| Item | Typical cost | Source |
|---|---|---|
| Septic tank unit only, about 2,800–3,100 litres (5–6 people). Equipment only: no delivery offload, excavation or installation. | about £1,180–£1,220 inc VAT | Clarkes of Walsham; Tanks For Everything, Oct 2026 |
| Larger septic tank unit only, 5,000 litres. Equipment only. | about £1,890 inc VAT | Tanks For Everything, Oct 2026 |
| Like-for-like tank replacement, supplied and installed, where the existing drainage field can be kept | roughly £3,000–£7,000 | Alton Facility Services; Clear Stream Drainage Solutions |
| New drainage field (sized from a percolation test), supplied and installed | £3,000–£7,000; one installer advertises from £3,500 (1–3 bed) and from £4,500 (4+ bed) | Clear Stream Drainage Solutions; Fastfix Drainage & Plumbing |
| Percolation test | around £300–£500 | Clear Stream Drainage Solutions |
| New septic tank with a drainage field, supplied and installed | from about £5,500 on a straightforward site; commonly £5,000–£15,000 | Fastfix Drainage & Plumbing; Clear Stream Drainage Solutions |
| First-time installation or difficult site (rock, clay, high water table, poor access) | £12,000–£18,000 or more | Homebuilding & Renovating; Clear Stream Drainage Solutions |
What's usually included
As an example, Fastfix's from-£5,500 package covers an EN 12566-1 certified tank, the drainage field, pipework, excavation, electrics if required, and compliance certification. The tank itself is the smaller part of the bill: merchants list a 5–6 person septic tank at about £1,200 inc VAT. Alton Facility Services says a proper quotation should list plant supply, excavation, pipework, electrical works, commissioning, waste disposal, tanker attendance and reinstatement separately.
Extras that change the price
- A new drainage field: £3,000–£7,000, averaging about £5,000 (Clear Stream Drainage; Fastfix from £3,500). If your old field has failed or was never compliant, this is often the biggest extra.
- Switching to a treatment plant: where you can't use a drainage field or must discharge to a watercourse. See the treatment plant costs.
- Decommissioning the old tank: emptying it via a registered waste carrier, then removing or filling it.
- Difficult ground or access: rock, clay, high groundwater, steep or narrow sites.
- Approvals: building control fees, and in Scotland a SEPA registration fee. SEPA lists £198.55 for a septic tank registration for up to 10 homes. Registration in Wales is free.
Repairs instead of replacement?
Not every problem needs a new tank. An overdue empty, a blocked pipe or a failed pump can cause similar symptoms, so ask an installer to inspect before you commit to a full replacement. 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
In England, a septic tank that discharges to a ditch or stream must be upgraded. The usual options are an add-on conversion unit, a new treatment plant, or a new drainage field.
| 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).
Get a local price
Ground conditions vary enormously, even between neighbours. Comparing two or three site-visit quotes is the most reliable way to see what your job will cost.
Frequently asked questions
What's the cheapest way to fix a failing septic tank?
If only the tank has failed and the drainage field still works, a like-for-like replacement (roughly £3,000–£7,000, according to installers' published guides) is usually cheapest. If it discharges to a ditch in England, an add-on conversion unit (around £5,000 + VAT fitted) can be cheaper than a new treatment plant. If the drainage field has failed too, expect to add £3,000–£7,000.
Is septic tank replacement covered by home insurance?
Sometimes, if sudden damage is the cause, but gradual wear usually isn't covered. Check your policy wording or ask your insurer.
Who pays for septic tank replacement when selling a house?
It's negotiable between buyer and seller. In England a septic tank discharging to surface water must be dealt with as part of the sale, so get quotes early.