Plenty of Cornish plots, from farm conversions to infill sites in small villages, aren't close to a mains sewer. If that's your situation, how you deal with foul water and rainwater is one of the first things to settle, because it affects the layout of your site, your budget and sometimes your planning and Building Regulations applications. This guide gives a general overview of septic tanks, treatment plants, cesspools and soakaways, and the checks that usually come before any groundworks start.
Mains drainage or private drainage?
Connecting to the public sewer is normally the preferred option where it's reasonably available, and Cornwall Council and the Environment Agency will usually expect you to consider it first. In Cornwall the sewerage provider is generally South West Water, so it's worth asking whether a connection is possible and what it would involve, including crossing third-party land.
If a connection isn't practical, you'll need a private system. The main options are:
Septic tank. A sealed chamber that separates solids from liquid. The liquid then flows to a drainage field (soakaway) where the ground treats it.
Package treatment plant. A small mechanical unit, usually needing electricity, that treats sewage to a higher standard before it's discharged to a drainage field or, in some cases, to a watercourse.
Cesspool. A sealed tank with no outlet that simply stores waste and has to be emptied regularly. It's usually a last resort because of the ongoing tanker costs.
Which one suits your site depends on ground conditions, the size of the property, distance to watercourses and the rules that apply where you are.
The general binding rules and permits
In England, small sewage discharges from private systems are covered by the Environment Agency's general binding rules. They set out standards for the system, how it's installed and maintained, and where it can discharge. As a general guide:
A septic tank shouldn't discharge directly into a ditch, stream or other surface water. It should discharge to a drainage field.
Treatment plants may be allowed to discharge to surface water if the rules are met.
Some situations call for an environmental permit instead, such as larger discharges or sites in sensitive locations like groundwater protection zones.
If you're replacing or altering an existing system, you may need to check that it still meets current rules.
These rules change from time to time, so check the current Environment Agency guidance for your site, and ask the designer or drainage specialist to confirm what applies.
Building Regulations Part H
Approved Document H of the Building Regulations covers drainage and waste disposal. It sets out standards for foul and surface water drainage, including pipe falls, access points, protection from pests and the design of private treatment systems and drainage fields. Building Control will usually inspect drainage at key stages, often before trenches are backfilled, so it's important to leave pipework visible until it has been signed off.
Our Building Regulations guide for Cornwall explains how inspections generally work across a project.
Percolation tests and ground conditions
A drainage field only works if the ground can absorb treated water. A percolation test measures how quickly water drains away in the soil at the proposed depth, and the result helps decide the size of the drainage field and whether it's viable at all. In Cornwall, ground varies enormously, from quick-draining sandy soils near the coast to heavy clay and shallow rock inland, and a high water table can rule out some locations.
You'll usually need:
A percolation test, carried out in the right season and in the right location.
A site assessment showing the position of watercourses, boreholes, springs, buildings and boundaries, as there are minimum separation distances.
Space for a drainage field, which can be larger than people expect, plus room for a replacement area in the future.
If the site fails the test, alternatives such as a different treatment system or a sealed tank may be needed, so it's wise to test before you buy land or finalise a design. Our guide to groundworks and foundations in Cornwall covers ground investigation more widely.
Surface water and soakaways
Rainwater from roofs and hard surfaces needs a separate route. Building Regulations generally expect surface water to go to a soakaway first, then to a watercourse, then to a sewer, so a soakaway is often the starting point on rural sites. A soakaway is typically a pit or crate system that lets water seep into the ground, sized using the roof and paving area and a soil infiltration test. It generally needs to be a safe distance from buildings and boundaries, and shouldn't be placed where it could affect a neighbour or an unstable slope.
Where soakaways aren't practical, other sustainable drainage options can help, such as permeable paving, swales, or attenuation tanks that release water slowly. A resin driveway can be laid as a permeable surface, which may reduce the amount of water you need to manage.
Planning your system into the project
Drainage affects design as much as budget. Good practice is to:
Check drainage options before committing to a plot or a final design.
Put treatment units and drainage fields where machines can reach them for installation and emptying.
Keep them clear of planned extensions, driveways and trees.
Make sure your planning application shows the proposed arrangement, as councils often ask for details of foul drainage.
Arrange regular emptying and servicing. Septic tanks typically need emptying about once a year, and treatment plants need servicing to the manufacturer's schedule.
Whether it's new builds, a barn conversion or a replacement dwelling, drainage should be part of the early conversation. If you're planning a rural project around Bodmin or Launceston, this is especially worth sorting out early.
Septic tank and drainage FAQs
Do I need planning permission for a septic tank?
Not always on its own, but foul drainage is usually considered as part of a planning application for a new home or conversion. Check with Cornwall Council for your site.
Can I install a septic tank myself?
Installation should meet Building Regulations and the general binding rules, so most people use a competent installer. Building Control may need to inspect the work.
What if I'm buying a house with an old septic tank?
Existing systems may not meet current rules, particularly if they discharge to a watercourse. A survey and advice from a drainage specialist are sensible before you buy.
How big does a drainage field need to be?
It depends on the percolation test result, the number of people the property is designed for and the system used. A designer or specialist will calculate it for your site.
Talk to us about your project
We can help you plan groundworks and drainage alongside the rest of your build. Call Cornish Building Company on 01208 816991 or contact us to talk through your site.
