If property has been extended (think you said it has!) then always possibility an inspection chamber has been 'buried' under flooring. (One of my pet hates!) Chambers are there for a reason, if neighbours have one then i'd try and identify which way the drains run. (
Should run in a straight line between chambers.....)
If it looks like there may be a chamber somewhere under your floor then now may be the time to locate it. It's acceptable to have a cover internally, but
must be correct type, with a cover that seals and is screwed down shut. It is a PITA to be called out to a drain problem, where no-one has any idea where the chambers are! Had occasions where a drain is blocked, and with no chambers and no idea where it might go, so we've had to dig, find the drain, then fit a chamber at an appropriate point for future needs.