Septic tank, sewage treatment plant or cesspool: which off-mains system to choose in the UK
Off the mains in the UK you choose between a septic tank with a drainage field, a sewage treatment plant and, as a last resort, a cesspool [1]. A septic tank with drainage field costs about £5,500 installed on average [2]; a treatment plant usually £6,000–12,000 [3], but it is the only system allowed to discharge to a stream [4]. All prices are approximate, as of September 2026.
Updated 2026-09-29 · every regulation and price links to its source at the bottom of the page. A guide, not a construction design.
Which off-mains systems are allowed in the UK?
Whatever you choose, a new tank or plant must meet BS EN 12566, shown by CE marking, a certificate or a British Water listing [5], and be sized to British Water’s ‘Flows and Loads’ guidance [6].
- Septic tank with drainage field. The tank only settles solids; the effluent must then be treated in the soil through a drainage field, a drainage mound or a constructed wetland (reed bed) [7]. It may discharge only to ground.
- Sewage treatment plant (package plant). A powered tank that treats sewage biologically. It may discharge to a watercourse that flows all year, up to 5 m³ a day [8] [9], or to a drainage field 20% smaller than one after a septic tank [10].
- Drainage mound or engineered ground. For soil that is unsuitable for a normal field; allowed only outside the floodplain, built to BS 6297:2007, with engineered soil giving a percolation value of at least 15 s/mm [11].
- Cesspool. A sealed tank with no treatment, only when nothing else is possible. It must hold at least 18,000 litres for two users plus 6,800 litres for each extra user [12]: for four people that is 31,600 litres (example calculation), and every litre leaves by tanker.
How much does a septic tank or treatment plant cost in the UK?
| What | Typical cost (approximate, as of September 2026) |
|---|---|
| Septic tank installed with drainage field | About £5,500 on average [2] |
| Septic tank only | £800–1,500: plastic £800–1,200, glass-reinforced plastic £1,000–1,400, concrete £1,200–2,000 or more [2] |
| Factory-made 2,800-litre tank for 4–5 people | About £930–1,105 ex VAT [13] |
| Site survey and percolation test | £250–500 [2] |
| Building control approval | £150–400 [2] |
| New drainage field | £1,500–4,000; replacing a failed one £1,500–5,000 [14] |
| Sewage treatment plant installed | £6,000–12,000, plus about £200 a year for electricity [3] |
| Treatment plant unit only, 5 people | About £2,080 ex VAT (£2,496 inc VAT), without installation [15] |
| Emptying and maintenance | About £200 a year [2] |
Plastic, fibreglass, concrete or brick: which tank?
Approved Document H accepts factory-made tanks of glass-reinforced plastic, polyethylene or steel built to BS EN 12566-1. It also describes tanks built on site: engineering brick at least 220 mm thick on 1:3 cement–sand mortar, or in-situ concrete at least 150 mm thick of C25/P mix [16].
The catch: in England the general binding rules require new systems to meet BS EN 12566 [5], and a tank built on site has no CE marking or certificate. We have not found official guidance reconciling the two, so check with building control and the Environment Agency before building a brick or concrete tank.
Precast concrete rings are familiar from soakaways, but for sewage the Environment Agency says: “You should avoid using non-standard systems such as boreholes, shafts, concrete rings or similar structures for new discharges to ground.” [17]
When is a sewage treatment plant worth the extra cost?
The purchase price is only part of it. A plant that needs electricity must cope with up to 6 hours without power or have an uninterruptible supply [18]. Natural Resources Wales says most plants need professional servicing every 12 months and detailed checks every 6 months [19]. Expect around £200 a year for electricity (approximate, as of September 2026) [3].
- You have no suitable soil but a stream that flows all year: only a treatment plant may discharge to surface water [4]. A short partial drainage field before the outfall is allowed only with a plant, with the perforated section no more than 10 m from the bank [20].
- Your plot is tight: after a plant the drainage field area uses a factor of 0.20 instead of 0.25 [10]. Example calculation: a four-bedroom house (six people) on soil with Vp 30 s/mm needs 45 m² of trench floor after a septic tank and 36 m² after a plant.
- Your existing septic tank discharges to a watercourse and must be upgraded [4].
Size a septic system for your plot →
What is the law and what is just common practice?
| Point | Status |
|---|---|
| Tank or plant to BS EN 12566, drainage field to BS 6297:2007 | Required for new systems in England [5] |
| Sizing to British Water ‘Flows and Loads’ | Required for systems installed since 1 February 2006 [6] |
| Building Regulations approval | Required for a new tank or plant; planning permission sometimes too [21] |
| Emptying before the tank is full | Required; ‘at least once a year’ is a recommendation [22] |
| Repairing a faulty system, by a competent person | Required [23] |
| Service contract for a treatment plant | Advice from Natural Resources Wales, not a legal rule [19] |
Frequently asked questions
Is a septic tank cheaper than a sewage treatment plant?
