← All guides · Reviewed 2026-08-05
Septic tank costs: emptying, installation and replacement
Off-mains drainage costs vary a lot with the property, so treat everything here as a ballpark and get a quote for the real figure. Our cost calculator gives an instant estimate from your own tank size, bedrooms and access.
The ranges at a glance
| Job | Typical range |
|---|---|
| Emptying (per visit) | £280–£360 (Cornwall, Aug 2026 — see below) |
| New septic tank + drainage field | £4,500–£8,500 |
| Sewage treatment plant, installed | £8,000–£15,000+ |
| Replacement / 2020-rules upgrade | £6,000–£12,000 |
| Drainage field on its own | £3,500–£7,000 |
Emptying
The routine cost most homeowners actually face. The long-run range is £180–£320 per visit, but that is not what Cornwall is paying at the moment: ringing round the county in August 2026 for a straightforward job with good access produced quotes of £300, £350 and £350. Budget £280–£360 until disposal capacity loosens.
The reason is that the constraint is not emptying your tank — it is where the tanker is allowed to unload afterwards, and the number of sites available has been cut sharply. It is seasonal, and it means booking outside July–September is worth more than negotiating. Full explanation in septic tank emptying cost in Cornwall.
Beyond that, price moves with tank size and how easily the tanker can reach it — remote or awkward-access properties cost more because the job takes longer and may need extra hose run from the road. Note too that you are paying for the visit and the disposal run rather than the volume, so a part-full tank costs close to a full one.
Most family tanks need emptying roughly once a year — though the real range is 6 months to 2 years. See how often should you empty a septic tank to work out your own interval.
This is the one cost that recurs forever, so it is worth knowing where you sit. A cesspit — which stores rather than treats — typically needs emptying several times a year instead, and that difference compounds.
New septic tank installation
A new septic tank with its drainage field typically runs £4,500–£8,500 for an average home, and more for larger properties or difficult ground.
The tank is the predictable part. The drainage field is what moves the price: soil that drains poorly needs a bigger field, and a bigger field means more excavation, more stone and more pipe. This is why a percolation test comes before a firm quote — nobody can price the field until they know how the ground behaves.
More detail: new septic tank installation.
Sewage treatment plant installation
A packaged treatment plant usually costs £8,000–£15,000+ installed. It is more than a septic tank because the unit is more sophisticated, needs an electrical supply, and carries an ongoing servicing commitment.
It is the common choice when a septic tank cannot legally be used — most often where there is no viable drainage field, or where the discharge has to go to a watercourse. Septic tank vs sewage treatment plant covers which applies to you.
Replacement and 2020-rules upgrades
Replacing a failing or non-compliant septic tank typically lands in the £6,000–£12,000 range. What decides it: whether you move to a treatment plant or add a drainage field, and how much old kit has to come out and be disposed of.
Not sure whether you actually need to replace yours? See do I need to replace my septic tank? — the answer is no more often than people expect. If you are selling, buying or selling a house with a septic tank explains when the upgrade becomes unavoidable.
Drainage field / soakaway
A new or replacement drainage field on its own is often £3,500–£7,000, driven mainly by ground conditions and the size the soil requires.
Worth knowing: a failed drainage field is usually the consequence of a tank that went too long between empties. It is the most expensive routine failure in off-mains drainage, and the most preventable. See septic tank soakaways and drainage fields explained.
The four things that move the price
- Access. Can a tanker or digger get close, or is it a long, tight or remote approach? In rural Cornwall this is the most common reason two similar properties get different quotes.
- Property size. More bedrooms means more capacity and a larger system.
- Ground conditions. Soil type and water table strongly affect drainage-field cost, and they are the least predictable input until tested.
- Permits and sensitive locations. A Source Protection Zone, or a discharge above the volume thresholds, can require a permit rather than falling under the General Binding Rules.
Getting a real number
Ranges are useful for budgeting and useless for deciding. For a figure tailored to your property, use the cost calculator, or check where your system stands against the rules first with the free compliance checker — it is worth knowing whether you are pricing a routine job or a compliance-driven one before you call anyone.
Sources: GOV.UK — Permits you need for septic tanks · GOV.UK — Find out if you need a permit
Common questions
How much does it cost to empty a septic tank?
Budget £280-£360 per visit for a typical domestic tank on current Cornwall prices — quotes collected across the county in August 2026 came back at £300-£350 for a straightforward job with good access. The usual long-run range is lower, around £180-£320, but disposal capacity is tight: the number of sites where tankers can unload has been cut sharply, so operators drive further and price that in. The other variables are tank size and how easily a tanker can reach it. Because you are paying for the visit and the disposal run rather than the volume, a part-full tank costs close to a full one.
How much does a new septic tank cost to install?
Around £4,500-£8,500 for an average home, including the drainage field. The tank itself is the predictable part; the drainage field is what moves the number, because poorly draining soil needs a larger and more expensive field.
How much is a sewage treatment plant?
Usually £8,000-£15,000+ installed. It costs more than a septic tank because the unit is more sophisticated, needs an electrical supply, and requires ongoing servicing. It is the usual choice where a septic tank cannot legally be used — for example where there is no viable drainage field.
What does it cost to replace a non-compliant septic tank?
Typically £6,000-£12,000, depending on whether you move to a treatment plant or add a drainage field, and how much old equipment has to be dug out and disposed of. Replacements driven by the 2020 rule change — discharging directly to a watercourse — sit in this range.
Why do septic tank quotes vary so much?
Four things: access, property size, ground conditions and whether a permit is needed. Ground conditions are the biggest single unknown, which is why a percolation test usually happens before anyone can give a firm price for a new drainage field.
Is it cheaper to repair or replace a septic tank?
Repair, where the tank itself is sound and the problem is localised. But if the drainage field has failed, replacement of the field is the work — and at that point it is worth pricing a full upgrade, because much of the cost is the same groundwork either way.
Need this sorting in Cornwall?
Kernow Septic covers Cornwall's towns and rural parishes. Call or run the free tools.