulations.

If you’re looking to alter the garage’s exterior features such as the windows, you’ll need to submit a planning application as this will end up changing how your home looks. You could always consider a pre-planning application to ensure your garage conversion project is on the right track for success.

Drawing up a Design for Your Garage Conversion

The first place to start with all garage conversions is conducting an assessment of the existing structure, in particular the soundness of the foundations, walls and roof.

This will go a long way to revealing the extent of works required to create a comfortable living environment – so it’s a key part of the early design stages that’ll dictate what you’re able to do with the design later on. If the garage you’re converting is in an especially dilapidated state, it may be cheaper to knock down and replace the structure with a newly-built, efficient version. By doing this you’ll have the opportunity to specify features such as sustainable insulation or a proper ventilation system.

Garage conversions require careful designing to find the right result. The project’s details will depend on the scale of the scheme, how you want to use it and integrate it into the property, and what your budget will stretch to. Speak to a design professional as they’ll be able to help you decide on a project route that best maximises your cash before you can begin designing a high-spec scheme.

For higher-end garage conversions, working with an architect could help to identify creative ways to maximise the potential of your garage conversion and establish a space that flows naturally into your home. Consider how you’ll want to use the space and how frequently, too, as this should help you decide how much of an investment your garage conversion will be.

Another popular route is to use a design and build company that specialises in converting garages. The results can be fantastic and many will take your scheme through planning and building control as part of their fee. Plus, their experience on-the-ground can help to ensure a smooth project and a predictable budget that you can see through to the end.

Garage Conversions: The Ultimate Guide to Design, Planning & Costs for a Garage Conversion

This residential garage conversion development project by MW Architects involved converting a set of old coach houses, with the garage on the lower storey transformed into comfortable, liveable homes. Photo: Taran Wilkhu

If your garage is integrated or attached, the space should be fairly easy to work into your home’s main structure and accommodation. You could knock through the wall to join up with an existing zone, for instance – perhaps enlarging a hallway or creating a front-to-back kitchen-diner.

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

MW Architects‘ coach house garage conversion features a light-filled interior, with the garage-looking front retained on both inside and out to bring the initial structure’s character into the modern day. Photo: Taran Wilkhu

A detached garage, meanwhile, lends itself to segregated uses, such as an annex or quiet home office. A single garage will offer around 15m2 of floor space; more than enough for a new living space, separate playroom, guest bedroom, or even an accessible downstairs shower and WC.

At around 30m², double garage conversions will give you more flexibility. It could house a bigger living room, ensuite bedroom, well-sized kitchen-diner or an annex. Alternatively, you could retain a single parking space inside your garage by erecting a suitably insulated and fire-rated partition, and then fitting out the rest of the room for habitation.

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Do Garage Conversions Require Planning Permission?

With many garage conversions – particularly when dealing with integral garages – most of the conversion 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.

Garage Conversions: The Ultimate Guide to Design, Planning & Costs for a Garage Conversion

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

Straightforward garage conversions like this are often considered permitted development (PD), so won’t usually need formal planning consent.

In some cases, 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.

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

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

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.

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 application will also be necessary if you want to significantly alter the 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 My Garage Conversion Need 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.

CASE STUDY London home upgraded with a garage conversion

After buying a rundown Victorian terrace house, Alice Constable Maxwell and George Walker have undertaken a full renovation that doesn’t scrimp on style. As part of the project, the couple decided to undertake a garage conversion to the attached space, maximising the floorplan.

Measuring just over 19m², the home’s attached garage was a dark, damp and redundant space ripe for repurposing. “We found out the property was once the milkman’s house and the garage was where he stored his pony,” says Alice. The couple decided to transform it into useful, functional living space – a task that could be completed under permitted development rights.

As well as providing the family with extra living space, converting the garage was a great way of boosting the value of the property. In fact, it’s thought that a garage conversion can increase the value of a home by as much as 20%. Undertaking such a scheme is highly cost-effective, too, since the shell is already in place.

For the transformation, YPP Services laid a concrete subfloor, with a waterproofing membrane beneath. Extra insulation wasn’t needed and the old, side-hinged garage doors were replaced with a modern, sliding and folding model that can be opened to allow bicycles to be carried in and out with ease. A built-in front door – which the couple utilises as their main entry point – makes for convenient access.

see the home

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 Slab for Garage Conversions

When it comes to beginning a garage conversion, 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.

inside garage conversions

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 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.

Insulating Garage Walls for a Comfortable Garage Conversion

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.

Thermafleece is a natural British sheepswool insulation, with products available for use in floors, walls and roofs

Roof Insulation for Garage Conversions

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 Windows & Doors to a Converted Garage 

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 garage conversion costs 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.

matching timber front and garage doors from Urban Front

If you’re extending over your garage to create another storey above, you might consider upgrading the garage’s exterior to help it blend with your home’s exterior. This garage has been modernised with Urban Front‘s up-and-over automated garage door in their Rondo design, made in Iroko wood to match the front entrance

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.

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

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.

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