Guide
Sewage treatment plant installation cost: every stage, priced
Updated
Sewage treatment plant installation cost is most of the bill. Premier Tech, which makes treatment plants, puts the installation work at £6,000 to £11,000 on average in the UK and a whole off-mains project at usually £8,000 to £15,000, before the plant is chosen.
Stage by stage
| Stage | Average cost in the UK |
|---|---|
| Trial hole test | from around £1,000 |
| Percolation test | £500 |
| System design | £500 to £2,000 |
| The treatment system | £2,000 to £4,000 |
| Installation: excavation, groundworks, equipment and labour | £6,000 to £11,000 |
| Total project | usually £8,000 to £15,000 |
Source: Premier Tech Aqua: estimated costs to install a septic system in the UK
What a public body paid
Winchester City Council published the contract for supplying and installing a package treatment plant at Itchen View: £44,220 including VAT, signed 28 April 2025, with Proseptic Limited. That is a larger, non-domestic plant, but it is a real price paid, read on the notice itself.
Source: Find a Tender: supply and installation of a sewage treatment plant, Itchen View
What pushes the price up
- Ground conditions found by the trial hole and percolation test, which decide the size of the drainage field.
- Access for a digger, and how deep the excavation has to go.
- A pumped outfall where the ground will not take the water by gravity.
- Discharging to a watercourse, which needs a plant rather than a septic tank under the general binding rules.