It usually turns up in the buyer’s survey. A line about the drainage, a question from their solicitor, and a sale that was going fine now has a condition on it.
If that is where you are, this is what the rules actually say. It covers England, where the Environment Agency sets the rules. Wales and Scotland have their own.
The rule that catches most sellers
The rules for septic tanks and small treatment plants are called the general binding rules.
The one that matters most when you sell is about where the waste water goes. If your septic tank discharges straight into a watercourse, such as a ditch, stream or river, it does not meet the rules. A septic tank only separates the waste. The water coming out of it is not clean enough to go into a watercourse.
The Environment Agency says you must put that right as soon as possible, with plans in place to do the work within a reasonable timescale, usually 12 months.
The three ways to put it right
The Environment Agency gives three options:
- Connect to the public foul sewer, if there is one close enough.
- Replace the septic tank with a small sewage treatment plant. A treatment plant treats the waste water properly, so it can go to a watercourse.
- Install a drainage field, also called an infiltration system. That is a run of perforated pipes in trenches, so the water soaks through the ground and gets treated on the way.
Which one suits your property depends on how far you are from a sewer, the ground, the space you have, and where the water can go. That is what we look at first.
Who pays when you are selling
The rules do not say it has to be the seller. The Environment Agency says the buyer and seller should agree who is responsible for replacing or upgrading the system, as a condition of sale.
In practice, the sale usually waits until somebody has sorted it, or at least until there is a quote and a plan the buyer’s solicitor is happy with.
What you have to tell your buyer
Whatever system you have, when you sell you must tell the new owner in writing that there is a sewage discharge. The Environment Agency says to include:
- a description of the treatment system and drainage system
- where the main parts are, including the discharge point
- any changes that have been made to them
- how they should be maintained, and the maintenance manual if you have one
- maintenance records, if you have them
If you do not have most of that, it is worth getting the system looked at before the buyer’s solicitor asks.
Looking after it once it is in
Septic tanks and treatment plants need the sludge removed before it gets past the maximum capacity. The Environment Agency says at least once a year, or in line with the manufacturer’s instructions.
What to do now
Call us on 01942 321 423. Tell us what the survey found and roughly where the tank is. We will assess what you have, explain what it needs to meet the rules, and tell you how quickly the work can be completed.
Source: GOV.UK, general binding rules for small sewage discharges to a surface water and to the ground. Checked October 2026. The rules can change, so check the GOV.UK pages for the latest version.