Drainage and septic tanks
What should you ask if an extension seems to stand over where a public sewer might run?
Ask your solicitor what the drainage and water search shows, and ask the surveyor whether they could see any sign of a sewer, a manhole or a build-over. A home survey does not produce a water-company build-over agreement. Looking at a rear extension cannot prove the pipe was bridged lawfully.
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?
Public sewers are often in the ground the extension now covers, especially on older streets where the sewer runs along the back. Building over one without agreement can matter when you come to sell or when the pipe has to be repaired. Approved Document H is about drainage design for building work. It is not the water company’s consent, and the surveyor will not have applied for that consent.
The search and the seller’s paperwork are the solicitor’s tools. The survey is useful if it found a manhole inside a room, a crack where a drain passes, or a floor that could not be inspected. Those are condition points. Do not convert them into a fine you invent. If the search says a sewer is under the extension and no agreement appears, ask the solicitor what they recommend before you exchange.
What should you ask your surveyor?
- Did you see manholes, gullies or a route of drain that the extension now covers?
- Are you stating that a sewer is under the extension, or that you could not tell?
- What should the solicitor check that you cannot check from the garden?
What are the typical next steps?
- Read the drainage search with your solicitor.
- Ask the seller for any build-over or building-regulations papers for the extension.
- Do not cut a hole in the new floor to find a pipe before you own the house.
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
Does a manhole in the extension prove there is no agreement?
No. It proves a chamber is there. The agreement is a document. Ask for the document.
Can the surveyor move the sewer on the plan?
No. They report the building. The water company and your solicitor deal with the sewer’s status.