Septic tank replacement and installation cost
Roughly £3,000–£7,000 for a like-for-like replacement, and commonly £5,000–£15,000 for a new tank with a drainage field.
Sewage treatment plant cost
Units from about £2,200 inc VAT (equipment only), and commonly £7,500–£15,000 supplied and installed.
What drives the price
- Ground conditions: rock, clay, peat or a high water table make excavation harder and can change the system you need.
- Discharge route: a new drainage field typically costs £3,000–£7,000 (Clear Stream Drainage; Fastfix from £3,500).
- Access: narrow lanes, steep gardens or long tracks slow the work down, and some islands need ferry freight.
- Size: the number of people, holiday letting, or several properties on one system.
- Compliance work: upgrading to meet current rules, decommissioning an old tank, approvals.
- Location: remote sites, islands and long tracks add delivery and travel costs. Clear Stream Drainage says Cornwall and Devon properties often face higher costs than the national average.
Emptying costs
| Who | Price | Source |
|---|---|---|
| Scotland: Scottish Water household de-sludge, up to 9 m³ (2026–27) | £254.40 scheduled; £353.10 unscheduled; £509.70 urgent | Scottish Water |
| Northern Ireland: NI Water domestic desludge, up to 4.5 m³ (2026–27) | one free empty every 12 months, then £108 (£142 for difficult access) | NI Water; NI Water |
| England example: private tanker firm in Sussex (prices from September 2024) | £170 for up to 1,000 gallons, rising to £380 for up to 4,000 gallons | South East Tankers |
Upgrading under the general binding rules
| Option | Typical cost | Source |
|---|---|---|
| Add-on conversion unit fitted after the existing tank (Marsh Uni:Gem, certified to EN 12566-6). Equipment only. | about £2,510–£3,010 inc VAT | Drainage Superstore; BPPlastics, Oct 2026 |
| Conversion, supplied and installed | typically around £5,000 + VAT | The Sewage Guy |
| Replacing the septic tank with a new treatment plant | commonly £7,500–£15,000 | Fastfix Drainage & Plumbing; Alton Facility Services; The Sewage Guy |
| Keeping the tank but discharging to ground through a new drainage field instead | £3,000–£7,000 for the field | Clear Stream Drainage Solutions; Fastfix Drainage & Plumbing |
In-tank retrofit kits installed after 2007 need an Environment Agency permit. Add-on units need EN 12566-6 certification to discharge to surface water without one (GOV.UK; WTE).
Why we don't publish "local prices"
We haven't seen reliable published data for septic tank prices by county, so we don't make it up. The best way to get a local price is to compare two or three quotes from installers who've visited your site. That's exactly what our free matching service does.
Frequently asked questions
Are these prices quotes?
No. They're published guide prices from installers, merchants, a manufacturer and the water authorities. Your installer will quote after looking at your site.
Do prices include VAT?
Merchants' equipment prices above are shown including VAT. Some installers quote plus VAT; for example, The Sewage Guy quotes conversions at around £5,000 + VAT. Always check whether a quote includes VAT and which items are listed separately.