The short answer
A new septic tank installation in the UK typically costs around £4,200–£6,000, with an average job near £5,500 including the tank, excavation, labour and a compliant drainage field where one is needed. A sewage treatment plant supplied and fitted is usually higher, commonly £9,000–£11,000. Whole projects often land between £8,000 and £15,000 once groundworks are factored in, so the realistic overall range is roughly £2,000 to £15,000. Groundworks — not the tank itself — are usually the largest part of the bill.
Price depends mainly on the system type, your ground conditions, site access, and whether a new drainage field is required. The figures below are typical installed prices from UK cost guides, not quotes.
Typical UK costs
- Septic tank installed£4,200–£6,000
- Average septic tank job~£5,500
- Treatment plant supplied & fitted£9,000–£11,000
- Drainage field£3,000–£7,000
- Tank unit alone£800–£1,500
- Overall project range£2,000–£15,000
What drives the price
- System type: a septic tank is cheaper than a sewage treatment plant, which has moving parts and treats effluent to a higher standard.
- Groundworks & access: rocky soil, clay, high groundwater or restricted access for a digger all slow excavation and raise cost.
- Drainage field: a new soakaway or drainage field, designed and percolation-tested, is often a large share of the total.
- Tank size: larger tanks cost more to supply and install but need emptying less often.
| Item | Typical figure | Notes |
|---|---|---|
| Septic tank installed | £4,200–£6,000 | tank, excavation, labour, drainage field |
| Treatment plant supplied & fitted | £9,000–£11,000 | plant typically £2,500–£5,000 of that |
| Drainage field | £3,000–£7,000 | varies with ground conditions |
| Overall project | £8,000–£15,000+ | complex sites can exceed this |
Indicative UK figures. Sources: Checkatrade, MyJobQuote 2026 cost guides.
Why a treatment plant often costs more
A sewage treatment plant uses an air pump and a biological process to treat effluent to a standard clean enough to discharge to a watercourse, where a basic septic tank cannot. That added engineering, plus the electrical connection and commissioning, is why a fitted plant commonly runs £9,000–£11,000 against £4,200–£6,000 for a septic tank. If your site has nowhere to put a compliant drainage field, a treatment plant may be the only route to compliance — which can make it the lower-cost option overall once you account for the alternative.
Frequently asked questions
How much does it cost to install a septic tank?
A septic tank installation in the UK typically costs around £4,200–£6,000, averaging near £5,500 including the tank, excavation and a drainage field. A sewage treatment plant supplied and fitted is usually £9,000–£11,000.
Why is the cost range so wide?
Because sites differ. Ground conditions, access for excavation, and whether a new drainage field is needed all move the price. Groundworks are usually the biggest single cost, so a percolation test gives the accurate figure.
Is a septic tank or a treatment plant cheaper?
A septic tank is usually the lower cost to install. A sewage treatment plant costs more because it treats effluent to a higher standard, but on some sites it is the only compliant option.
Sources & further reading
Figures on this page are typical UK ranges drawn from published sources and depend on your specific site. They are guidance, not a quotation.