Drainage and septic tanks
What is a package sewage treatment plant, and how is it different from a septic tank?
Government guidance calls a package treatment plant a system that treats the liquid so it can go to ground or, unlike a septic tank, to surface water where the rules allow. A septic tank alone is not that plant. A surveyor may not be able to tell them apart from a cover in the lawn.
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?
The difference matters because the general binding rules treat a discharge to a ditch or stream differently from a discharge to a drainage field. The label on the lid, the installer and the paperwork are how you tell. A home survey that says “private drainage” is flagging the question. It is not answering it.
Ask the seller for the make, the manual and the service record. Treatment plants need power and servicing. A septic tank needs emptying and a working drainage field. The maintenance cost is whatever the service firm charges. Ask them. Do not use a neighbour’s annual contract as your figure.
What should you ask your surveyor?
- Could you tell a treatment plant from a septic tank, or only that drainage is private?
- Did you see power to a plant, or an air blower?
- Where do you think the outfall is, and how sure are you?
What are the typical next steps?
- Get the manual and the last service through the solicitor.
- If the type is unknown, ask an installer to identify it before you discuss upgrades.
- Check the electric supply if a plant needs power. A dead plant is not treating anything.
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 treatment plant never need emptying?
Do not assume that. Many still need sludge removed. The manual says. The survey does not.
Can you connect to a mains sewer instead?
Sometimes, if a main is nearby and the water company will agree. That is a separate project with its own cost. Ask the company. The survey cannot order a connection.