What the septic tank rules mean when you buy a house

If the house you're buying isn't on mains drainage, its sewage system has to meet a set of Environment Agency conditions called the general binding rules. The rules say what the system must do. They don't say who pays to put it right when it doesn't. That gets settled during conveyancing, between you and the seller

Everything below applies in England. Wales, Scotland and Northern Ireland have their own regulators and their own rules.

What the general binding rules require

The general binding rules let a small domestic sewage discharge run without an environmental permit, as long as it sticks to the conditions.

Volume limits

A septic tank or treatment plant draining to the ground through a drainage field can discharge up to 2 cubic metres a day. A small sewage treatment plant discharging to surface water, like a ditch or stream, can discharge up to 5 cubic metres a day.

Go over the limit and you have a choice. Connect to the public foul sewer (your local water company can advise), or change the system so it meets the rules. If neither works, you need a permit from the Environment Agency, and a permit isn't guaranteed. Where the risk of pollution is too high, the Environment Agency can refuse one or attach conditions that force improvements.

The day-to-day conditions

Volume isn't the only test. The system has to take domestic sewage only and mustn't pollute surface water or groundwater. It also has to meet the British Standard that applied when it was put in, be installed and run properly, and have enough capacity for the home it serves.

It also needs emptying. The sludge has to be removed before the tank goes past its maximum capacity, and the guidance says that should happen at least once a year, or as often as the manufacturer recommends.

Newer systems face extra rules

If the discharge started on or after 1 January 2015, it can't meet the general binding rules at all if there's a public foul sewer within 30 metres of any boundary of the property it serves. In that case the expectation is that you connect.

Discharges that started on or after 2 October 2023 need a permit if they're within 50 metres of another system that discharges under the general binding rules.

Septic tanks that drain into a watercourse

This is the rule that catches most buyers out. A septic tank must not discharge straight into a watercourse, whether that's a river, stream or ditch.

If one does, the owner must, as soon as possible, do one of three things:

• Connect to a public foul sewer
• Replace the septic tank with a small sewage treatment plan
• Install a drainage field so the tank discharges to the ground instead

The Environment Agency expects plans to be in place to do the work within a reasonable timescale, usually 12 months.

You'll still find plenty of advice online saying the upgrade had to happen by 1 January 2020 or when the house was sold, whichever came first. That's out of date. The deadline has passed, and the guidance no longer works that way. The duty is ongoing, which is actually worse news for an owner, because there's no future date to wait for.

It's also worth knowing there are three routes, not two. A new drainage field is often the cheapest fix where the ground drains well, and it's the one people forget about when they assume a full treatment plant is the only answer.

Who pays for an upgrade

Here the guidance is a recommendation, not a rule. When a property with a septic tank draining into a watercourse is bought or sold, the buyer and seller should agree who'll replace or upgrade the system, and should make that a condition of the sale.

In practice it becomes a negotiation, and it usually ends one of three ways. The seller knocks money off the price. The buyer's solicitor holds back a retention until the work is done and signed off. Or the seller does the work before you exchange contracts and shows you proof.

Get a proper quote before you start haggling. The cost depends a lot on the ground, how easy it is to get machinery in, and whether a new drainage field needs percolation testing. The difference between an easy site and a tricky one is big enough that arguing over a guessed figure wastes everyone's time.

What the seller must tell you

This part isn't optional. When a property with a sewage discharge is sold, the seller must tell the new owner in writing that the discharge exists. That written notice has to include:

• A description of the treatment system and drainage system
• Where the main parts of the system and the discharge point are
• Details of any changes made to the system
• How the system should be maintained, plus the maintenance manual if they have one
• Maintenance records, if they have them

So there's a real difference in how firm the two duties are. Agreeing who pays for an upgrade is something the guidance says you should do. Handing over that written information is something the seller must do. A scribbled note about roughly where the tank is doesn't cut it.

The questions to ask

Responsibility sits with the operator, which normally means the owner. Where one system serves several homes, the responsibility is usually shared too, so a shared tank means shared liability. That's the version most likely to come up late in a sale and cost the most to sort out.

Make sure your solicitor raises these specific enquiries with the seller:

• Where does the system discharge, and into what?
• When was it installed?
• Was building regulations approval obtained?
• Can you see the emptying (desludging) records for the last few years?
• Is the system shared, and if so, with whom?
• How far away is the nearest public foul sewer?

Our guide to the enquiries buyers ask sellers covers the wider list, and we've written separately about the other property catches when buying in the countryside.

If a system went in before 1 January 2015 and has no building regulations approval, the local council is the place to ask about approval and planning permission. That conversation is much better had by the seller before the house goes on the market than by you three weeks before exchange.

The one question that settles most of this

Where does the outflow go? Ask the seller to show you the discharge point on the ground. If it runs into a ditch or stream, you're buying a system that has to be replaced or upgraded, and the price should reflect that.

Our complete guide to conveyancing explains when drainage enquiries get raised and how long answers usually take. If you haven't picked a solicitor yet, it's worth comparing conveyancing quotes from firms that deal with rural properties regularly.