Can I keep my existing septic tank when I extend?

Sometimes. It depends on the condition of the tank, whether it discharges lawfully, and whether the extra bedrooms push the system beyond its designed capacity. It should be assessed rather than assumed.

How much space does a drainage field need?

The area follows the percolation result and the number of bedrooms, so it varies widely. On tight plots the field, not the house, is often what limits the scheme.

Is a treatment plant always better?

It is more tolerant of difficult ground and produces a cleaner effluent, but it uses electricity, needs servicing and costs more to own. Where a drainage field works, a well built tank and field is a simple, long lived solution.

Building control inspects the installation as part of the works, and where a discharge to a watercourse is involved the environmental permitting regime applies as well.

Services · Cornwall

Off mains drainage in Cornwall, what rural sites have to prove before anything else works

A very large share of Cornish properties are not connected to a public foul sewer, and for anyone extending a rural cottage, converting a barn or building on a paddock, foul drainage is not a detail to resolve later. It affects where the building can sit, how many bedrooms it can have, and in some cases whether the scheme is possible at all. It is worth settling in the first fortnight rather than the last.

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Septic tank or treatment plant

A septic tank separates solids and discharges partially treated effluent to a drainage field where the ground finishes the job. A package treatment plant aerates the effluent and produces a cleaner discharge that may be released to a watercourse under permit. Tanks are simpler and cheaper to run but need suitable ground and space. Plants tolerate tighter or wetter sites but need power and servicing. The ground normally decides, not preference.

Septic tanks may not discharge directly to a watercourse under the general binding rules
Drainage fields need a percolation test and a minimum area based on the result
Separation distances apply to buildings, boundaries, watercourses and any private water supply
Plants need an electrical supply, an access route for tankering and a service regime

Percolation testing is the gate

The test measures how quickly water drains through the soil in the actual location of the proposed drainage field, and the result sets the size of the field. Cornish ground varies enormously across a single field, from free draining sandy loam to clay to shattered rock over granite. Testing early is cheap. Discovering after planning approval that the ground will not accept a field, and that the only remaining option needs a discharge permit, is not.

Extensions and additional bedrooms

Adding bedrooms increases the design population of the system, and an existing tank sized for a two bedroom cottage may not lawfully serve a four bedroom house. Building control will ask. Where the existing installation is old, undocumented or clearly failing, the honest route is to design the replacement into the project budget rather than hope the question is not raised at inspection.

What the application should show

A site plan with the tank or plant position, the drainage field layout, separation distances, levels and the outfall or discharge point answers most questions in one drawing. Where a permit is required, the timeline for that runs alongside the planning application rather than after it, and knowing that at the start keeps the programme honest.

Frequently asked

Straight answers.

Can I keep my existing septic tank when I extend?: Sometimes. It depends on the condition of the tank, whether it discharges lawfully, and whether the extra bedrooms push the system beyond its designed capacity. It should be assessed rather than assumed.
How much space does a drainage field need?: The area follows the percolation result and the number of bedrooms, so it varies widely. On tight plots the field, not the house, is often what limits the scheme.
Is a treatment plant always better?: It is more tolerant of difficult ground and produces a cleaner effluent, but it uses electricity, needs servicing and costs more to own. Where a drainage field works, a well built tank and field is a simple, long lived solution.
Who signs it off?: Building control inspects the installation as part of the works, and where a discharge to a watercourse is involved the environmental permitting regime applies as well.

Rural site with no mains drainage?

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