Replacing a failing tank, swapping to a treatment plant or putting in a new drainage field: what it costs, what the rules say where you live, and an installer who works in your area to price it.
Ask for a site visitDo I need to replace mine?
A free matching service, not an installer.
Distances from council guidance on Approved Document H, Building Regulations Part H. A diagram of the principle, not a design for your plot.
The answer depends on three things: which nation you are in, what is in the ground, and where it drains. Pick yours.
In England a septic tank must not discharge straight to a stream, river or ditch. The rules expect it to be replaced or upgraded as soon as possible, normally with plans in place within about 12 months: a sewage treatment plant to the same watercourse, a new drainage field, or a sewer connection. If you are selling, agree in writing with the buyer who will do it. Environment Agency: discharges to surface water.
A septic tank draining to a proper drainage field can stay under the general binding rules if it puts no more than 2 cubic metres a day to ground, is emptied at least once a year, is not polluting, is not in Source Protection Zone 1, and the outlet is not a soakaway, well or borehole installed after December 2007. Failing any of those means a repair, a new field or a permit. Environment Agency: discharges to the ground.
A treatment plant may discharge up to 5 cubic metres a day to a watercourse that flows all year. It must be maintained by a competent person and emptied at least yearly. Discharges started since 2015 also need to be clear of protected sites and of a public sewer within 30 metres. Environment Agency: discharges to surface water.
The same ground rules apply as for a septic tank: 2 cubic metres a day, a drainage field to BS 6297 rather than a newer soakaway pit, yearly emptying and servicing, and no Source Protection Zone 1 without a permit. Environment Agency: discharges to the ground.
Natural Resources Wales says a discharge to surface water must come from a package treatment plant, and that includes systems in use before 6 April 2010. Replace the tank with a plant, move the outlet to a drainage field, or connect to a sewer. Natural Resources Wales: registering your system.
In Wales a septic tank to a drainage field can usually be registered free with NRW if it puts less than 2,000 litres a day to ground, the property is at least 30 metres from a public foul sewer, and it is not in Source Protection Zone 1 or within 50 metres of a well, spring or borehole used for drinking water. Natural Resources Wales: registering your system.
A package treatment plant discharging less than 5,000 litres a day to a watercourse can usually be registered free with NRW, provided no public sewer is within 30 metres and the site is clear of sensitive areas. Natural Resources Wales: registering your system.
Register a plant draining to ground with NRW if it is under 2,000 litres a day and clear of Source Protection Zone 1 and drinking water sources. Registration is free and normally one-off. Natural Resources Wales: registering your system.
England's general binding rules do not apply in Scotland. What matters is the SEPA authorisation, and SEPA assesses any new or changed discharge when you register it. Registrations held on 31 October 2025 became authorisations under the Environmental Authorisations (Scotland) Regulations, and from 1 March 2026 standard conditions apply: sewage treated to the level previously authorised, the system and outfall maintained, and no significant impact on the water. An unregistered discharge needs registering. SEPA: septic tanks and private sewage treatment systems.
Discharges to land via a soakaway are covered by SEPA registration, with categories set by the number of properties and the soil's percolation value. If yours was never registered, a new registration is £198.55 at the time of writing. SEPA: septic tanks and private sewage treatment systems.
Upgrading from a septic tank to a package treatment plant needs no new SEPA application. Keep the registration, service the plant as the maker says and de-sludge it at least yearly. SEPA: changing your registration.
Keep the SEPA registration current, service the plant, and check the soakaway or infiltration system for ponding. Moving the outfall more than 10 metres or changing to a watercourse needs a new registration. SEPA: changing your registration.
A cesspit (cesspool) is a sealed tank with no outlet, so the discharge rules do not apply. It must not leak, and it fills quickly, so emptying costs are high. Many owners replace one with a treatment plant or septic tank to cut tanker bills, which needs Building Regulations approval and a suitable outlet.
Most decisions turn on the outlet. A septic tank survey traces it with a dye test or a camera, checks the tank and the field, and tells you whether the system meets the rules where you live. That is the first step for buyers and sellers in particular.
