OP you have got feathers ruffled heer!! Going back to your query. You can go back 3m under normal PD or 6m under LRE rules (currently due to expire end of May but expected to be extended) 45 degree doesn't apply to single storey. You can build within 3m of a public sewer/drain SW or foul provided you don't load it without involving the WA. BC will be keeping an eye on you and will advise if you need consent from WA. For example I designed a job a couple of years ago where there's a pipe within a couple of feet of the foundation. Snag was it's 3m deep so we had to pile so that no load was applied to the pipe. You can move pipes but as said above you pay a fee. You can however normally use your own contractors with WA inspecting. You would probably need 4 chambers by the sound of things and the falls might be an issue. Different WAs have different rules about how much pipe you can build over so your best bet is to check their website.