Like home extensions, a garage conversion to make a living space is a complex building project and you should seek professional advice from an architect or building contractor and check with your local council building control team before starting. Particular attention will need to be paid to the following when drawing up plans:
A garage conversion doesn’t always need planning permission as it often falls under ‘permitted development’. But it’s important to check permitted development rights still apply before starting any work, even if just for your client’s sake.
Converting a garage will always need a building regulations application. A Building Notice application where detailed plans aren’t needed will usually do, although a Full Plans application with a fully detailed specification will give you and your customer more security about what work needs to be carried out before you commence on site.
You must ensure the foundations are strong enough to carry any additional masonry loads. A new inner skin on the external wall or filling in the garage door with brick & block and a window will all add extra weight. Check the condition and suitability of the existing foundation or floor when the door infill area is excavated. If there isn’t a foundation below the existing garage door you could carry the new wall off a suitable lintel – talk to your building control surveyor.
Check the existing walls for stability and make sure there aren’t any defects. If satisfactory, then your building control surveyor is likely to consider it suitable for structural purposes. If it’s single skin with piers and the piers are being removed, the wall will be weakened and should be tied to a new inner leaf using remedial wall ties.
If you’re dealing with a wall of single leaf construction, you must treat to make it weatherproof. Treatment options include tanking the wall using a vapour permeable membrane linked to a damp proof course or membrane at floor level, or providing a lightweight blockwork inner leaf with insulation in the new cavity. In all wall types, care must be taken to ensure the floor membrane laps with the DPC in the existing walls.
You must insulate the walls, roof and floor to habitable standards, your building control surveyor or designer will help you choose the right insulation. Remember that you may need to add or increase ventilation to the roof voids. Where lining the garage with an independent stud partition, the insulation should be fitted tightly between the studs – using insulated plasterboard is best to avoid cold bridging.
Windows must have openable vents of an area equal to 1/20th of the floor area of the room. You also need 5,000mm2 of trickle vents to provide background ventilation. And if the new room can only be accessed via another room, a window with a clear opening of 450 x 733mm is a must for means of escape.
Any party wall between the garage and an adjoining property will need to be insulated to stop sound transmission. Check the existing wall construction – anything less than 200mm of dense block-work will usually need further work, which may include additional masonry or specialist independent acoustic partitions – your building control surveyor will be able to advise you further.
The requirements of Part P – Electrical safety in dwellings will apply to the new conversion, so ensure any installations are compliant and certified to BS 7671.
Your building control surveyor should be able to provide practical guidance on site.