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Hi All. I was speaking to the building control officer this morning and discussed altering the soil stack and relocating the waste stack on the back of the house so I do not need to have penetrations through my new extension roof. He agreed that I could take my main roof gutter water over the new roof (as long as I show the area and check the gutter pipe sizing tables etc.) he also agreed I could cut my soil stack low, take a 92.5° bend from above the T jct. where the WC is connected, run it along horizontally within the extension roof void and pop it out the wall rather than via the roof - that is easy and will mean just boxing out the pipe within the extension which is not a big deal. I MAY move it round the corner too but for now, that is the plan. So the next pipe to deal with is the 3 inch cast iron waste stack, as I can run gutter water over the new roof I can therefore take the waste stack down lock stock and barrel, this services the main gutter, bath and sink. Looking at the bathroom wastes and with some PB removed in the kitchen below it looks like I could run a piece of waste pipe between 2 floor joists, pickup the bath and sink and core through the wall and come out on the wall round the corner from where the waste currently exits - within about 750mm of this is the waste pipe from the kitchen, so some digging will soon reveal a pipe I can build a manhole onto and connect the new waste pipe. See sketch attached, yellow is existing, blue is proposed, NTS and just rough... I don't really want to add a 110mm stack down the side of the house, to catch a sink and a bath I think it is just excessive anyway, so I would like to use 82mm pipe, it doesn't appear that common and there are not many parts available for it but it looks like I can get all the bits I need. Plan would be, new 82mm waste pipe on the outside wall, then a T branch (if I need air admittance) or 92.5° deg bend through the wall, mortar that in, then push a stub of 82mm into that and then downsize the rest of the run with a 40mm to 82mm pushfit boss, I would also run the 40mm pipe further into the 82mm than needed so that it effectively just poured down the external pipe and reduces any likelihood of leaks from the boss in the future. How does this sound? This is simple and non invasive enough that I could do this little project almost immediately so I am not faffing about with pipes when trying to put up my timber kit. What are you guys doing for your external bath and sink wastes when not using 110mm? Most google hits show these huge ugly 110mm stacks... The other option being considered is move the soil stack round the corner (much more digging) and then tying in the toilet and the bath and sink into the one pipe, but that is another story.