I want to put up a single-storey extension with flat roof across the back of my house (semi-detached). Neighbour's foul water drain runs right underneath where my extension will go (e.g. from my neighbour's garden, over our boundary, under my garden and into communal sewer the other side of my proposed extension). Will it be okay to build, or do I need to get drainage survey done / apply for drainage easement? What happens legal-wise if the drain blocks / collapses in the future? Also, do you know what is the maximum distance I can build out away from my house? 3m, 3.5m, 4m are figures I've heard mentioned. Obviously there may be a loss of daylight issue - can the neighbour legitamately object, and what would they need to prove? Any advice appreciated!