Installation costs at a glance
The two market guides agree closely at the middle of the range and differ mainly on how far the tails stretch. MyJobQuote splits the market by system size.
| System | Cost | Source |
|---|---|---|
| Typical installation | £4,200–£6,000 | Checkatrade 2026, includes tank + standard excavation |
| Average | Around £5,500 | MyJobQuote 2026 |
| Small system | £2,500–£5,000 | MyJobQuote 2026 |
| Medium system | £4,500–£7,000 | MyJobQuote 2026 |
| Large system | £7,000–£10,000+ | MyJobQuote 2026 |
| Time on site | 2–7 days | MyJobQuote 2026 |
What the price includes
At the £4,200–£6,000 level you should expect the tank itself, the excavation, and a standard installation: the tank bedded in, pipework connected, and the site reinstated to a reasonable condition. Checkatrade’s guide is explicit that this band assumes standard conditions. Before comparing quotes, confirm in writing what each installer includes — delivery, digger time, backfill, connections, and any drainage field work are the usual dividing lines.
What blows the budget
Ground conditions, access, the drainage field and building control move the final price more than the tank sticker price does. A tank costs roughly the same wherever it is dug in; the ground around it does not:
- Ground conditions — rock, high water table or poor ground can mean extra excavation or a different installation method.
- Access — if the digger and tank delivery cannot reach the site, the job slows down and costs more.
- Drainage field — a new or replacement drainage field is often priced separately and is one of the biggest add-ons.
- Building control — notification and inspections are part of a proper job; a competent installer will include or itemise them.
If a quote looks far outside the £4,200–£6,000 band, ask what site condition drove it. If you need a sense of what size tank your household requires, use the septic tank size calculator.
Treatment plant alternative
Where discharge rules demand a higher treatment standard, a sewage treatment plant is the alternative: supply and installation often runs £9,000–£11,000, per Checkatrade’s domestic treatment plant guide. See septic tank vs treatment plant for when that becomes necessary.
Replacement is a different job
Replacing an existing tank is not the same as a fresh install — the old tank has to come out, and the condition of the drainage field decides whether you need a full new system. See septic tank replacement cost.