Reports for a scheme served by a private treatment plant

A dwelling that cannot reach a public foul sewer needs three things the council will ask for before it validates or determines the application: evidence that mains connection is not reasonably practicable, a percolation test carried out properly, and a drainage field or outfall design that respects the separation distances. Get those three right and a package treatment plant is an ordinary solution that gets approved every week. Get the percolation test done in August on a dry site and the whole strategy collapses at the building regulations stage, long after permission has been granted.

The planning question and the environmental permitting question are separate, and both have to be answered.

When mains drainage is unavailable and what follows

National guidance sets a clear order of preference for foul drainage, and it starts with connection to the public sewer. The right to connect to a public sewer is given by section 106 of the Water Industry Act 1991, so the test is not whether a sewer exists nearby but whether connection is reasonably practicable in cost and engineering terms.

Where it is not, the order runs: a package treatment plant discharging to a watercourse, then a package treatment plant or septic tank discharging to ground through a drainage field, then, as a last resort, a sealed cesspool. Councils dislike cesspools because they need emptying by tanker for the life of the building, and many will refuse a scheme that relies on one where any alternative exists.

One rule catches people out immediately. Under the general binding rules for small sewage discharges in England, if a public foul sewer is available within 30 metres of the property boundary, you are expected to connect to it rather than discharge.

The drainage hierarchy and evidencing each step

Evidence beats assertion. The submission should contain, in order:

1. A written enquiry to the sewerage undertaker with its reply, showing the location of the nearest public sewer and its capacity.
2. A plan showing the route a connection would take, with levels, and a note on whether it would need to cross third party land.
3. A quotation or costed estimate for the connection, including any pumping station and its future maintenance.
4. A statement of why that solution is not reasonably practicable, referenced to the figures above.
5. The proposed alternative, with its design.

Third party land is often the deciding factor, because you cannot compel a neighbour to grant an easement. Say so plainly if that is the position, with the correspondence attached.

A Foul Sewage Assessment is the document that houses this, and on a site with a watercourse it sits alongside a drainage strategy dealing with surface water.

General binding rules against a permit

Small sewage discharges in England operate either under the general binding rules or under a permit from the Environment Agency. The general binding rules cover discharges up to stated daily volumes, with lower limits to ground than to surface water, provided the conditions are met.

Those conditions include the 30-metre sewer rule, a requirement that equipment meets the relevant British and European standard and is installed to the manufacturer's instructions, restrictions in sensitive locations, and an obligation to maintain the system and to tell the buyer when the property is sold.

A permit is needed where the volume exceeds the thresholds, where the discharge is in a designated sensitive area such as a groundwater source protection zone, or where the Agency decides the rules do not adequately protect the environment. Permits take time and cost money, so establish which route applies before the layout is fixed rather than afterwards.

Two further points. A septic tank discharging directly to a watercourse has not been acceptable for new installations for years, and existing ones have had to be replaced or upgraded. And new discharges to surface water need a treatment plant, not a septic tank, because the effluent quality from a septic tank is too poor.

Percolation testing and drainage field design

The test procedure is set out in Approved Document H of the Building Regulations, and it has three parts that get skipped.

First, a trial hole deep enough to establish the water table and the soil profile, dug in the wettest part of the year. Second, percolation test holes, presoaked overnight, with the time recorded for the water level to fall through the specified range, repeated and averaged to produce the percolation value. Third, the drainage field area calculated from that value and the population served, using the formula in the Approved Document, which differs for a septic tank and for a treatment plant.

Timing decides the result. A test carried out in a dry summer on clay will produce a percolation value that suggests the ground drains when it does not, and the failure appears the first winter after occupation. Where the programme will not allow a winter test, say so in the report and design conservatively.

Where the ground will not accept effluent at all, the answer is a discharge to a watercourse, a mound system or a change of approach, not a smaller field.

Separation distances from watercourses and boreholes

The Approved Document sets out minimum distances for a drainage field, including separation from watercourses and permeable land drains, from any groundwater abstraction point, and from buildings, with a further separation between the treatment unit itself and habitable parts of any building. Check the current edition for the figures, because they are the numbers building control will apply.

Plot the constraints before you plot the house. On a small plot, the drainage field envelope, the 30-metre sewer check, boundary distances and the position of any well or borehole often leave one viable location, and it may not be where the garden was going to go. The same discipline applies on a shared access, as covered for a self-build plot on a shared private drive.

Ask the neighbours about private water supplies. A spring or borehole serving one house at the end of a lane may not appear on any dataset, and a discharge upstream of it is a serious problem.

Maintenance obligations and conditions

Expect the permission to carry a condition requiring the system to be installed as approved and maintained thereafter, and sometimes requiring the details to be submitted before work starts.

The practical obligations sit outside planning. Desludging on the manufacturer's cycle, a maintenance log, access for a tanker with a hard standing and a turning area, an electrical supply for the aeration unit, and an alarm. Where a plant serves more than one dwelling, somebody has to own and manage it, which usually means a management company and a deed. That arrangement is easier to set up before the first house is sold than after.

Include tanker access in the site layout. A treatment plant that a tanker cannot reach is a maintenance failure waiting to happen, and it is the kind of detail an officer notices on a rural scheme.

The document list for an unsewered site

For a single dwelling or a small scheme on an unsewered site, expect to submit:

• Planning Statement covering the drainage hierarchy and the policy position.
• Foul Sewage Assessment with the undertaker's correspondence, the cost comparison and the chosen solution.
• Percolation test results with trial hole logs, dates and the calculated field area.
• Drainage layout drawing showing the treatment unit, the field or outfall, separation distances and tanker access.
• Surface water drainage details, kept separate from the foul strategy.
• Flood Risk Assessment where the site is in Flood Zone 2 or 3 or is one hectare or more.
• A note on the permitting route, stating whether the general binding rules apply or a permit is being sought.

Rural schemes usually stack other constraints on top, and the evidence base for holiday lets in redundant farm buildings overlaps at exactly this point, because a change of use raises the loading on an existing system.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. For specialist technical reports, qualified professional input may be required. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. Enter your postcode on PlanScope to see which reports your site needs.