Drainage and septic tanks
A toilet runs constantly. Is that a collapsed drain?
No. A cistern that never shuts is usually a valve or a washer in the cistern, and it wastes water. The survey did not dismantle the cistern and did not camera the drain. Running water is not, by itself, a collapsed sewer.
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?
You can hear a continuous trickle into the pan. The flush valve may not seat, or the inlet may overfill so that water spills down the overflow into the bowl. Both are cistern faults. The underground drain may be fine. The surveyor notes the noise and any staining. They do not strip the siphon on a home survey, and they do not infer a broken pipe from a noisy cistern. Approved Document H is not a diagnosis of this washer. A standard visual inspection does not become a drain test because a ball valve hisses.
A plumber can usually see the fault once the lid is off. That small repair is a different conversation from a drainage firm and a camera. If the pan also backs up when other fittings are used, say that separately. Backup is a drain symptom. A lonely trickle is not. Do not ask the seller for a sewer replacement because the cistern runs. Do not ignore it for ever either: it wastes water and can stain the pan. There is no camera grade to attach, because no camera was used. Keep the sentence on the cistern unless the survey saw something else.
What should you ask your surveyor?
- Which WC runs, and did you see overflow into the pan or only hear it?
- Did any other fitting back up?
- Are you keeping this off a collapsed-drain conclusion?
What are the typical next steps?
- Do not price a new sewer for a running cistern.
- Ask a plumber to look at the cistern if you need it stopped.
- Mention backup only if it actually happened. Do not add it for effect.
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 running toilet prove the septic tank is full?
No. The cistern is mains water into the pan. A full tank is a different inspection the survey has not done.
If the overflow drips outside, is that the same fault?
It can be the same inlet fault, discharging outside instead of into the pan. It is still not a camera survey of the drain.