I would be really grateful for advice here. I had an extension built and as such the length of sewage pipe from downstairs loo near front door to manhole at back extended by 3m. I'm not sure if builder connected this to the run of pipes joining the soil stack with a y shaped thing but this connection must be past the stack now as the stack got moved round to the side of the house. When the stack was at the back of house it may well have run straight to it. I know where the pipe runs from toilet and the stack is 'round the corner' from it if that makes sense. Would this situation cause foul smells intermittently? Seems worse after rain and I just realised run off from gutters on extension runs into foul near kitchen grid. But its not only when it rains that its smelly. Drains were surveyed nothing detected in way of leaks. Drain man said he thinks fall is 'adequate' . Smell from toilet not sink. My husband has checked the seal. I have a compact flushing toilet maybe not enough water flow for 10m? distance to main drain? I dont think it was as bad 1) when we had larger cistern and 2) when our ensuite above down stairs loo was in constant use (shower & toilet broken due to age and cant afford replacement yet) Would this be solved by getting some sort of stack/valve for downstairs loo? Are you allowed 2 stacks - can't find info on whether a second affects air pressure? Or will the increased through flow of water when we use ensuite again stop stagnation and smells? Smells come into en suite when trap dries out but resolves when I fill up. Hope this all makes sense. I had a plumber round but not a whiff that day as per the law of sod! Thanks in anticipation!!