Garage conversion knock-through: structure and approvals

A garage conversion knock-through is an alteration to the opening between the garage and the house. The wall may carry roof, floor or room loads, and the work can affect lintels, steelwork, fire separation, floor levels, damp details and services. Do not remove masonry until the proposed support and approval route are clear.

Check what the wall is doing

Look above and below the proposed opening, not just at the wall face. The engineer or designer may need to consider roof trusses, first-floor loads, lintels, padstones, foundations, cracking and services hidden in the wall. A wider opening usually needs a more deliberate support detail than a small doorway.

Coordinate the construction detail

• Show the opening width, support, bearings and any temporary works.
• Resolve the floor-level difference between the house and garage.
• Maintain required fire separation and protect penetrations.
• Reroute electrics, plumbing, heating and ventilation before the wall is opened.
• Specify how the new floor, damp protection and threshold meet the house.

Building control may require calculations, drawings, inspection stages and evidence that the finished work meets the relevant requirements. A Party Wall question can also arise where the wall or excavation affects a neighbouring owner; that is a separate check.

Make the brief useful

Send the engineer photographs of both sides, the room above, dimensions, proposed room use, known services and any previous alterations. Read the engineer guide for when professional input is needed and the integral garage guide for the wider conversion context.

Sequence the opening safely

A knock-through is not only a demolition task. Temporary support, service isolation, dust protection, steel installation, fire stopping, making good and inspection need to be sequenced. Ask who designs the support, who checks the bearings, when building control attends and how the new threshold and floor connect to the house. Never rely on a contractor cutting first and resolving the load path afterwards.

Inspect before closing the work

Photograph the support, bearings, fire stopping, cable routes and service diversions before plasterboard covers them. Keep the engineer’s calculations, building-control comments and any approved revision with the final drawings. This record is useful for future maintenance as well as the completion file.