Foul sewage or water supply assessment: which one you need
Two short documents, two different undertakers, two different failure modes. On a single plot the foul sewage answer usually decides whether the scheme works at all.
A foul sewage assessment deals with wastewater leaving your site: where it goes, whether the public sewer can take it, and what happens if there is no sewer. A water supply assessment deals with clean water arriving: whether the network has capacity, what pressure and flow you can expect, and what reinforcement might be needed. Both are short documents on a single plot, and the foul side is almost always the one that decides whether the site is developable.
Self-builders discover this in the wrong order. The plot is bought, the design is drawn, and then the percolation test fails or the sewer turns out to be 400 metres away and uphill.
Wastewater out against water in
Keep the two questions physically separate in your head, because the evidence, the consultee and the fallback positions are all different.
| Foul sewage assessment | Water supply assessment | |
|---|---|---|
| Question answered | Where does the wastewater go | Can the site be supplied with water |
| Main consultee | Sewerage undertaker | Water undertaker |
| Statutory route | Connection or requisition under the Water Industry Act 1991 | Supply and main requisition under the same Act |
| Regulator involved | Environment Agency for private discharges | None routinely |
| Common failure | Failed percolation test, no sewer within reach | Low pressure, long main extension cost |
The same company often performs both roles, which is why applicants assume one enquiry covers both. It does not, and the two departments answer on different timescales.
The foul drainage hierarchy
Planning Practice Guidance sets a clear order of preference. Connect to the public foul sewer first. Where that is not reasonably practicable, use a package sewage treatment plant. Only where neither is possible should a septic tank be considered, and a septic tank discharging to a watercourse has not been acceptable since the current general binding rules took effect.
You are expected to show why you have not used a higher option. "There is no sewer nearby" needs a distance, a plan and, ideally, a budget cost for the connection. Section 106 of the Water Industry Act 1991 gives a right to connect to a public sewer, and sections 98 onwards allow a sewer to be requisitioned, so distance alone is not automatically a reason to skip to a treatment plant.
The evidence a council expects for a non-mains solution:
- A plan showing the nearest public sewer with the distance measured.
- Percolation test results carried out to the recognised method, with the trial hole positions marked.
- A drainage layout showing the treatment plant, the drainage field and the separation distances to the dwelling, the boundary and any watercourse or borehole.
- Confirmation of whether the discharge is covered by the general binding rules or needs an environmental permit.
- Where a permit is needed, the application reference or an explanation of the timetable.
Run the percolation test before you exchange on the plot. A site with heavy clay and a high water table cannot take a conventional drainage field, and the alternatives are expensive.
Private treatment and the general binding rules
Small sewage discharges in England are regulated by the Environment Agency, and modest discharges that meet the general binding rules do not need a permit. The rules set volume limits that differ between discharges to ground and discharges to surface water, and they include requirements on the treatment standard, the position of the outfall and maintenance. Check the current rules on gov.uk, because the volumes and conditions have been revised more than once.
Where the discharge exceeds the limits, or where the site is in a sensitive location such as a groundwater source protection zone, a permit is required and the timetable lengthens considerably. Build that into the programme rather than treating it as a post-permission detail.
Nutrient neutrality adds a further layer in affected catchments. Where a catchment is subject to nutrient restrictions, a new dwelling on a private treatment system can need mitigation before permission can be granted at all, and the calculation depends on the treatment standard achieved.
The water supply side
The water undertaker has duties to provide a supply and to allow connection, and a developer can requisition a new main where one is needed. The practical issues are cost and pressure rather than principle.
Ask the undertaker for a point of connection enquiry early. The response tells you where you can connect, what pressure and flow are available at that point, and whether any network reinforcement is required. A long main extension along a private lane, with a wayleave to negotiate, can cost more than the drainage works.
Water efficiency has become a planning matter too. The Building Regulations set a standard for water consumption per person per day, with a tighter optional standard that councils in water stressed areas can apply through local plan policy. Check the development plan, because meeting the tighter figure changes the sanitaryware specification and should be stated in the application rather than discovered at Building Regulations stage.
Who the consultee is and when
The sewerage undertaker is consulted on applications involving building over or near a public sewer and on proposals with drainage implications. The Environment Agency is consulted on certain discharges and on sites where groundwater is at risk. The lead local flood authority deals with surface water drainage, which is a separate subject again and should not be mixed into the foul assessment.
Surface water and foul water must be kept apart on the drainage strategy. Combining them is one of the more common reasons a drainage submission is returned, and the construction stage arrangements then have to keep them separate on site as well.
Where both are asked for at validation
Councils increasingly list both on the local validation checklist for new dwellings, particularly outside settlement boundaries. Read that list before you assemble the pack. Some authorities accept the drainage section of the standard application form as the foul sewage assessment for a single dwelling, while others want a standalone document with test results attached.
A short worked illustration. A single plot at the edge of a village sits 180 metres from the nearest foul sewer, along an adopted road. The sewer connection is priced and comes in below the cost of a treatment plant with a permit, so the application proposes connection, and the assessment simply records the distance, the quote and the point of connection. That takes one page. The alternative version of the same site, with no sewer within a kilometre and clay soils, needs percolation results, a drainage field design, a permit application and a nutrient calculation. Those figures are illustrative rather than drawn from a real scheme.
Draft both documents from the same site answers using the 66 report types, and see PlanScope pricing if you want to edit and version them as the undertaker responses come in. To start, enter your postcode on the PlanScope home page.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.
Frequently asked questions
Do I need both a foul sewage assessment and a water supply assessment?
On many new dwelling applications, particularly outside settlement boundaries, councils ask for both. The foul assessment covers wastewater disposal and the water supply assessment covers clean water capacity. Check the local validation checklist, because some authorities accept the drainage section of the application form for a single dwelling while others want standalone documents.
What is the drainage hierarchy for foul sewage?
Connect to the public foul sewer where reasonably practicable. If that is not possible, use a package sewage treatment plant. A septic tank is the last option and cannot discharge to a watercourse under the current general binding rules. You are expected to show why a higher option in the order was not used, with distances and costs rather than assertions.
Do I need a permit for a package treatment plant?
Not always. Small sewage discharges in England that meet the Environment Agency's general binding rules do not need a permit, but the rules set volume limits that differ for discharges to ground and to surface water, along with treatment and maintenance conditions. Discharges above the limits, or in sensitive locations, need an environmental permit.
When should I do a percolation test?
Before buying the plot if you can, and at the latest before commissioning architectural drawings. A failed percolation test on heavy clay or a high water table rules out a conventional drainage field, and the alternatives cost considerably more. Test results with trial hole positions marked on a plan are what the council will want to see.
Who pays for a new water main to reach my plot?
The developer generally does. A new main can be requisitioned from the water undertaker under the Water Industry Act 1991, with the cost falling on the person requiring it, subject to the statutory calculation. Ask for a point of connection enquiry early so you know the pressure available and the likely reinforcement cost before committing.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.