The existing garage floor is likely to be strong enough for general domestic use, but may need to be upgraded to ensure it is adequate in terms of damp-proofing and thermal insulation. It may also be desirable to change the level of the floor to match the levels in the existing home.
The simplest way to achieve this would be to upgrade the existing concrete floor. Alternatively, if levels permit, a new timber floor could be constructed over the existing concrete floor.
The existing concrete floor can be used as a base, however a new damp proof membrane (DPM) will need to be introduced. DPMs come in solid or liquid form, the latter being a practicable solution for a garage conversion. Manufacturers will be able to advise. A suitable gauge damp proof membrane (DPM) and thermal insulation must be provided. These can be laid over the sand blinding or on top of the concrete.
Thermal insulation may be required and can be placed on top of the membrane (if a liquid membrane is used care should be taken to ensure the two materials do not react with each other - a separation layer may be needed). The exact details will vary depending on which products are used.
The floor can be finished with a layer of screed or a timber covering ("floating floor") the exact specification of which will depend on the insulation material used beneath. A screed is likely to need to be in the order of 75mm thick and should include a reinforcement mesh to prevent it cracking.
Care should be taken to ensure any existing airbricks for the main house are not obstructed by this work. If so, they should be extended through the new floor to external air.
Suspended Timber Floor
The existing floor level to the house may be quite high above ground, and in cases such as this it is more practicable to use timber joists, with a void underneath. A minimum gap of 150mm should be kept between the existing concrete ground and the underside of the timber. The timber floor joists must be sized correctly depending on their length. They are then laid across the shortest span from wall to wall with a gap underneath.
An intermediate wall with a small footing may be needed to reduce the span and keep the thickness of the floor joists to a minimum. A damp proof course (DPC) should be placed on the underside of the timber. Insulation is then placed between the joists (thickness required depends on the product used). Air vents should be placed underneath to provide ventilation to the void and the air should be able to travel from one side of the building to the other.
In some areas, the ground could have a certain amount of contamination where gases form. If this is the case then this gas needs to be ventilated and a gas membrane will be required to stop it from entering the building.
Advice on this can be found in the Building Research Establishment (BRE) / Environment Agency report: BR 414 - Protective measures for housing on gas-contaminated land, 2001
For Radon gas, there is guidance the Building Research Establishment report: BR 211 - Radon: Guidance on protective measures for new dwellings, 1999