Drainage and septic tanks
A rainwater harvest tank in the garden was not opened. What should you avoid calling it?
Avoid calling it a septic tank, a treatment plant, or a cesspool. It is a store for roof water, if that is what the pipes show, and the survey did not open it. You do not know whether it is clean, cracked, or connected the way the seller describes.
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?
Harvest tanks sit underground or against a wall, with a rainwater inlet and a supply to taps or a garden. The surveyor may see a lid, a pump switch, or a pipe from a downpipe. They do not lift a heavy lid as a matter of course, and they do not test water quality. A standard inspection is visual. Approved Document H is building-regulations guidance for drainage work, not a service of this tank, and not a camera grade. If the lid stayed shut, the sentence is “not opened”. Do not invent silt levels you did not see.
Foul water should not be led into a rain store. The survey can only say that if a foul pipe was visible. If it was not, do not accuse the system of a misconnection you have not seen. Also do not use septic discharge rules on a tank that is only collecting a roof, unless foul drainage is actually involved. Ask the seller for the layout and whether a pump serves the house. A pump that was off was not tested. The tank can still overflow in rain whether or not the surveyor ran a tap. Keep this note on rainwater. The foul system, if it is separate, stays in its own paragraph.
What should you ask your surveyor?
- Did you see a harvest tank, and did you open it?
- Which pipes enter it, as far as you could see?
- Are you keeping it off the septic-tank description?
What are the typical next steps?
- Do not open a heavy lid alone.
- Ask the seller what the water is used for and where overflow goes.
- Do not file the tank as a cesspool because it is buried.
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 pump switch mean the water was tested?
No. A switch means there is a pump. The survey did not test the water or the pump.
If the tank overflows into a soakaway, has the soakaway been sized?
Not by this visit. Overflow and crate size are different. The survey measured neither.