Foul sewage assessment or drainage strategy: which you need
Two flows, two regulators, two documents. How the right to connect works, when a package treatment plant is acceptable, and what councils ask for at each scale.
A Foul Sewage Assessment deals with what leaves the building: toilets, kitchens, bathrooms, and where that flow goes. A Drainage Strategy deals with what falls on the site: roof water, paving, roads, and how that is stored, treated and released. Two flows, two regulators, two sets of evidence. Self-builders routinely submit one and are asked for the other, usually a fortnight after validation.
The split matters because the people who say yes are different. Foul connection is a matter for the water and sewerage undertaker under the Water Industry Act 1991. Surface water is a matter for the lead local flood authority and the council, working to national standards and local requirements.
Foul flows against surface water
Modern systems are separate. Foul goes to the public foul or combined sewer, or to a private treatment system. Surface water goes to the ground, to a watercourse, or as a last resort to a sewer.
Combining them is the error that generates objections. Connecting surface water to a foul sewer adds volume to a treatment works and contributes to storm overflow spills, and undertakers refuse it as a matter of course. Where a site drains to an old combined sewer, the strategy should still separate the flows on site so that a future separation is possible.
| Foul Sewage Assessment | Drainage Strategy | |
|---|---|---|
| Flow | Wastewater from the building | Rainfall on roofs and hard surfaces |
| Main decision maker | Water and sewerage undertaker | Lead local flood authority and council |
| Governing framework | Water Industry Act 1991 and environmental permitting | National standards, local requirements and Building Regulations |
| Typical evidence | Capacity response, connection enquiry, treatment plant details | Infiltration testing, storage calculations, discharge rates |
| Common sticking point | Network capacity and build over agreements | Discharge rate and maintenance |
The right to connect and how it works
Section 106 of the Water Industry Act 1991 gives an owner or occupier the right to connect drains and private sewers to the public sewer. The right is not unqualified: the undertaker can refuse where the mode of construction or condition of the drain is defective, or where the connection would prejudice the sewerage system.
The process runs on its own timetable, separate from the planning application. Make a pre-development enquiry to the undertaker early, because the response tells you whether capacity exists, whether an off-site upgrade is needed, and whether the point of connection is where you assumed. On small sites the answer is usually straightforward. Where it is not, the undertaker may ask for a drainage impact assessment and the council may impose a condition preventing occupation until network improvements are complete.
Two related consents catch self-builders out. Building over or close to a public sewer needs the undertaker's agreement, and the position of public sewers is not always where the site plan shows a manhole. And where a new sewer will serve more than one property and is to be adopted, an adoption agreement under section 104 is needed, with the design meeting the undertaker's published standards from the start.
Package treatment plants and the binding rules
Where no public sewer is available within a reasonable distance, a private system is the fallback, and national guidance sets a hierarchy: connection to the public sewer first, then a package treatment plant, then a septic tank.
Small sewage discharges are controlled through general binding rules made under environmental permitting legislation. Meeting the rules means the discharge can be made without a permit, subject to conditions covering the treatment standard, the volume, the location and the maintenance regime. A discharge that cannot meet the rules, including anything in a sensitive location or above the volume threshold, needs a permit from the Environment Agency.
For a discharge to ground, the drainage field has to be designed on the basis of percolation testing, and the results govern the area required. Test before you fix the house position. A drainage field needs separation distances from buildings, boundaries, watercourses and any water supply, and on a tight plot the field is often what decides where the house sits.
Evidence the council expects with the application includes the percolation test results, the proposed treatment plant specification, a plan showing the drainage field with separation distances, and confirmation of the maintenance arrangement. A Foul Sewage Assessment that says "package treatment plant to be installed" with no testing behind it will come back.
What the council asks for at each scale
Requirements scale with the development.
- A single dwelling connecting to a mains sewer usually needs no more than the answer on the application form and confirmation that a connection is available.
- A single dwelling on a private system needs the percolation evidence and plant details described above.
- A small site of several plots usually needs a foul drainage layout, the undertaker's capacity response, and a surface water strategy with infiltration testing and storage calculations.
- A major scheme needs both documents in full, with the lead local flood authority consulted formally on surface water.
For the surface water side, the Drainage Strategy has to follow the hierarchy: infiltration where ground conditions allow, then discharge to a watercourse at a restricted rate, then a sewer connection by agreement. Infiltration testing to the recognised method, carried out at the depth and location of the proposed feature rather than in a convenient corner, is what supports the first option.
Where flood risk pulls the two together
On a site in Flood Zone 2 or 3, or on a site of one hectare or more, a Flood Risk Assessment sits alongside both documents and has to agree with them. A drainage strategy proposing infiltration on a site with a high water table, in a flood risk assessment that says the water table is high, is the kind of internal contradiction that gets picked up at consultation.
Groundwater is the common thread. High groundwater rules out infiltration for surface water and constrains a drainage field for foul. Where both are ruled out, the scheme needs a discharge route and a pumping arrangement, and that changes the cost enough to matter at land purchase.
Sequencing for a self-build
Do the investigations before the design is fixed, not after permission. Percolation and infiltration testing, the undertaker's pre-development enquiry and a groundwater check together cost a small fraction of a self-build budget and determine the position of the house, the driveway and the garden.
The other sequencing decision is whether to apply in outline or in full, which changes how much drainage detail is needed at the first application, as set out in outline or full permission for a small residential site.
Conditions to expect on either route are familiar: no development until a detailed foul and surface water drainage scheme has been approved, no occupation until it has been implemented, and a maintenance requirement thereafter. Where a network upgrade is needed, expect a condition tying occupation to its completion.
PlanScope drafts both documents structured to the sections councils expect, flagging every site fact it cannot know, including test results and undertaker responses, rather than inventing them. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Enter your postcode to see which drainage documents your site is likely to need.
This is general guidance, not legal advice. Confirm requirements with your local planning authority, your sewerage undertaker and the Environment Agency before relying on it.
Frequently asked questions
Do I need both a foul sewage assessment and a drainage strategy?
On anything beyond a single dwelling connecting to an existing mains sewer, usually yes. Foul and surface water are separate flows with separate decision makers, and councils treat them as separate evidence. A single dwelling on mains drainage often needs no more than the answer on the application form and confirmation that a connection is available.
What is the right to connect to a public sewer?
Section 106 of the Water Industry Act 1991 gives an owner or occupier the right to connect drains and private sewers to the public sewer. The undertaker can refuse where the drain is defective or where the connection would prejudice the sewerage system, so make a pre-development enquiry early to confirm capacity and the point of connection.
When can I use a package treatment plant?
Where no public sewer is available within a reasonable distance. National guidance sets a hierarchy of mains sewer, then package treatment plant, then septic tank. The discharge must meet the general binding rules covering treatment standard, volume, location and maintenance, or it needs an environmental permit from the Environment Agency.
What evidence supports a drainage field?
Percolation testing carried out to the recognised method at the depth and position of the proposed field, a layout plan showing separation distances from buildings, boundaries, watercourses and water supplies, and the treatment plant specification. Test before fixing the house position, because on a tight plot the drainage field often decides the layout.
Can surface water be connected to the foul sewer?
Undertakers refuse it as a matter of course, because it adds volume to treatment works and contributes to storm overflow spills. The surface water hierarchy puts infiltration first, then discharge to a watercourse at a restricted rate, then a sewer connection by agreement. Where a site drains to an old combined sewer, separate the flows on site anyway.
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.