Foul Sewage Assessment: capacity evidence and the sections

A Foul Sewage Assessment answers one question in a defined order: where is the wastewater going, and why is that the right answer. National guidance sets a hierarchy, and the report has to work down it, evidencing each option it rejects before reaching the one it proposes.

Councils hold up applications on this more often than developers expect, usually because the assessment asserts that a mains connection is impractical without a word from the sewerage undertaker to support it.

This is general guidance rather than legal advice. Local requirements vary, so check the validation list before you submit.

When a Foul Sewage Assessment is a validation requirement

The application form asks how foul sewage will be disposed of. Any answer other than a straightforward mains connection tends to prompt a request for supporting evidence, and many local validation lists require the assessment for all new dwellings.

Sites where the report is unavoidable: rural plots with no sewer in the road, conversions adding units to an area with known capacity constraints, sites in a nutrient sensitive catchment, and anywhere the council has recorded flooding from the foul or combined system.

A Foul Sewage Assessment sits alongside the surface water work rather than inside it. Keep the two separate, because combining foul and surface water in one narrative is the fastest way to end up proposing a connection the undertaker will not accept.

The foul drainage hierarchy the report must follow

The order is settled:

1. Connection to the public foul or combined sewer.
2. A package sewage treatment plant, discharging to a watercourse or to ground.
3. A septic tank with a drainage field, discharging to ground.

Work down it in the report and show your reasoning at each step. Distance to the nearest public sewer, the cost and practicality of a connection, land ownership along the route, levels and the need for pumping are all legitimate reasons to move down the hierarchy. Preference is not.

The cost argument needs care. A connection is not unreasonable simply because it is expensive relative to a treatment plant, and councils look for a quantified comparison rather than an impression.

Getting a capacity response from the sewerage undertaker

Every sewerage undertaker runs a pre-development enquiry service. Use it early, because responses take weeks and the answer shapes the layout.

Ask for the location and depth of the nearest public foul sewer, whether capacity exists at the receiving works and in the network, whether any off-site reinforcement is needed, and what the undertaker's position is on the proposed connection point.

Three statutory routes sit behind the answer. Section 106 of the Water Industry Act 1991 gives a right to connect a drain or sewer to the public system, subject to notice and the undertaker's conditions. Where no sewer exists, the requisition provisions can require one to be provided. And new sewers intended for adoption are dealt with through an adoption agreement, which is worth starting in parallel with the planning application on anything above a few plots.

Where the building sits over or close to a public sewer, a separate build over agreement is needed. That is not a planning matter and it will still stop the build.

Package treatment plants and the general binding rules

Small sewage discharges are regulated by the Environment Agency. Where the discharge is below the stated volume thresholds and the site is not in a sensitive location, the general binding rules apply and no permit is needed, provided the conditions are met. Above those thresholds, or in a groundwater source protection zone or other sensitive area, a permit is required.

The report should name which route applies and why, state the daily discharge volume assumed and how it was calculated, and confirm the plant is certified to the relevant standard. Include the maintenance regime, because plants that are not desludged fail, and councils increasingly condition maintenance.

Nutrient sensitive catchments add a further layer, since the nutrient budget depends on the treatment route chosen. In those catchments the drainage decision and the mitigation strategy have to be designed together.

Septic tanks and why the report must justify them

A septic tank provides settlement only. The treatment happens in the ground, through a drainage field, which is why the ground has to be suitable.

New discharges from a septic tank to a surface watercourse are not allowed under the general binding rules, and existing tanks discharging that way have had to be replaced or upgraded. So the proposal is always a drainage field, and the report has to show the field will work.

That means percolation testing to the method in the Building Regulations drainage guidance, trial holes at the depth of the proposed field, groundwater levels recorded, and the separation distances respected: distance from any watercourse or permeable drain, from any groundwater abstraction point, and from any building. Fields cannot sit under vehicle access or hard surfacing.

Ground conditions and percolation test reporting

Report the raw data, not just the conclusion. The trial hole logs, the water levels observed, the test times for each fill, the calculated value, and the resulting drainage field area with the arithmetic shown.

Two honest caveats belong in the report. Tests carried out in a dry summer will overstate performance on clay soils, so state the date and the antecedent weather. And a single test point on a sloping site tells you little, so test at more than one location where the field is large.

Where infiltration fails, the fallback is a treatment plant discharging to a watercourse, which needs a riparian owner's consent and possibly a permit. Identify that fallback in the report rather than leaving it for a discharge of conditions application eighteen months later.

Conditions councils attach to foul drainage

Expect a condition requiring full foul drainage details to be submitted and approved before development starts, and another preventing occupation until the system is installed and operational. Where off-site works are needed, a Grampian condition preventing occupation until they are complete is common.

Two practical points. Discharge of conditions applications take up to eight weeks, so submit the details well before you need them. And keep the foul drainage evidence consistent with the surface water evidence in the Drainage Strategy and with any Water Supply Assessment, because contradictions between the three documents are what consultees notice first.

Get the undertaker's enquiry submitted before the layout is fixed. You can enter your postcode on PlanScope to see which reports your site is likely to need.

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