The building, and what it does to the job
Across this patch the twenties and thirties barely register, so the standard semi galley is not the shape we are fitting. Repeated stock means repeated units, and if they are sound the rest is straightforward. What is behind the old top is checked, not assumed.
The addition wall is half a brick thick in places and has usually had a damp course injected and been re-plastered, which leaves it neither flat nor consistent. Sockets and switches are cut to measured positions with the faceplates off, not to the positions shown on a drawing. What the wall can take decides how tall the stone goes.
Level has to hold across the whole room rather than along one wall, or a floor that falls end to end shows as a step where two pieces meet. A mitred external return around it has to come from the same position in the slab, or the pattern visibly steps at the corner. Full height wants a flat wall behind it.
