Garage conversions: the ultimate guide to costing, planning and designing a garage conversion

Garage conversions are the perfect cost-effective solution for anyone who wants more space but doesn't want to extend. Find out how to successfully convert a garage into extra living space

Garage conversions: by OB Architecture
(Image credit: Martin Gardner)

Before you begin planning a garage conversion, first consult an estate agent about whether converting your garage could have a negative impact on your property's price. If you'll be living in your home for years to come, it may well be worth proceeding whatever they say, but if garages are highly sought after in your area, think carefully before converting yours.

Before you start work on the conversion, consider what you’ll do with everything currently kept in the garage. If you store a lot of garden gear in it, you may need to factor a shed into your plans; if it’s where you keep your chest freezer, you’ll need to work out where it can be moved to – or whether you can live without it. 

If you really have no room anywhere else for these things, perhaps a partial garage conversion (or a conversion that has lots of storage space set aside within it) will work better for you.

Do you need planning permission for a garage conversion?

In the majority of cases, the work involved in converting a garage will be classed as permitted development

However, if you live in a Conservation Area, a location where development can be restricted to protect the appearance of the surroundings, or on a new estate with strict guidelines (some new homes are built with a condition that the garage remains as parking, so you'd need to apply to change its use), it’s worth checking what’s allowed. 

Detached garages are much more likely to require planning permission for a change of use.

Use our guide to planning permission for more details.

Do you need insurance to convert a garage?

Yes: if you are project managing the garage conversion yourself, it is highly sensible to arrange conversion insurance. If you fail to insure yourself properly, and there is any loss or damage caused during the works, it's unlikely that your home insurer will cover you will your home is undergoing alteration or renovation.

Conversion insurance will cover the work being carried out and the existing structure: in other words, your house. It should also cover materials, plant tools and equipment (stolen or damaged during works, these can cost thousands to replace). 

This type of insurance should also cover public liability and employer's liability – should any of the workers be injured on site, for example – do check as this is an important element needed to protect yourself. 

Ensure that the conversion insurance is ready to go from the moment work starts until the very end of the job when the new room is in use. 

Architect or garage conversion specialist?

If you're planning a garage conversion, using an architectural designer or a specialist garage design and build contractor, will help put your ideas into fully formed plans, and give you much needed expert design input. If the garage is attached or integral to your home, they will also help you consider the best position for access, and put plans in place to redirect this if required. 

To find an architect, visit architecture.com or use an architectural technologist (see ciat.org.uk). The starting point for your design is to draw up a floorplan. Expect to pay from as little as £1,200 right up to £3,000 for an architect's services, depending on the complexity of the design.

Specialist garage conversion companies can save you time in putting together applications and will be experienced in getting the best from this type of project, but any good builder will be able to do the job. 

Ideally, work with someone who has been recommended and is a member of an accredited body, such as The Federation of Master Builders. Find reliable contractors with the help of our guide.

Planning a garage conversion: can you DIY convert a garage?

Planning a garage conversion on a budget? Some people choose to come up with a design themselves and carry out all the work they can on a DIY basis – a good option for those with limited funds and the spare time to get stuck in.

You can do most of the garage conversion yourself, which is ideal if you’re on a tight budget, but your work will be regularly inspected by building control officers and anything that’s not up to scratch will have to be undone at your own expense. Get an idea of what's involved in project managing an extension with the help of our guide.

Get written quotes (not estimates) from three different companies, with a clear specification of what’s included. Using an architect can give a more creative result.

spare bedroom in a garage conversion

Converting an unused garage in a high value area such as London and adding extra living space can be a sure way of adding significant value

Garage conversion structural essentials

When planning a garage conversion, there are some important structural considerations that you need to tackle early on.

Replacing the garage door

The garage door will have to be replaced with a new wall and/or window. This may mean new foundations, but there are other options, including having lintels set just below ground level that bear on sound masonry or existing foundations each end. Expect to pay, including a small window, around £1,300 for this.

If you live in a conservation area you might find that your local council want you to retain the original door. There are ways to do this, including building a false interior wall behind the doors which, ideally, will have glazed sections. This isn't a bad place to site a kitchen sink, for example (see below for inspiration).

Garage converted into a home office seen from the exterior by Bliss Interiors

(Image credit: Bliss Interiors)

Upgrading the garage roof

The garage roof will almost certainly need to be upgraded to ensure it is water-tight. Use new tiles and materials that match those of the main roof. If you can swap a flat roof for a pitched one it will make the conversion less obvious from outside, and the new raised pitch may have space for rooflights. Doing much more than general repairs to the roof will have to be approved by your local building control department.

Internal wall work in a garage conversion

An internal structure of stud walls built inside your garage will have insulation added to increase energy efficiency. Expect to pay around £750 per stud wall.

