Converting a garage into habitable space looks simple from the outside — box up the door opening, plaster the walls, lay a floor — but the existing garage slab and structure were built to a completely different standard than a habitable room requires. A specification that doesn't deal with floor level, damp-proofing and thermal upgrade is pricing a stud wall and a lick of paint, not a legal room.
This is one of the smaller-value conversions in cash terms, which is exactly why corners get cut on quotes: the total sum looks affordable even when the technical detail underneath it is thin.
Floor level and structure
Garage slabs are typically laid without a damp-proof membrane and often sit below the house's finished floor level, with a step down at the former door threshold. The spec should state how the new floor build-up — insulation, damp-proof membrane, screed or floating floor — will raise and level the floor to match the adjoining house, and whether the existing slab needs breaking out or can be built up over.
Where the new floor build-up raises levels significantly, the spec should confirm resulting headroom under the garage's existing roof structure, since ceiling height can become tight once insulation and floor buildup are added.
Damp-proofing
Because most garages have no damp-proof course in the walls at the same level as the house, and no damp-proof membrane in the slab, the spec must state how rising and lateral damp will be dealt with — typically a new DPM lapped up the walls, or a damp-proof course installed at the correct level, verified against the existing house DPC.
This is not optional for habitable use, and a specification silent on damp treatment should be treated as incomplete.
Thermal upgrade
Garage walls and roofs were built to a shed standard, not a thermal one. The spec should state the insulation being added to walls, floor and roof/ceiling, with enough thickness to meet the applicable Building Regulations thermal performance for a habitable extension of the existing dwelling, and should confirm the window and door specification (typically double or triple glazed to match).
Where the original up-and-over door opening is being infilled, the wall construction filling it needs its own insulation and weatherproofing detail — a common area where a cheap quote just boards over the opening without proper build-up.
Services and ventilation
The spec should confirm how heating is being extended into the new room (radiator off the existing system, or independent), where new sockets and lighting circuits are run from, and whether trickle ventilation or extract ventilation is provided, particularly if the room will contain a shower or kitchenette.
What cheap garage conversion quotes tend to skip
Damp-proof membrane and DPC works, building control fees for a change of use, and matching the new floor level properly to the house are the most common omissions. Rendering or re-cladding the external wall left exposed by removing the garage door is another frequently missed item.
- Damp-proof membrane and DPC tie-in to the house
- Floor build-up to match house finished floor level
- Building control notice for change of use to habitable room
- External wall finish where the garage door opening is infilled
- Extended heating circuit or independent heat source
Common questions
Does a garage conversion need building control approval?+
Yes. Converting a garage into habitable space is a change of use with structural, thermal, fire and drainage implications, and needs to be signed off under Building Regulations.
Why does my garage floor need to be rebuilt rather than just carpeted over?+
Garage slabs generally lack a damp-proof membrane, so carpeting or boarding directly over one traps moisture and risks damp and mould in the new room. The floor build-up needs a proper membrane and insulation layer.
Can I convert a garage myself and just get it signed off afterwards?+
It's possible but riskier — if the completed work doesn't meet the regulations, you may be required to open it up again for inspection or carry out remedial work, which costs more than getting it specified correctly first.
Get a garage conversion specification that meets building control
We check floor level, damp-proofing and insulation against building control requirements before we quote.