Liquid Screed for Full-Time Granny Annexes

Where step-free access is required, we work back from the external threshold height, accounting for insulation and screed depth so the internal floor and any accessible wet room sit flush without upstands or lips. This calculation needs to happen before the slab is set out, since the finished floor level has to be agreed with the groundworker and, where relevant, an occupational therapist or accessibility consultant advising on the design.

Getting this wrong is one of the more costly mistakes on an annexe project, because a threshold that ends up a step higher than intended can compromise the whole purpose of the building. We check drawings against manufacturer guidance for wet room formers and door thresholds before confirming a build-up.

Because an annexe is likely to be occupied continuously, the underfloor heating and screed specification are geared towards steady, dependable warmth rather than the more occasional heat-up cycles of a garden office or seasonal room. Pipe centres and manifold position are agreed with the heating installer before the pour, with particular attention paid to bathroom and wet room zones that may need tighter spacing.

Commissioning follows a gradual warm-up schedule once the screed has cured sufficiently, and we advise on realistic timescales so the resident isn't waiting longer than necessary to move in once the floor and finishes are complete.

An annexe with its own kitchen and bathroom typically needs its own drainage, water and electrical supply, all of which cross the floor zone at some point. We ask to see first-fix drawings before quoting so we understand where pipework and cabling penetrate the slab and screed, and can plan around them rather than discovering clashes on pour day.

Depending on the annexe's use — whether it's treated as an extension of the main dwelling or a self-contained unit — building control requirements can vary, and thermal elements and accessibility provisions are usually signed off before the floor is covered over.

The most frequent issue is a threshold level fixed too late, after the slab has already been cast, leaving no room for the intended screed depth or wet room fall. Confirming finished floor levels at design stage avoids this entirely.

A second common pitfall is underestimating how much depth a level-access shower former needs relative to the surrounding screed, which can leave an awkward step if not planned early with the bathroom fitter and heating installer together.