Septic tank installation in Cornwall
A large share of Cornish homes sit off mains drainage: farmhouses, converted barns, chapel conversions and holiday lets down lanes where no sewer will ever reach. Many of their septic tanks were put in decades ago, some draining to the nearest stream, and replacement work is steady across the county.
Two things make Cornwall harder than average: ground that often drains badly, and a coastline full of protected waters that limits where treated effluent can go.
New septic tank and drainage field£4,000 to £10,000a guide, not a quote
Ground and water in Cornwall
Much of Cornwall is granite (around Bodmin Moor, St Austell, Carnmenellis and Land's End) or the slates and shales known locally as killas. Weathered granite can drain well; killas and the clay-rich soils over it often do not, and many sites have a high winter water table. That is why a percolation test, dug in the wet season if possible, is the first job on most Cornish replacements rather than a formality.
Where the ground fails the test, the usual answer is a sewage treatment plant discharging to a watercourse. The Environment Agency's binding rules for surface water discharges do not cover a new discharge in or within 500 metres of a designated bathing water or protected shellfish water, and Cornwall has a long list of both along its coast and estuaries. Near those, a permit or a sewer connection is likely. Check the outlet location on the government's Magic map before a design is fixed.
Holiday lets
A holiday cottage is sized differently from a family home. Under the rules, commercial and holiday properties use British Water's Flows and Loads guidance, adding every source together. A barn sleeping ten in summer may need a bigger plant than its bedroom count suggests.
What to watch for in Cornwall
- Outlets near bathing waters, estuaries and shellfish waters
- Killas and clay soils that fail percolation tests
- High winter water tables in valley bottoms
- Holiday lets sized for summer occupancy
- Narrow lanes that limit digger and tanker access
Building Regulations approval for a new tank or plant goes to Cornwall Council building control (or an approved inspector). The Isles of Scilly have their own council.
Work installers take on in Cornwall
- Septic tank replacement: Old, cracked or non-compliant tank out; a new tank or a treatment plant in.
- Sewage treatment plant installation: Powered treatment that can discharge to a stream or ditch where a septic tank cannot.
- Drainage field installation: The trenches that return treated effluent to the ground, sized from a percolation test.
- Septic tank survey: An inspection for a sale or purchase, or before deciding whether to replace.
Typical prices
| Job | Typical price |
|---|---|
| New septic tank with a new drainage field | £4,000 to £10,000 |
| Sewage treatment plant, installed | £6,000 to £12,000 |
| New drainage field | £1,500 to £4,000 |
| Site survey and percolation test | £250 to £500 |
Questions from Cornwall
How much does septic tank installation cost in Cornwall?
Published prices are national: about £4,000 to £10,000 for a new septic tank and drainage field, or £6,000 to £12,000 for a treatment plant. Difficult ground and access in parts of Cornwall can push jobs towards the top of those ranges.
Can my septic tank drain into a stream in Cornwall?
Not any more in England. A septic tank discharging to a watercourse must be replaced or upgraded, usually with a treatment plant or a drainage field. Near bathing or shellfish waters even a treatment plant may need a permit.
Do I need to register my septic tank in Cornwall?
No. England has no register for small discharges that meet the general binding rules. If your system cannot meet them, you need an Environment Agency permit.
Is a treatment plant better for a Cornish holiday let?
Often, because it can discharge to a watercourse and copes with variable loads when sized correctly. It needs power and a yearly service, so plan for that in the letting budget.
Ask for a site visit in Cornwall
Three short steps: where the property is, what the system needs, and how to reach you. About two minutes.
A free matching service, not a off-mains drainage installer. We look for one who works in your area and name them to you first. Nothing is shared until you say yes.
Thanks, your enquiry is in.
- We look for an off-mains drainage installer who works in your area and takes on this kind of job, then email or call to tell you who it is.
- Your details reach that firm only if you say yes.
- They arrange a site visit, test the ground if needed and give you a written price. No marketing from us, ever.