Upgrading external walls in a garage conversion

External garage walls will usually be single course brickwork, which won’t meet building regulations for moisture and insulation. This can be solved by adding an interior, insulated stud wall built off a damp-proof course on a couple of courses of bricks. ‘Set 75mm clear of the original wall, this stud wall can easily accommodate services and insulation prior to plasterboarding,’ says Designer Nigel Lewis of Space & Style Home Design.

Ventilating, heating and insulating a garage conversion

Ventilation: Shower rooms and kitchens must meet building regulation requirements with extractor fans, but you might also like to include an extractor if you’re planning on using the room as a home gym. Otherwise, an opening window and background ventilation in the form of air bricks or trickle vents will be needed.

Insulation: Building regulations require walls, windows, the floor and roof to be insulated to a certain level. It’s important to get this right — poorly done, it will result in a cold, damp room. 

With pitched roofs, insulation may be two layers of 150mm glass fibre quilt, one between the joists, another over; flat roofs tend to need one layer between the joists of rigid PUR insulation board and another below, with a 50mm air gap above for ventilation. The second layer underneath will drop the ceiling height, however. 

Dry-lining the walls and fitting sealed double-glazed windows are also probable requirements. New windows won’t usually need planning permission but will need to be energy-efficient and double-glazed to be approved by building control. They may also need to be able to be opened right up, too, to comply with fire safety regulations (see above). Match the style to the house’s existing windows to help the conversion fit in.

If floor height is an issue (particularly if you've had to lower the ceiling to insulate it, fitting multi-foil laminate insulants rather than using polystyrene floor insulation can help reduce the thickness of the under layer. 

Heating: If your boiler won’t cope with another radiator, or it’s not practical to run the new pipework, consider electric underfloor heating, which is relatively easy to install.

Bear in mind that bedrooms, home offices and playrooms will need more insulation and better heating than a utility room, while a home gym or shower room will need better ventilation than a living room.

Electrics and plumbing in a garage conversion

Plumbing: If you're planning on installing a toilet or shower, double check the location of the existing water and soil pipes – doing so may help you keep garage conversion costs down, if you can put your fittings in a convenient place. 

Bear in mind the extra costs involved in installing a radiator – and the extra strain put on your boiler. Factor in between £1,000 and £3,000 for gas and plumbing works.

Electrics: Ask your electrician or builder if rewiring the garage for lights and electric radiators will mean that your mains (fused at 100 amps) will be put under strain, too. They may suggest installing an additional mains supply and separate consumer unit, which can add significantly to garage conversion costs. 

Another option is to put the garage on the existing consumer unit, perhaps with its own miniature circuit breaker. Consider adding a minimum of one new 20-amp circuit, too.

If yours is a detached garage conversion, wiring can be run through an underground conduit, although a new connection may be needed if it is to be a separate dwelling, an annexe, perhaps. A good electrician will always be able to advise.

Expect to pay around £100 to have a new electrical socket fitted; this price will be negotiable the more electrical work you are having done.

Kitchen diner in a garage conversion

This garage conversion is now a spacious kitchen diner

(Image credit: Clear Architects)

Fire safety for integral garage conversions

What if you have an integral garage but without an internal door to it from the house. Can you fit one if you're not converting it?

The answer is yes, but two fire safety requirements apply:

The door needs to be an FD30S fire door with a self-closing device fitted. Thirty-minute fire-resisting doors are best bought as a set with the frame because the latter is made from thicker timber than standard and is rebated to fit the necessary intumescent (which means it swells when heated) smoke seal to the sides and the head. Hence, you can’t use normal door frames for this purpose.

Internal doors to garages are the only doors in homes that need to be self-closing. Those with a checking action that pauses in the almost-shut position before fully closing are best, avoiding crushed fingers. The floor within the garage must be at least 10cm lower than the finished floor within the house to prevent fuel spillage running into the house. If it isn’t 10cm lower, the door sill will need to be raised by brick or concrete block to create this 10cm step over, or the floor of the garage should be sloping outwards to beneath the garage car doors.

Garage conversion build schedule

  • Check structure is suitable for conversion
  • Produce design drawings
  • Confirm whether planning permission is required
  • Apply if consent is required
  • Produce detailed design and building regulations drawings
  • Arrange conversion insurance to cover the new works and the existing structure; most home insurers will exclude loss or damage while the property is undergoing alteration or renovation.
  • Notify local authority Building Control of commencement
  • Strip garage back to retained structure
  • Structural alterations, including knocking through to house
  • Damp-proof new floor, if necessary
  • Insulate walls and new floor, plus the roof if single storey
  • Pour new floor, if required
  • Install new window and doors
  • First fix joinery, wiring and plumbing
  • Plaster and dry-line
  • Fit new doors, skirting, architrave, fixed floor finishes and light fittings
  • Decorate

Designing a garage conversion >>