Garage conversion foundations: slab, walls and door infill

A garage conversion does not automatically need new foundations, but the existing slab, footings and walls must suit the proposed work. Door infills, new openings, floor excavation, cracking or changes to roof and wall loads can all require investigation before the build-up is finalised.

Separate the slab from the foundations

The concrete garage floor and the foundations supporting the walls do different jobs. A slab may be sound for storage but unsuitable for a new insulated floor, or it may conceal settlement and moisture. The wall footing may be adequate for the existing structure but still need to be understood where a door opening is infilled or widened.

What to inspect

• Look for settlement, cracks, movement, hollow areas and previous repairs.
• Establish the floor level in relation to the house and external ground.
• Check whether the door threshold has a proper support and weathering detail.
• Confirm the construction of the wall above and beside the opening.
• Investigate drains, services and any nearby excavation before breaking ground.

Opening-up should be planned and made safe. A specialist or structural engineer can advise on the location and extent of investigation rather than relying on a single exposed corner.

Infill is a foundation and weathering question

Where the garage door becomes masonry and a window, the new section needs a stable base, damp protection, a durable connection to the existing construction and suitable support above the opening. The finish should resist water and movement at the junction. A thin cosmetic panel over an unsuitable threshold is not a substitute for a designed wall section.

Record what is found

Photograph the exposed construction, note dimensions and record changes from the original proposal. Ask building control what must remain visible for inspection. If the design changes after opening-up, update the drawings and quote before the work is covered.

When proportionate opening-up is justified

Foundation questions are usually triggered by a new or widened opening, cracking, settlement, an infill detail or a significant change in load. The aim is not to dig everywhere: agree the locations, purpose and reinstatement before opening-up begins. Record levels, photographs and findings, then make sure the final floor, wall and support details reflect what was actually found.