Drainage and septic tanks
A manhole position on an old plan now sits under the extension slab and the cover was not found. What pipe condition under that slab is unknown?
The pipe condition under that slab is unknown. An old plan puts a manhole where the extension slab now is, and the cover was not found. The survey has not lifted a cover that is not there, not camera-surveyed the run, and not seen the pipe the slab covers. It can record the clash between the plan and the floor. It cannot say the pipe is intact, collapsed, or even still on that line. Unknown means unknown. The slab is the reason.
Last reviewed 24 October 2026. Written for buyers in England and Wales unless this answer says otherwise. No named surveyor has signed this page. It is general guidance, not a report on your house, and not legal advice.
What is the fuller explanation?
Extensions sometimes swallow a chamber. The plan is a drawing of where a cover once was, or was supposed to be. Concrete does not let a surveyor check the bore, the benching or the joints. A home survey does not become a drain camera because a plan looks awkward. The only point is that the length under the slab was not seen, so its condition is not in the report. The rest of the drainage, if a cover elsewhere was lifted, is a different length.
Tell the solicitor the plan position and the missing cover, and that the survey has not approved the extension's drainage. If you need the pipe's condition, a drainage contractor would have to reach it, which may mean opening the slab. Do not describe the hidden length as clear. The survey only records what was seen, and under the slab nothing was.
What should you ask your surveyor?
- Did the old plan place a manhole where the slab now is?
- Was the cover not found, so the pipe under the slab was unseen?
- Have you left the condition of that buried length unknown?
What are the typical next steps?
- Treat the pipe under the extension slab as unseen.
- Do not describe that length as clear.
- Ask a drainage contractor if the run must be found and camera-surveyed.
Costs vary with the building and with who you instruct. Where this page does not cite a published figure, ask the surveyor, a relevant specialist, or your solicitor for a written quote. Do not treat a forum average as the price.
Also asked about this
If another chamber in the garden is clean, is the slab length clean too?
No. A clean chamber is that chamber. The length under the slab was not seen, and the cover there was not found. The survey has not carried a camera between them. Condition does not leap under concrete. The slab length stays unknown.
Does the missing cover prove the extension has no building-regulations approval?
No. A missing cover proves the cover was not found. The survey has not produced, or refused, a building-regulations certificate. Pipe condition under the slab is unknown. Approval papers, if they exist, are a separate search. The slab does not decide them.