Documents where a septic tank serves the existing property

Where an existing property is served by a septic tank, the planning application needs to show that the drainage arrangement will still work after the development, and that it complies with the general binding rules for small sewage discharges. The septic tank development documents councils and the Environment Agency expect are a survey of what is there now, percolation test results, a drainage field design sized for the increased load, and a statement of whether an environmental permit is needed. Adding bedrooms to a house on a septic tank is a drainage project as much as a building project.

Start with the discharge point, because that single fact decides whether the existing system is lawful at all.

Why existing drainage affects the application

National policy sets a drainage hierarchy for foul water. Connect to the public sewer where that is reasonably available, then a package treatment plant, then a septic tank, and the applicant has to show why each higher option was rejected.

So the first question on an unsewered site is how far the nearest public sewer is and what a connection would cost. Councils expect a reasoned answer rather than an assertion, and a sewerage undertaker's pre-development enquiry response is the evidence that settles it.

The second question is capacity. A septic tank sized for a three bedroom house serving a six bedroom house with an annexe will not perform, and the planning condition that follows will require an upgrade anyway. Work out the population equivalent for the completed development before you design the layout, because the drainage field takes land that the garden design probably assumed was available.

The binding rules and when a permit is needed

Small sewage discharges in England are governed by general binding rules administered by the Environment Agency. Meet them and no environmental permit is required. Fail any one of them and you need a permit under the environmental permitting regime.

The volume limits are the starting point: up to 2 cubic metres a day for a discharge to ground, and up to 5 cubic metres a day for a discharge to surface water. Beyond those figures a permit is needed regardless of anything else.

Then the rule that catches most older properties. A septic tank must not discharge directly to a surface water such as a ditch, stream or river. Since January 2020 those arrangements have had to be replaced, either by connecting the tank to a drainage field so the discharge goes to ground, or by replacing the tank with a package treatment plant whose cleaner effluent may discharge to surface water. If the property you are developing still discharges straight to a ditch, that has to be dealt with as part of the scheme, and the detail of the rules is set out in the treatment of package treatment plants and the general binding rules.

The siting rules matter as much as the volumes. A drainage field must not be within 50 metres of a well, borehole or spring used for drinking water, nor within 10 metres of a watercourse or any other permeable drain, nor within 15 metres of a building, and it must not sit in a groundwater source protection zone 1. Where the site is in a source protection zone, the wider document set described for a scheme in a source protection zone applies as well.

Upgrading to a treatment plant

A package treatment plant produces a cleaner effluent than a septic tank because it treats the sewage biologically rather than simply settling it. That is why it can discharge to surface water within the binding rules while a septic tank cannot.

Three practical consequences follow. A plant needs a power supply, so the layout has to carry a cable to it and the running cost falls on the occupier. It needs servicing at the manufacturer's stated interval, and councils increasingly condition a maintenance arrangement. And it needs vehicle access for desludging, which means a tanker route within a stated hose distance of the tank, drawn on the site plan.

A plant discharging to surface water also needs a defined outfall that you have the right to use. Discharging into a ditch on a neighbour's land without an easement is a problem that planning permission does not solve.

Drainage field sizing and percolation testing

The drainage field is the part applicants underestimate, and it cannot be designed without testing.

Percolation testing follows the standard method: trial holes at the proposed depth, pre-soaked, then timed to establish a percolation value, repeated at several positions across the proposed field. The value feeds a calculation of the area required for the design population. Clay soils give long percolation times and very large fields, and in some cases fail outright, which is when a mound system or a different discharge route has to be considered.

Two tests are needed, not one. A trial hole to establish groundwater level and soil profile, and the percolation test itself. Groundwater within a short distance of the base of the field rules out the design, and testing in a dry August gives a misleading answer, so record the date and the preceding weather.

Submit the raw results rather than a conclusion. A Foul Sewage Assessment with the trial hole logs, the percolation figures, the calculation and the resulting field layout is what allows an officer to condition the scheme rather than refuse it for lack of information. The same evidence base is described for unsewered sites generally in the guidance on foul drainage, package plants and permits.

Position against buildings and boundaries

The drainage field has to go somewhere that works and stay available for the life of the development.

Keep it clear of buildings by the required distance, out from under hard surfacing and parking, away from tree root zones that will block the pipework, and off land you might later want to develop or sell. Levels matter: a field needs a shallow even fall and cannot be placed uphill of the tank without pumping.

Two layout mistakes recur. Putting the field in the only part of the garden that the new house overlooks, so that the occupier later covers it with a patio. And splitting a plot so that the drainage field for one dwelling sits in the curtilage of another, which needs an easement and a maintenance agreement rather than goodwill.

Show the field, the tank, the vent position, the tanker access and the measured distances on the submitted site plan. An officer who can read the 50 metre and 15 metre distances off the drawing has no reason to ask for more.

Conditions that commonly follow

Expect three conditions on an unsewered scheme.

First, a condition requiring details of the foul drainage arrangement to be submitted and approved before development begins, or before occupation. Get ahead of it by submitting the design with the application, because the percolation testing takes time and cannot be done in a wet week.

Second, a condition requiring the system to be installed in accordance with the approved details and retained and maintained thereafter. Where a treatment plant serves more than one dwelling, the maintenance arrangement is usually secured by a planning obligation rather than a condition.

Third, a condition requiring any necessary environmental permit to be obtained. That is a reminder rather than an authorisation, and the permit application runs on its own timescale with the Environment Agency.

One honest uncertainty. Councils differ on how much drainage detail they want at application stage, and some are content to condition everything. Submitting the percolation results anyway costs little and removes the risk of a refusal on inadequate information, which is the outcome that wastes a season.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. Browse the PlanScope report types to see which drainage documents an unsewered site usually needs. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.