On a related note, out of curiosity, if I build an extension over an inspection chamber manhole (leaving it accessible under an inset bit of flooring/insulation), will my new room smell of shite?
You can’t be serious? a manhole inside your house won’t pass planning consent, check the submitted drawings for the extension, it will have an new inspection hole outside and the original one inside sealed, you would also presumably have insulation over it/under the flooring.
Presuming it’s a soil pipe running from toilet then you can get gas back up the pipe from the septic tank, whether it seeps out the manhole cover is down to how well it’s sealed I guess, if you lift it though inside the house and it’s a stinky one the whole house will spontaneously vomit then move out for a week.