The three failures that make a converted garage feel converted
Garage conversions are among the best-value square metres available on a suburban house, and they are also the alteration most likely to feel second-rate afterwards. Three technical failures account for almost all of it, and each is a design decision rather than a workmanship one. The first is the floor level. Garage slabs sit lower than the house floor and often fall towards the door for drainage. If the conversion simply insulates and finishes over the existing slab, the finished floor ends up below the adjoining rooms and the doorway has a step down into what is meant to read as part of the house. The remedy is to design the build-up to arrive level: insulation and screed thickness chosen to make up the difference, or where that would leave inadequate head height, the slab broken out and relaid at the right level. The second is the thermal line. A garage wall is frequently single skin, with no insulation and no cavity, and the roof is often uninsulated and of a different construction from the house. Insulating the elements but leaving the junctions unresolved produces cold bridges exactly where they show — along the wall-to-floor junction, at the reveals of the new window, and where the garage roof meets the house wall — and the symptom is condensation and black mould in the corners of a room that is otherwise warm. Continuity of insulation matters more than its thickness. The third is damp. The garage was never intended to be dry in the way a habitable room is, so its damp proof course may be missing, at the wrong level, or discontinuous with the house. The conversion needs one continuous barrier from the new floor membrane into the wall course and across the junction with the house, and where the external ground level outside the garage is high — which it often is, because the drive was resurfaced — that has to be lowered too. Get those three right and the room reads as original; get any one wrong and it never quite does.