Usually: about £5,500 installed on average for a septic tank with drainage field [2] against £6,000–12,000 for a plant, plus its running costs [3] (approximate, as of September 2026).
Can I install a concrete ring septic tank in the UK?
Concrete rings and shafts should be avoided for new discharges to ground [17], and new tanks must meet BS EN 12566 [5].
Do I need a treatment plant to discharge into a stream?
Yes. A septic tank may not discharge to surface water; a treatment plant may, and only to water that flows all year [4] [9].
How much does a new drainage field cost?
Around £1,500–4,000, or £1,500–5,000 to replace a failed one (approximate, as of September 2026) [14].
When is a cesspool allowed?
Only when no other option is practicable; it must hold at least 18,000 litres for two users [12].
Sources
- Approved Document H (Building Regulations 2010, England), 2015 edition — H1, Requirement H1(1) (UK) · regulation · checked 2026-09-29 · source
- MyJobQuote — Septic Tank Installation Cost Guide (UK Prices 2026), Average cost; cost breakdown (UK) · price · checked 2026-09-29 · ✓ · source
- MyJobQuote — Septic Tank Installation Cost Guide (UK Prices 2026), Sewage treatment plants (UK) · price · checked 2026-09-29 · ✓ · source
- Environment Agency General Binding Rules: small sewage discharges (England), Surface water — Use the correct treatment system (UK) · prohibition · checked 2026-09-29 · source
- Environment Agency General Binding Rules: small sewage discharges (England), Make sure your treatment system meets the relevant British Standard (UK) · regulation · checked 2026-09-29 · source
- Environment Agency General Binding Rules: small sewage discharges (England), Make sure your treatment system is installed and operated correctly, and has enough capacity (UK) · regulation · checked 2026-09-29 · source
- Approved Document H (Building Regulations 2010, England), 2015 edition — H2, H2 para 1.2, 1.15 (UK) · regulation · checked 2026-09-29 · source
- Environment Agency General Binding Rules: small sewage discharges (England), Rules that apply to all discharges — Only discharge 2 cubic metres or less a day (UK) · regulation · checked 2026-09-29 · source
- Environment Agency General Binding Rules: small sewage discharges (England), Surface water — Make sure the surface water has flow; Do not discharge to an enclosed lake or pond (UK) · prohibition · checked 2026-09-29 · source
- Approved Document H (Building Regulations), H2, H2 1.34-1.37 (UK) · regulation · checked 2026-09-29 · source
- Environment Agency General Binding Rules: small sewage discharges (England) (UK) · regulation · checked 2026-09-29 · source
- Approved Document H (Building Regulations 2010, England), 2015 edition — H2, H2 para 1.13–1.14, 1.59–1.61 (UK) · regulation · checked 2026-09-29 · source
- Tanks Direct — Klargester Alpha 2800 L septic tank; JDP — Marsh 2800 L horizontal septic tank (UK) · price · checked 2026-09-29 · ~ · source
- MyJobQuote — Septic Tank Installation Cost Guide (UK Prices 2026), Drainage field costs (UK) · price · checked 2026-09-29 · ✓ · source
- JDP — 5 Person GRAF One2Clean Sewage Treatment Plant (UK) · price · checked 2026-09-29 · ~ · source
- Approved Document H (Building Regulations 2010, England), 2015 edition — H2, H2 para 1.19–1.20 (UK) · regulation · checked 2026-09-29 · source
- Environment Agency General Binding Rules: small sewage discharges (England), If you use a non-standard system (such as a well, borehole and soakaway) (UK) · recommendation · checked 2026-09-29 · source
- Approved Document H (Building Regulations 2010, England), 2015 edition — H2, H2 para 1.55–1.56 (UK) · regulation · checked 2026-09-29 · source
- Natural Resources Wales — Running and maintaining a septic tank or small sewage plant, Maintaining and servicing your PSTP (UK) · recommendation · checked 2026-09-29 · ? · source
- Environment Agency General Binding Rules: small sewage discharges (England), Surface water — Make sure any partial drainage field is close enough to the watercourse (UK) · regulation · checked 2026-09-29 · source
- Environment Agency General Binding Rules: small sewage discharges (England), Get building regulations and planning approval (UK) · regulation · checked 2026-09-29 · source
- Environment Agency General Binding Rules: small sewage discharges (England), Get your treatment system regularly emptied (UK) · regulation · checked 2026-09-29 · source
- Environment Agency General Binding Rules: small sewage discharges (England), Get your treatment system regularly maintained (UK) · regulation · checked 2026-09-29 · source
🌐 🇫🇷 Selon la réglementation française · 🇩🇪 Nach deutschen Vorschriften · 🇺🇸 US rules · 🇵🇱 Według przepisów polskich · 🇺🇦 За нормами України