Well, today's fun and games with the new extension has been the discovery that the rainwater drain running slap bang down the middle of it (which was going to be removed and replaced with a soakaway) is in fact also used for the neighbour's rainwater.
Terraced house - extension is in the gap between the two kitchens - up to the boundary line.
both houses have a rainwater gulley in the inside corner between the kitchen & the main body of the house.
The drain from mine comes off at 45 deg, running for about a metre, then turns to follow the boundary line - about 30cm or so from it. About 30cm along the straight run is another join... running back to the neighbour's gulley.
This is a bit of a blow - the plan had been to remove the whole drain & divert the rainwater to a soakaway (as per plans & building control requirements). It runs at perhaps a spade's depth below the surface - it may well run over the top of the foundations, but would be buried under concrete floor.
What's normally done in these circumstances? This strikes me as potentially very messy - neighbour's cooperation is not entirely guaranteed to be forthcoming & practically if I actually want this building work to be finished this year then I will probably have to pick up the tab for any rerouting work.