The drainage question that decides most Hinckley extensions is one almost nobody asks at the design stage: what kind of system is the house on.
Older parts of the town are largely on combined drainage, where foul waste and rainwater share a single pipe. Newer development is on separate systems, with rainwater kept apart from foul and taken somewhere else entirely. The two behave differently and they are governed differently, and a house that has been extended once already may well be on a hybrid of both without anybody having recorded it.
This matters because an extension adds roof. A modest single storey rear extension can add twenty square metres of new catchment, and in a downpour that water has to go somewhere. Adding it to a combined system that is already at capacity is how a neighbour further down the run ends up with a problem they did not have last year. Where the system is separate, rainwater cannot simply be teed into the foul run for convenience, however much easier it would be on the day.
Because of that we work out where the new water is going before the foundations are designed rather than after the roof is on. Sometimes the answer is a soakaway, which only works if the ground will genuinely take it and is worth a percolation test rather than an assumption. Sometimes it is an attenuation solution, and sometimes it is a properly consented connection.
On the older streets near the centre, where runs are shared between houses and the records are thin, we would rather put a camera down an existing drain for an hour than design around a guess. Our drainage work page covers surveys, repairs and new connections.