Non-mains drainage and the general binding rules

Non mains drainage rules sit in two places at once. The Environment Agency regulates the discharge itself, through the general binding rules for small sewage discharges or through an environmental permit. The local planning authority deals with whether the development is acceptable, including whether the proposed drainage is workable on that site. They are separate decisions, and satisfying one does not satisfy the other.

A planning permission does not authorise a discharge, and compliance with the binding rules does not make a house acceptable in planning terms.

When a private system is the only option

National practice guidance sets out a hierarchy for foul drainage. Connection to the public sewer comes first. Only where that is not reasonably available does a private system follow, with a package treatment plant preferred over a septic tank.

Reasonable availability is a judgement about distance, levels, physical obstacles and cost. Where a public sewer runs 400 metres away downhill with no obstruction, an authority will expect a serious look at connection, including the option of requisitioning a sewer under the Water Industry Act 1991. Where the nearest sewer is two kilometres away across third party land, the question answers itself.

Evidence the point rather than asserting it. A short note recording the location of the nearest public sewer, taken from the undertaker's records, with the levels and the reason connection is not practical, prevents the objection before it arises. The broader process for establishing that position is set out in the approach to a Foul Sewage Assessment.

The general binding rules in outline

The general binding rules allow a small domestic sewage discharge without an individual permit, provided the discharge stays within stated limits and meets a set of conditions. They are published by the Environment Agency in two versions, one for discharges to the ground and one for discharges to surface water.

The core limits are volumetric. A discharge to the ground is covered up to two cubic metres per day. A discharge to surface water is covered up to five cubic metres per day. Above those figures a permit is required.

Alongside the volume limit, the rules require the system to be installed and maintained properly, to be of an appropriate standard, to serve domestic sewage only, and not to cause pollution. They also require the operator to be able to identify the system and to keep it in working order, which in practice means knowing where the tank is and having it emptied.

One rule catches many rural properties. A septic tank may not discharge directly to a surface water body. Where an existing property has that arrangement, the discharge has to be replaced, upgraded to a treatment plant, or redirected to a drainage field.

Where a permit is required instead

The binding rules do not cover everything. A permit application to the Environment Agency is needed where any of the following applies.

• The discharge exceeds the volume limits for ground or surface water.
• The discharge is to the ground within a groundwater source protection zone 1.
• The discharge is close to a protected site or a sensitive water body, in the circumstances the rules set out.
• The discharge is not domestic sewage, or serves a use the rules do not cover.
• The Environment Agency has told the operator that a permit is required for that particular discharge.

Permit determination takes time and carries a fee. Where a scheme is likely to need one, start it in parallel with the planning application rather than after, and say in the application that it is in hand. An authority is more comfortable granting permission where the regulatory route is identified than where the drainage note simply says a treatment plant will be installed.

Discharges to ground and to surface water

The two routes behave differently and the choice is usually made by the ground rather than by preference.

A discharge to the ground goes through a drainage field, which distributes effluent through perforated pipework in trenches so that the soil completes the treatment. It is not a soakaway and it is not a hole filled with rubble. Where the ground will not accept the flow, the field does not work and the system surfaces, usually in the second wet winter after occupation.

A discharge to surface water requires treated effluent from a package treatment plant, because a septic tank discharge is not treated to a standard that permits it. It also requires a watercourse that flows, and an outfall the applicant has the right to construct and maintain.

Sizing for occupancy rather than bedrooms

Systems are sized by population equivalent, the daily load the plant is designed to treat. The building regulations approach uses the number of bedrooms as a proxy, which is convenient and frequently wrong at both ends.

Size on the expected occupancy of the building, with a sensible allowance for change of use over the life of the system. A five bedroom farmhouse occupied by two people still needs capacity for the household that buys it next. A holiday let with high turnover has a different loading pattern from a family home, with sharp peaks and periods of no flow at all that affect the biological process in a treatment plant.

Where the system serves more than one property, the arrangement needs a management structure as well as a design. Who empties it, who pays, who holds the permit, and what happens if one property sells. Councils increasingly ask that question, and an answer written into the application saves a condition being drafted for you.

Percolation testing and separation distances

The percolation test is the technical heart of a ground discharge, and it is the step most often skipped or fudged.

A proper test digs trial holes at the depth of the proposed drainage field, in the location proposed, soaks them overnight, then measures the time for water to fall between set levels, repeated across several holes and averaged. The result gives a value used to calculate the required trench length. A test carried out in high summer on ground that is waterlogged in February will produce an answer that flatters the site, so record the date and the conditions.

Separation distances in the building regulations guidance protect wells, buildings and watercourses. Drainage fields are kept a stated distance from any watercourse or permeable drain, a considerably larger distance from any point of groundwater abstraction, and clear of buildings and of ground subject to vehicle loading. Take the figures from the current Approved Document rather than from memory, and plot them on the site plan as circles so the available area is obvious.

On a small plot the distances often leave nowhere for the field. That is a design constraint to discover before the layout is fixed, not after.

Planning conditions on private systems

Expect a condition requiring full details of the foul drainage system to be submitted and approved before development starts, and a further condition preventing occupation until the system is installed in accordance with the approved details.

Some authorities go further, requiring evidence of the permit or confirmation that the binding rules are satisfied, a maintenance schedule, and in shared arrangements a management company or a deed. Where the site is close to a designated water body, the authority may also ask about nutrient loading, and in catchments subject to nutrient neutrality that becomes a substantial issue in its own right.

Read the trigger points. A condition requiring the system to be installed before any development starts, rather than before occupation, would be unusual and impractical, and is worth correcting during determination.

What the assessment should contain

Keep it short and specific. The nearest public sewer and why connection is not practical. The proposed system type, its standard and its population equivalent. The discharge route, to ground or to surface water. The percolation test results with dates and hole locations, or the outfall details and the right to discharge. The separation distances plotted on a plan. The regulatory route, binding rules or permit, with the application reference if one has been made. And the maintenance arrangement.

Where the site also has surface water constraints, keep the foul and surface water cases separate within the Drainage Strategy rather than merging them, because the consultees are different and the tests are different. The level of scrutiny that applies to the surface water side is described in what a consultee checks in a drainage model output. Where the property also relies on a private water supply, deal with that in a Water Supply Assessment and check the separation between the supply and the drainage field early.

This is general guidance, not legal advice. Check the position with the Environment Agency, your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. To see which drainage documents a rural site needs, enter your postcode on PlanScope.