Drainage and septic tanks
What should you ask if a drain appears to run under the floors of the house?
Ask whether that is known or only suspected, and do not assume a leak under the floor is causing the cracks. Suspended floors sometimes have a drain beneath. A surveyor may see a gulley on one side and a chamber on the other and infer a line. They have not seen the pipe in the void unless they entered it. A camera, if you commission one, is what shows the condition of that length.
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?
A leak under a floor can wash out fill and, in some cases, mimic settlement. It can also do nothing visible for years. The Association of British Insurers’ subsidence advice is about foundation movement, which a suspected drain line does not prove. The honest survey sentence is “line not proved, void not inspected” or “camera recommended”.
Approved Document H does not forbid a drain under a building, and it does not bless one either. Build-over agreements with a water company are a solicitor’s question when a public sewer is under an extension. Ask your solicitor about the search, and ask a drainage firm about the pipe. Do not dig up the hall on a hunch.
What should you ask your surveyor?
- What did you actually see that suggests the route?
- Did you inspect the underfloor void?
- Is a camera the investigation you want, or a legal search?
What are the typical next steps?
- Separate the water-company search from the camera. They answer different questions.
- Do not cut the floor before the camera has been tried from the chambers.
- If there are cracks as well, show both to the surveyor’s recommended investigator. Do not pick a cause yet.
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 every terrace have a drain under the floor?
No. Many runs are outside. The inference has to be explained. Ask.
Will a camera always pass under the house?
Not if there is no access or the pipe is collapsed at the start. The firm should say what they could not reach.