Garage conversions

If you need more space and you do not want to move house you may wish to consider converting your garage. It is important that your conversion project is carefully planned. This guide is not a substitute for professional advice but has been written to provide you with useful information about how the building regulations will affect your conversion.


If you have a brick or block garage attached to your house it is probably suitable for converting. When you are thinking about whether your garage is suitable for conversion you might like to consider:

  • Whether there are any known problems with your garage, are there any cracks in it? Is it damp? Does the roof leak? Has the floor been contaminated with fuel or oil?
  • How will you get into the new room? Have you got or can you put a doorway through to the garage from the house?
  • Will you have enough parking and storage area if you convert your garage?
  • Is there enough room in your garage to provide the accommodation that you require or would you be better extending the property?
  • Is your existing garage built from an unusual construction? For example, prefabricated panels, concrete frame etc.

If you can resolve all of these issues then your garage may well be suitable for conversion.

Technical issues - how to proceed

Garage conversions can be complex projects and unless you are experienced in construction you will need to get some professional advice.

Infilling the garage door opening

This tends to be the most visible part of your conversion from the outside and whatever you choose to infill the opening it will need some support. Some garages have a foundation that runs across the garage opening which you can use to support your infill. Unfortunately the only real way to tell if the front of your garage has an existing foundation is to dig a hole and find out. If there is no foundation under your garage door opening there are two main options.

  1. Dig a foundation 1m deep or to the same depth as the foundations of the existing garage, call us to inspect the foundation and then fill it with concrete.
  2. If the opening is only the width of a single garage door install two 150mm deep concrete lintels across the opening supported by the existing foundations. Your Building Control Surveyor will be happy to provide you with more advice about which is the best option for you when we get to site. With the foundations in place the garage door opening can now be filled in. There are several options for how this can be done. The opening can be filled in with brickwork to match the house and a window. People generally narrow the garage door opening slightly as a full width window can appear out of proportion. To keep the damp out and to provide insulation it is best to use a cavity wall and your new window should be double-glazed. Other options include installing a lightweight timber framed panel with a weatherproof external surface and insulation, this can be quicker and cheaper and has the advantage that it is easier to remove if you, or any future owners of the house, ever wanted to reinstate your garage. Whichever option you choose it is important that the infill panel provides adequate weather resistance and insulation and that all of the new work is tied into the existing construction. Your Building Control Surveyor will be happy to provide you with advice about this.

Raising the floor level

Garage floors are generally lower than the floor in the main house and they often slope towards the garage door. For these reasons garage floors are generally raised as part of a conversion. There are two main ways of raising a garage floor. Whichever way you choose it is important to consider insulation and damp proofing.

Option 1 – Concrete: using this method a polythene membrane is placed over the garage floor, floor insulation is laid down, a second polythene membrane is installed and the floor level is brought up to the same level as the house using concrete or sand and cement screed.

Option 2 – Timber: this method involves placing treated timber floor joists onto a damp proof membrane placed over the existing concrete floor, placing floor

insulation between the joists and covering the floor joists with floor boards or tongued and grooved chipboard. The floor can then be finished with carpet, laminate or any other decorative surface.

Lining the external walls

The walls used to construct garages are not normally up to habitable standards, they are sometimes formed from a single thickness of brickwork and even when built from cavity masonry they are often uninsulated. When converting your garage the walls will need to be upgraded. There are three main areas of concern when upgrading walls in a garage conversion: weather and damp resistance, insulation and sound resistance. The upgrading scheme that you choose will be influenced by the original construction of your garage walls, these can be broadly divided into two categories.

Cavity walls: if your garage is built from cavity walling, weather resistance and damp proofing are unlikely to be a problem. These walls generally have damp proof courses and providing that your wall is in good condition and is not showing signs of water ingress or rising damp, the wall will simply require insulating and a plaster finish ready for your decoration. There are two options for insulating the wall: either the cavity can be injected with cavity wall insulation or an insulated lining board can be fixed to the inner face of the wall prior to plaster boarding or plastering. Various boards are available and your Building Control Surveyor will be happy to provide advice as to which boards are suitable for your project.