General guidance from the regulators' published rules, not legal advice. Northern Ireland has its own rules.
The outlet usually decides. Where the ground drains well, a septic tank and drainage field is the simpler, cheaper system; where it does not, or the only outlet is a stream, a plant is the answer.
| Compare | Septic tank | Treatment plant |
|---|---|---|
| What it does | Settles solids; bacteria partly break them down without air | Settles, then pumps air through so bacteria treat the effluent further |
| Effluent leaving it | Still needs the soil of a drainage field to finish treatment | Cleaner; can go to a flowing watercourse or a smaller field |
| Can it discharge to a stream? | No in England and Wales. Scotland: only as SEPA authorises | Yes, within the rules or a registration |
| Power | None | Permanent supply for the blower and alarm |
| Installed price | £4,000 to £10,000 with a new field | £6,000 to £12,000 |
| Yearly upkeep | Emptying, typically £120 to £350 | Emptying, a service and power, about £350 to £800 |
| Drainage field size | People × Vp × 0.25 m² | People × Vp × 0.2 m² |
Typical installed prices from published UK price guides, autumn 2026, read November 2026. Ground, access and the length of the drainage field move a job within these ranges more than the tank does.
| Job | Typical price |
|---|---|
| New septic tank with a new drainage field Small to large tank, two to seven days on site | £4,000 to £10,000 |
| Replacement keeping a working field | £4,000 to £6,000 |
| Sewage treatment plant, installed | £6,000 to £12,000 |
| New drainage field Large or difficult sites up to £7,000 | £1,500 to £4,000 |
| Site survey and percolation test | £250 to £500 |
| Emptying At least yearly under the binding rules | £120 to £350 |
Installed prices are ranges published by UK trade cost guides between April and September 2026, summarised by us. Running costs are a treatment plant maker's published estimate. None states clearly whether VAT is included. A guide, not a quote.
Each nation regulates small sewage discharges differently. Building Regulations Part H applies to the installation itself.
Tell us about the property and we look for an installer who works in your area. We tell you who they are before passing anything on.
The installer traces the outlet, checks access and digs test holes to see how fast the ground drains.
Tank or plant sized for the bedrooms, a building notice or plans to building control, and a check against the binding rules, or a SEPA or NRW registration.
Usually two to five days on site: excavation, a concrete base, the unit set level, pipework and the drainage field or outfall.
Ground reinstated, plant commissioned if there is one, and a written record of the system for you and any future buyer.
Installers work across rural England, Scotland and Wales. These counties have pages because their rivers, rock or rainfall change the design.
Published UK guides put a new septic tank with a drainage field at about £4,000 to £10,000, depending on size and ground, and a sewage treatment plant at £6,000 to £12,000. Try our cost estimator.
You need Building Regulations approval for a new septic tank or treatment plant, and possibly planning permission. In England the discharge must meet the general binding rules or have an Environment Agency permit. In Scotland it needs SEPA authorisation and in Wales registration with Natural Resources Wales.
In England, the Environment Agency's general binding rules say a septic tank must not discharge directly to surface water. Owners of tanks that do must replace or upgrade them; the guidance highlighted 1 January 2020 for properties sold with such a tank. Tanks draining to a working drainage field were not affected. See the septic tank regulations.
A well installed modern plastic or GRP tank, emptied on time, is commonly expected to last several decades; the drainage field usually fails first, often because solids were carried over from a tank that was not emptied.
Council guidance on Approved Document H puts the tank at least 7 metres from the habitable part of a building, within 30 metres of access for the emptying tanker, and the drainage field at least 15 metres from any building.
The work must still meet Building Regulations and the discharge rules, so you would need building control approval and a percolation test. Most owners use an installer because the digging, levels and drainage field design are where systems fail.
Three short steps: where the property is, what the system needs, and how to reach you. About two minutes.
A free matching service, not a off-mains drainage installer. We look for one who works in your area and name them to you first. Nothing is shared until you say yes.