CMM Design Services Ltd

1 - Design and Space Planning:
At five metres long by two and a half metres wide, the internal space of most garages is longer and thinner than most rooms in a house. To achieve a more natural shape, consider using stud or block walling to convert the garage into two rooms, typically a toilet, shower or store room.
Consider also how you plan to use the rooms and have some drawings prepared.

2 - Planning Permission:
Planning permission is unnecessary if you don’t plan to alter the structure of the building.
A garage conversion is permitted in most circumstances. However:
If you live in a Listed building or a Conservation Area, planning permission may be required for even minor modifications.
Some new build homes were built with a condition requiring the garage to remain as parking, so an application to remove it becomes necessary.
Standalone garages are more likely to require ‘change of use’ planning permission when converted to habitable rooms.

3 - Building Regulations:
The change of use from a garage to a habitable room will mean compliance with Building Regulations, including delivery of a building notice to your council. Building Regulations apply to: moisture proofing, ventilation, insulation, fireproofing, escape routes and structural soundness.
As a result, almost any design decision must take them into account.
For example:
When you divide up the garage, a new room is created. This room is subject to a set of Building Regulations that require an escape route and ventilation separate from the main room.
Alterations such as an infill wall replacing the original garage door will also be subject to Building Regulations concerning the foundations.
The building inspector will want to visually inspect windows, doors, fireproofing and foundations before he or she gives a certificate of completion.
Once the building inspector is satisfied, the completion certificate should follow within 28 days. It is often much sooner.

4 - Insulation and Damp-Proofing:
Insulation:
In pitched roofs, use two layers of 150mm glass fibre quilt, one between the joists, and the other layer laid over the first.
Flat roofs tend to need one layer between of rigid insulation board and another below. The space in between flat roof joists however can’t be entirely filled. A 50mm air gap must be left above for ventilation. The second layer underneath will drop the ceiling height a bit. Typically 150mm deep flat roof joists will receive 100mm of rigid insulation between the joists and 50mm beneath them.
As with floors, if the ceiling height is an issue (due to smaller than 150mm deep joists allowing less insulation between) multi-foil laminate insulants can help reduce the thickness of the under layer.

Damp-proofing:
The concrete floor may or may not have been cast over a damp-proof membrane (DPM). In recent years, integral garages would normally have included a DPM and certainly the walls would have a damp-proof course (DPC). But without plaster and screed finishes to conceal them, the two elements would not meet as they would in the house. Protecting the concrete floor with a polythene or paint-on DPM and dressing it up under your new finishes to the DPC layer will ensure that damp is not a problem.

5 - Plumbing and Wiring:
Make sure a thorough survey of the plumbing and wiring in the house and garage has been undertaken. Any wall you plan to pierce for doorways or windows needs special attention. Locate the main outflows for water and if you plan to install a toilet, the soil pipe.
Check the garage for wiring in the walls and ceiling. Rewiring the garage for lights and electric radiators will place additional strain on the household mains.
An additional mains supply can be installed which will also require the installation of a separate consumer unit.
Wiring to a detached garage can be run through an underground conduit. If it is to be a separate dwelling, a new connection may be required, depending on likely power usage. Always consult a qualified electrician.

6 - Flooring:
Floor insulation is not provided in an existing garage and including some in the conversion should be an essential part of the project, whenever it’s possible.
Garage floors are often lower than the house floor and a layer of insulation beneath a floating chipboard deck floor can be selected to bring the two levels together, particularly when the difference is up to 150mm or so. Chipboard sheets are at least a third of the depth of a floor screed, giving you depth for more insulation. Rolled out multi-foil laminate insulation is thin and good for draping over fixed-down battens when the difference in levels is not so great.

7 - Walls:
External walls are covered by Building Regulations and must meet requirements in terms of moisture-proofing and insulation. If the garage is integral to the house the exterior walls will usually meet building regulations. Otherwise consider a second block wall, or a stud wall, inside the existing exterior wall. Insulation, power and water pipes can be put behind this wall but this will decrease the interior dimensions of the space.
Interior walls between rooms must meet Building Regulations’ requirements for fireproofing. This can mean one or two layers of fireproof plasterboard on stud walls. For block walls this is unnecessary. Doors through interior walls need to be fireproof with a 30-minute rating. Additionally, Building Regulations may require a step in the floor at the doorway to prevent fire spreading along the floorboards.
If the garage will see a lot of use, especially if it is external to the house, consider additional insulation. This will decrease costs long term. Additionally, the insulation requirements for buildings have been rising and will continue to rise. Over-insulating now will make your property more saleable later when Building Regulations may require it, and it’s cheaper to add while the walls are under construction.
Replacing the garage door with an infill wall will require investigating the foundations to confirm their depth. Shallow fill foundations may require improvement to support the additional weight.
Consider replacing the door with a large window and panelling. The adjoining room to the door can be a store room, or the window can be the ‘front’ of the conversion. By saving the expense of reworking the foundations these options can be much cheaper. Some owners have built an interior-style block wall behind the existing garage door and insulated this, leaving the exterior appearance of the garage unaltered.

8 - Windows and Doors:
A second room will require ventilation to meet Building Regulations and an escape route to meet fire regulations. Building Regulations require a window 1/20 the floor area of the room, while fire regulations require a window with a 600mm base opening and a total area of not less than 0.45m². The window must also have trickle vents.
When installing windows employ a contractor accredited by BSI, CERTASS or FENSA (FENestration Self Assessment). Make sure the contractor shows you the certification.
Doors must meet the same requirements as windows in terms of their U value. In addition to uPVC doors with double glazed windows, wooden doors are also a good choice.
When putting in doors or windows in the walls of the garage, reinforcement may be necessary to the original garage wall. This will be in the form of a concrete lintel or a rolled steel joist (RSJ), depending on the size of the new opening. A lintel will need to overhang the edges of the window gap by 100 to 150mm minimum.
In standalone garages particularly, footings are often absent or inadequate and this makes adding a window and some panelling a more attractive idea than digging new footings and building an infill wall. However, the front opening of most garages is not a standard size for windows so additional building work may be necessary.
A block or brick wall filling some of the gap can bring the opening to a standard size without requiring additional footings, due to the decreased weight when compared to a full infill wall. Consult with the building inspector at the planning stage.