← All guides · Reviewed 2026-08-05
Do I need to replace my septic tank? (2026 UK rules)
You need to replace or upgrade your septic tank if it discharges directly to a watercourse — a stream, river, ditch or canal. That’s been banned since 1 January 2020 under the Environment Agency’s General Binding Rules. If your tank discharges to the ground via a properly working drainage field, and it’s in good working order, you generally don’t need to replace it. If you’re selling the property, a non-compliant tank will usually need to be sorted out as part of the sale.
When you do need to act
- Discharges to a watercourse. Since 2020, a septic tank sending effluent straight to a stream, river, ditch or canal is not allowed. You’ll need to either add a drainage field so it discharges to the ground instead, replace the tank with a sewage treatment plant, or — rarely, in rural Cornwall — connect to the mains sewer.
- Failing or polluting. A tank that’s leaking, overflowing, causing smells, or visibly polluting land or water needs fixing or replacing regardless of where it discharges.
- Selling with a non-compliant system. Buyers’ solicitors and surveys increasingly flag septic tank compliance. A watercourse discharge will usually need resolving before or as part of completion.
- Undersized or otherwise breaching the rules — for example a drainage field too close to a watercourse or borehole, or a discharge volume over the General Binding Rules limits (2 m³/day to ground, 5 m³/day to a watercourse).
When you’re fine
- Your system discharges to the ground through a properly functioning drainage field that meets the rules (see our General Binding Rules guide for the detail), or
- you have a sewage treatment plant, which is allowed to discharge to a watercourse within limits because it treats the effluent to a higher standard — see septic tank vs treatment plant.
Not sure which applies to you? That’s the single most common question we get, and it’s usually quick to answer once you know where the pipe actually goes — see discharge to a watercourse vs to the ground.
Your options if you must upgrade
- Add or repair a drainage field. Often the cheapest fix if the ground is suitable — it keeps your existing septic tank but routes the discharge to the ground instead of a watercourse.
- Install a sewage treatment plant. The usual route where there’s no room or suitable ground for a drainage field, since a treatment plant can legally discharge to a watercourse.
- Connect to the mains sewer. Only realistic where a connection is nearby and “reasonably practicable” — uncommon for rural Cornwall properties, which is exactly why most rely on a septic tank or treatment plant in the first place.
Costs
| Option | Typical cost |
|---|---|
| Upgrade (drainage field or plant, blended) | £5,000–£15,000 |
| Sewage treatment plant installation | £9,000–£13,000 |
| Drainage field only (where ground allows) | Cheaper — often £3,500–£7,000 |
See our full septic tank cost guide for what drives the price up or down, or run the cost calculator for a figure closer to your property.
Not sure if yours is compliant?
Most homeowners genuinely don’t know where their septic tank discharges to until they check. Our free compliance checker asks a few simple questions and tells you, in plain English, whether your setup is likely compliant and what — if anything — you need to do.
Sources: GOV.UK — General binding rules: small sewage discharge to a surface water · GOV.UK — General binding rules: small sewage discharge to the ground
Common questions
Are septic tanks banned in the UK?
No. Septic tanks themselves aren't banned — only discharging from one directly to a watercourse is banned, since 1 January 2020. A septic tank that discharges to the ground via a compliant drainage field is still allowed.
Do I need to replace my septic tank when I sell my house?
If it's non-compliant — for example discharging straight to a stream or ditch — yes, this will usually need to be resolved as part of the sale. A tank that discharges to the ground through a working drainage field doesn't need replacing to sell.
How much does it cost to replace a septic tank?
Typically £5,000–£15,000, depending on the option: adding a drainage field is usually cheapest where ground conditions allow, a sewage treatment plant install usually runs £9,000–£13,000, and connecting to mains (rare in rural Cornwall) varies with distance to the sewer.
Septic tank vs sewage treatment plant — what's the difference for compliance?
A treatment plant actively cleans effluent to a higher standard, so it's allowed to discharge to a watercourse within limits. A septic tank only provides partial treatment, so it must discharge to the ground via a drainage field — it can't legally discharge to a watercourse.
Need this sorting in Cornwall?
Kernow Septic covers Cornwall's towns and rural parishes. Call or run the free tools.