Solid brick walls: in garages these are generally only a single brick approximately 100mm thick and they often have intermediate piers that buttress the walls and give them additional strength. A single brick wall will not provide adequate weather resistance to a habitable room and a supplementary wall will need to be provided behind the original wall. This can be done either by building an additional skin of masonry to form a cavity wall, the cavity can be insulated as the wall is built, and the wall can then be dry lined or plastered. Alternatively an independent timber framed wall can be constructed with a cavity between the new framing and the existing wall. The frame should be constructed from treated timber and insulation should be provided between the timber studs. Once the frame is in place an insulated plasterboard finish can be applied ready for decoration. Occasionally garages are built with 225mm thick solid brick walls. If they are in good condition and have a damp proof course they will normally provide adequate weather resistance but they will need to be lined with an insulating board to improve their insulation. 

Party walls

If any of the walls of your garage are shared with a neighbour it is considered to be a Party Wall. These walls will need to be upgraded to reduce sound transfer between your new room and your neighbour’s property. Your Building Control Surveyor will be happy to provide you with advice as to how you can upgrade any Party Walls.

The ceiling

Unless your existing garage has an adequate ceiling you will need to provide one as part of your conversion. Plasterboard is the most common material used for ceilings as it offers good fire resistance and flame spread properties. Other materials can be used but they will generally need to be treated to improve their fire performance. If the garage is open to a roof you will need to provide insulation above the ceiling and the roof void will generally need to be ventilated above the insulation to reduce the risk of problems with condensation. In a pitched roof fibreglass insulation will normally suffice but with flat roofs, where space is confined, high performance insulation boards are often required. Your Building Control Surveyor will be happy to discuss this with you.


To maximise the usability of the room you will probably want to install heating; in most instances the most effective way of doing this is to extend the existing central heating system. You will need to check with your plumber or heating engineer to ensure that your existing boiler has sufficient capacity to serve any additional radiators and any new radiators should be fitted with thermostatic valves to control the room temperature. If it is not possible to extend the existing system, or, if you prefer an alternative method of heating, e.g. electric panel heaters, careful consideration should be given as to how these can be switched and controlled to ensure that they function efficiently.


If you are looking to include a sink, bathroom, shower room or cloakroom in your conversion it is important that you consider drainage at an early stage. Any new appliances will need to connect to your existing foul drainage system as they are not allowed to be connected into rainwater drains. When planning your layouts make sure that there are suitable routes for pipes to run to a point where they can connect to existing drains.

Windows and ventilation

Any new habitable rooms will need to be ventilated. Generally this is achieved by providing an opening window equivalent to 1/20th of the floor area of the room with a trickle vent at high level. All new windows must be fitted with highly efficient double glazed units and it is wise to make sure that they contain an opener with a clear area of at least 0.33m2 and 450mm wide which should be large enough for you to escape through in the case of fire. This is essential if the door out of your garage opens into a room other than your entrance hall. Special fire escape hinges should be fitted to this window to ensure that it can be fully opened if you ever need it. In bath or shower rooms an extract fan should be fitted and in rooms without opening windows extract fans should be fitted that are triggered by the light switch with overrun timers that allow the fan to remain on after the light is turned out.

Fire precautions

When you are investing money in your home it is a good opportunity to review the fire precautions that are available in the existing house. Mains operated smoke detection significantly improves fire safety in the home and the building regulations require that it should be installed where garages are converted to habitable rooms.


You are likely to require some electrical alterations as part of your conversion. Depending on the age and condition of your existing electrical system it is sometimes possible to extend existing circuits but sometimes new circuits and even a new distribution board will be required. It is a good idea to get advice from a competent electrician at an early stage. When appointing an electrician please ensure that they are able to issue you with BS7671 test certificates when they have completed their installation as these will be required before your building regulations Completion Certificate can be issued and you will incur additional costs if the test certificates have not been provided.