Drainage and septic tanks
What is the difference between a combined drain and separate foul and surface systems?
A combined system takes foul water and rainwater in one pipe. Separate systems use two. A home survey does not map the public sewer. It may see one gulley or two. Which system you are on is a water-company and search question. Connecting a new downpipe to the wrong one is how misconnections start.
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?
Victorian streets are often combined. Later estates are often separate. The surveyor should not pretend to have checked the undertaker’s map. Approved Document H is what designers use for new work, including where rainwater may go. Your solicitor’s drainage and water search is the document that usually says what serves the house, and even that can be wrong in the detail of private pipes.
If you are adding a bathroom or a large roof, the choice of pipe matters and may need building regulations. The survey of the existing house does not design that alteration. If the report only says “drains not tested”, do not upgrade it to “on a combined sewer”. Ask the search. There is no condition rating for the mere fact of a combined system.
What should you ask your surveyor?
- Are you describing what you saw in the garden, or the public system?
- Did you see rain and foul sharing a chamber?
- Is any of this a defect, or only a description?
What are the typical next steps?
- Read the water search before you write to the seller about the sewer type.
- Do not reconnect pipes until you know which system they should join.
- If the survey found a defect, keep that defect. Do not add the sewer type as if it were one.
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
Is a combined system a fail?
No. It is a type. A blockage or a misconnection is a defect. The type is background.
Does a soakaway mean the house is not on a sewer?
It may mean rainwater does not go to the sewer. Foul water may still go to a sewer or to a tank. Ask which.