Reading the pair, and using the neighbour as evidence
On a street of semi-detached pairs, the planning history of the pairs around you is a better guide to what will be approved than any general design guidance, and it is unusually easy to read because the buildings are effectively identical. Walk the road and record, pair by pair, what has been done: which have rear dormers and roughly what size, which have been converted hip-to-gable and whether one or both halves, which carry rooflights only, and which are untouched. Then pull the council’s planning register for those addresses to see which alterations were permitted development and which were applications, and — where they were applications — what was approved and what was refused. Three patterns usually emerge and each is directly useful. The first is a de facto standard rear dormer size that the street already carries, which is the safest thing to match. The second is whether hip-to-gable has been accepted here at all, and if so whether only on pairs where both halves were done. The third, visible only in the refusals, is the point at which this authority objects — usually a dormer running the full width, or one taken up to the ridge, or a gable on one half of an otherwise intact pair. Matching an existing alteration on the other half of your own pair is the strongest position of all, because the effect of your proposal on the composition is then demonstrably nil: the symmetry is restored rather than lost. It is worth measuring the neighbour’s dormer properly rather than eyeballing it, because a matching proposal cited with its reference number and dimensions converts a discretionary judgement into a comparison, and comparisons are decided faster.
