Contact everybody you can think of who might have an interest in the matter. Incidentally, I built an extension over a soil pipe leading from my house to the main drain in the back garden, and I simply fitted an airtight drain cover in my kitchen to allow for access in case of blockages. If the extension is built over a communal drain then you need to check it out. I'm an optimist, and if the extension has been there for a number of yars without problem I wouldn't worry too much; but I'd still check it out.