Foul drainage evidence with a package treatment plant

Package treatment plant evidence has to answer four questions in order, and councils refuse or delay applications that answer them out of sequence. Is a mains connection reasonably available? If not, where will the treated effluent go? Will the ground accept it, proven by percolation testing? And does the layout keep the drainage field the required distances from buildings, watercourses and water supplies?

Get the percolation test done before the layout is fixed. The drainage field is often the largest single element on a rural plot, and discovering that it needs twice the area you allowed for means redrawing the site plan.

When a treatment plant is acceptable in principle

The hierarchy in Approved Document H puts a connection to the public foul sewer first. Where a sewer is within a reasonable distance, commonly taken as around 30 metres, a connection is normally expected, and the cost of a longer connection has to be weighed rather than simply asserted as prohibitive.

Only where a mains connection is not reasonably available does a package treatment plant or septic tank with a drainage field come into consideration, and a cesspool, which stores rather than treats, sits below both and is rarely accepted for a new dwelling.

So the first page of the Foul Sewage Assessment should record the enquiry made to the sewerage undertaker, the distance to the nearest public sewer, the levels involved, and the undertaker's answer. A statement that there is no mains drainage in the area, with no enquiry reference, is the most common gap in these submissions.

Two further points on principle. A package treatment plant is a treatment system with a mechanical element and a power supply, producing a better effluent quality than a septic tank; a septic tank provides settlement only. And any new plant should be certified to the relevant European product standard for small wastewater treatment systems, with the certificate submitted.

The general binding rules and when a permit is needed

Small sewage discharges in England are regulated by the Environment Agency through general binding rules. Meet the rules and you do not need an environmental permit, though a new discharge normally has to be registered. Fall outside them and a permit is required, which is a longer and more expensive route.

The rules cover both the volume and the circumstances. Discharges above the stated daily volumes, discharges in certain sensitive locations, and discharges that cannot meet the conditions all need a permit instead. The current volume thresholds and the list of circumstances requiring a permit are published on the Environment Agency's guidance pages, and they differ between a discharge to ground and a discharge to surface water, so read the right one.

One rule has caught out many rural property owners: a septic tank discharging directly to a surface water such as a ditch or stream is no longer acceptable, and such systems have to be replaced with a treatment plant or connected to a mains sewer. If your site has an existing septic tank discharging to a watercourse, the application is an opportunity to resolve that rather than a reason to avoid mentioning it.

State plainly in the submission which route you are relying on: general binding rules with registration, or a permit application, with the reasoning.

Discharge to ground against discharge to water

These are different regulatory animals and the evidence differs.

Discharge to ground is normally preferred where the soil allows it, because the ground provides additional treatment. Where percolation fails, the fallback is a discharge to surface water, which requires a treatment plant and a watercourse that flows throughout the year. A seasonal ditch is not a reliable receptor, and an assessment that relies on one will be challenged.

Where the site is in a nutrient-sensitive catchment, the foul drainage solution interacts with the wider nutrient position and may need separate consideration.

Percolation testing and sizing the drainage field

This is the technical core of the submission, and it follows a defined method.

1. Dig trial holes at the proposed drainage field location, to the depth given in the guidance, and record the soil profile and any groundwater encountered.
2. Establish the groundwater table, which must sit a sufficient depth below the invert of the drainage field. Test at the wettest time of year where possible, because a summer test can be misleading.
3. Carry out the percolation test itself: fill the test hole, allow it to drain, then refill and time the fall over the measured section, repeating the test the stated number of times and in more than one hole.
4. Calculate the percolation value in seconds per millimetre as the average of the results.
5. Size the drainage field from that value and the design population, using the formula in Approved Document H.

The formula is published, so the arithmetic can be shown openly: the required trench floor area is the design population multiplied by the percolation value multiplied by a factor given in the guidance. A high percolation value, meaning slow draining ground, produces a large field. Clay soils frequently produce values outside the acceptable range altogether, at which point a drainage field is not viable and the solution changes.

Illustrative example. A four bedroom house is designed for a population of six. A percolation value in the middle of the acceptable range produces a trench area of a few hundred square metres once the formula is applied, which on a narrow plot competes directly with the garden, the parking and the root protection areas of retained trees. The figures here are illustrative, and your own come from the test results.

Two practical cautions. Do not run the test in freezing conditions or during prolonged heavy rain, as the guidance excludes both. And do not accept a single hole result on a site with variable ground, because the field has to work across its whole area.

Separation distances that decide the layout

The distances in Approved Document H constrain the site plan more than most people expect, and they should be drawn before anything else is positioned.

• at least 10 metres from any watercourse or permeable drain
• at least 50 metres from a well, borehole or spring used for drinking water supply
• at least 15 metres from any building
• clear of access roads, driveways and paved areas, since compaction and sealing destroy the field
• outside the root protection areas of retained trees, which is a separate calculation
• not within an inner source protection zone, where a discharge to ground is unlikely to be acceptable

Add the levels question. A gravity drainage field needs the invert at a workable depth below the plant outlet, and if the land falls the wrong way a pumping station enters the design, with its own power supply, alarm and maintenance obligations. The relationship between site levels, inverts and floor levels is the same discipline described in our note on finished floor levels in a flood risk assessment.

Surface water drainage is a separate exercise on the same site, and the two cannot share a system. The Drainage Strategy deals with rainfall, attenuation and the allowances applied for climate change, which our note on climate change allowances in a drainage strategy explains.

Maintenance and what happens at sale

A treatment plant is a mechanical asset with an owner's duty attached. The general binding rules require the system to be maintained in accordance with the manufacturer's instructions, desludged by a registered carrier at the stated interval, and kept in working order.

Keep a maintenance log from the first day. Service records, desludging receipts and the original certification form a file that matters twice: when the authority or the Environment Agency asks, and when the property is sold. On sale, the seller must give the buyer the details of the system, including its location, maintenance requirements and the discharge route.

For a self-builder this is worth planning. Put the plant where a tanker can reach it, specify an alarm that is audible from the house, and confirm the electrical supply arrangement with the plant on the drawings rather than leaving it to the contractor.

What to submit with the application

A complete package for a rural dwelling usually contains:

1. The sewer connection enquiry and the undertaker's response, with distances and levels.
2. A statement of which regulatory route applies, general binding rules or permit.
3. Trial hole logs, groundwater observations and the percolation test results with dates and conditions.
4. The drainage field calculation showing the formula and the design population.
5. A drainage layout plan with separation distances dimensioned, plus a long section with invert levels.
6. The plant specification and its product certification.
7. A maintenance statement, including access for desludging.

Where water supply is also off mains, a Water Supply Assessment covers the other half of the servicing question, and the two documents should be consistent about the position of any borehole relative to the drainage field.

Book the percolation test before the architect finishes the site layout. Enter your postcode on PlanScope to see which reports a rural self-build is likely to need.

This is general guidance, not legal advice; check the position with 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.