Foul drainage on unsewered sites: package plants and permits

Foul drainage on an unsewered site follows a fixed hierarchy. Connect to the public sewer where that is reasonably practicable. If not, use a package treatment plant or a septic tank discharging to a drainage field. Only if neither works does a cesspool become acceptable, and councils treat it as a last resort. On top of that sits the environmental regime, where a small sewage discharge either complies with the general binding rules or needs a permit from the Environment Agency.

Most refusals and most conditions on rural applications come from one omission. The percolation test was not done before the application, so nobody knows whether a drainage field is possible at all.

Establishing that mains connection is not reasonably available

The council will expect evidence, not an assertion that there is no sewer nearby.

Ask the sewerage undertaker for a plan of its assets around the site and a written view on connection. Then set out the practical position: the distance to the nearest public foul sewer, the levels between it and the site, whether pumping would be needed, whether third party land lies in between and whether an easement could realistically be obtained, and the indicative cost.

Distance alone does not settle it. A sewer 40 metres away across your own land is a different proposition from one 40 metres away on the far side of a neighbour's garden with a 3 metre level rise. Building regulations guidance treats a nearby public sewer as the starting point, so where you are arguing that connection is impracticable, the argument needs to be made in writing with figures.

Put that reasoning into a Foul Sewage Assessment rather than into a paragraph of the covering letter. Officers assess it against the hierarchy, and a document that follows the hierarchy in order is much easier to agree with.

Septic tanks, package plants and the general binding rules

The two systems are not interchangeable, and the difference matters for what you may do with the effluent.

The general binding rules for small sewage discharges in England set the conditions under which a discharge is allowed without a bespoke permit. The volume limits are the ones to remember: 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.

A septic tank must not discharge directly to a watercourse. That has been the position since the rules were tightened, and existing properties doing so have had to replace or upgrade. A package treatment plant discharging to surface water is permissible under the rules where it meets the relevant European standard, is properly maintained and stays within the volume limit.

New discharges also have to be registered with the Environment Agency where the rules require it, and the operator has ongoing duties on maintenance and desludging.

When an environmental permit is required

The general binding rules stop applying in defined circumstances, and then a permit is needed before the discharge starts.

• The discharge exceeds the volume limits, which typically means a larger scheme or a small cluster of dwellings.
• The site is in a groundwater source protection zone 1.
• The discharge is within a set distance of a well, spring or borehole used for drinking water.
• The discharge could affect a protected site such as a site of special scientific interest or a European site.
• The Environment Agency has told you a permit is required for that location.

Permit applications take time and carry a charge, so establish the position at feasibility stage. This is also where multi-plot schemes go wrong. Four houses on individual plants may each sit under the limit while the combined discharge to the same receptor does not, and the Agency looks at the discharge rather than at the number of applications.

Drainage field design and percolation testing

The percolation test is the single most useful thing you can do before designing anything. It measures how quickly water soaks away, expressed as a value in seconds per millimetre.

The method in the building regulations guidance is prescriptive. Dig trial holes in the location of the proposed drainage field, at the depth of the proposed trenches, fill and pre-soak them, then time the fall of water between fixed levels, repeating the test three times and averaging.

Two thresholds decide the outcome. Where the value is below the lower limit the ground drains too fast to treat the effluent, and where it is above the upper limit the ground will not accept the flow. Only ground within the range is suitable for a drainage field at all, and the value then sets the required area of the field for the number of people served.

Separation distances constrain the layout as much as the arithmetic. A drainage field is kept clear of watercourses and permeable drains, well away from any water abstraction point, and set back from buildings and boundaries. The water table must not rise to within a metre of the invert of the distribution pipes at any time of year, which is why a test in August can mislead badly on a site that is wet in February.

Where the ground fails, the options narrow to a plant discharging to surface water with a permit if one is available, a mound or raised system where the council accepts it, or a cesspool. Each costs more than a standard drainage field.

Groundwater protection and source protection zones

Groundwater is the reason the regime exists. Source protection zones are mapped around abstraction points used for public supply, with the innermost zone the most restrictive, and a discharge to ground inside it will not be acceptable without a permit and often not at all.

Check the mapping before you buy the land. The Environment Agency publishes the zones, and the same exercise picks up whether the site drains towards a designated site that would raise a habitats issue.

Private water supplies add a second layer. Neighbouring springs, wells and boreholes used for drinking water impose their own separation distances, and they are not always shown on any map. Ask locally and record what you are told.

What the foul sewage assessment must contain

Keep it short and evidenced. A rural application does not need forty pages.

1. The hierarchy addressed in order, with the mains connection position evidenced.
2. The proposed system, its capacity in population equivalent and its performance standard.
3. Percolation test results with the trial hole locations, dates, depths and calculated values.
4. The drainage field size calculation and a layout drawing showing separation distances.
5. Groundwater and designated site constraints checked, with the source consulted and the date.
6. The regulatory route: general binding rules with registration, or a permit application.
7. Maintenance arrangements and how they will be secured.

Where surface water is also being dealt with on site, keep it in a separate Drainage Strategy so the two are not confused. If part of the site lies in Flood Zone 2 or 3, the Flood Risk Assessment should confirm that the treatment plant and drainage field are not at risk of inundation, since a flooded drainage field discharges to the flood water. And where the site relies on a private supply, a Water Supply Assessment covers the other half of the picture.

Maintenance obligations and how they are secured

A treatment plant is a machine. It needs power, servicing, desludging and someone responsible for it, and a plant that is not maintained fails quietly rather than dramatically.

Councils secure that in three ways. A condition requiring details of the system and its maintenance regime to be approved before occupation. A note on the decision drawing attention to the environmental duties, which are enforceable by the Environment Agency regardless of planning. Or, for shared systems serving more than one dwelling, a management company or a maintenance obligation, because a shared plant with no named owner will eventually fail.

For a single house, keep the service records. They transfer with the property, and a purchaser's solicitor will ask for them along with proof of registration.

This is general guidance, not legal advice. The rules on small sewage discharges change, so check the current general binding rules and confirm the position with the Environment Agency and your local planning authority before relying on any of it.

Test the ground before you draw the house. Enter your postcode on PlanScope to see which reports an unsewered site is likely to need.

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