Foul Sewage Assessment: what each section has to prove
A Foul Sewage Assessment has to prove the waste water has somewhere lawful to go and that the receiving system can take it. Here is what each section must evidence, from the drainage hierarchy to adoption and connection triggers.
A Foul Sewage Assessment has one job: to prove that the waste water from your development has somewhere lawful to go, that the receiving system can take it, and that the arrangement will be in place before anyone moves in. The foul sewage assessment sections follow that logic, and each one has to prove something specific rather than describe an intention.
Most refusals and most requests for further information come from two gaps. No written evidence from the sewerage undertaker, and no explanation of why an off-mains solution was chosen when a public sewer runs within reach.
Establishing the existing arrangement
Start with what is there now. Not what the applicant believes is there, what the records and the site show.
The section should state the existing foul drainage arrangement for the site and for the buildings being replaced or converted, with evidence. Sewer records from the undertaker, a drainage survey where the connection is not obvious, manhole covers and invert levels recorded on site, and the location and condition of any existing septic tank or treatment plant.
Conversions deserve extra care. An agricultural building may have no foul drainage at all, and a farmhouse may discharge to a tank that has been in the ground since the 1960s with no permit and no record. Say so plainly. An assessment that quietly assumes an existing connection it cannot evidence will be picked apart by the undertaker's consultation response.
Record levels while you are there. Invert levels at the nearest manhole and finished floor levels at the proposal decide whether gravity drainage works, and retro-fitting that survey after a refusal costs a month.
The drainage hierarchy and why order matters
National guidance sets a clear order of preference for foul drainage, and departing from it without justification is the commonest reason an assessment fails.
| Rank | Option | What has to be proved |
|---|---|---|
| 1 | Connection to the public foul sewer | That a sewer is available, that capacity exists, and the point of connection |
| 2 | Package sewage treatment plant | That mains connection is not reasonably available, plus the discharge route and permitting position |
| 3 | Septic tank with drainage field | That options 1 and 2 are unavailable, plus percolation testing and ground conditions |
Section 106 of the Water Industry Act 1991 gives the owner of premises a right to connect drains to a public sewer, subject to the undertaker's ability to object on limited grounds including the mode of construction and prejudice to the sewerage system. That right is what makes option 1 the default. Distance and cost do not by themselves make a sewer unavailable, and an assessment arguing otherwise needs a costed comparison rather than an assertion.
Where a septic tank is proposed, deal with the permitting position head on. Septic tanks discharging directly to a surface water body are not allowed under the general binding rules for small sewage discharges, and systems outside the binding rules need an environmental permit. Percolation testing to the method in Approved Document H, with the test pit locations and results included, is what turns a drainage field proposal into an evidenced one.
Evidence of sewer capacity from the undertaker
Write to the sewerage undertaker early and put the reply in the appendices. A pre-development enquiry or capacity assessment, quoting the number of units, the expected foul flow and the proposed point of connection, is the evidence councils actually want.
Three outcomes are common. Capacity exists and the connection is straightforward. Capacity exists at a different point, which usually means a longer run and an easement. Or reinforcement is needed, which means a phased connection and a planning condition to match.
Calculate the flow properly rather than quoting a rule of thumb. Population equivalent derived from bedroom numbers, a stated per-head daily flow, and a peaking factor, with the source of each figure named. Foul flows for non-residential uses need a different basis, and a mixed scheme should show the two separately.
Where the site sits in a nutrient neutrality catchment, the foul drainage route becomes part of a much larger argument about phosphorus and nitrogen loading, and the assessment has to state which treatment works receives the flow and what its permit says. That work belongs alongside the ecology evidence rather than buried in a drainage appendix.
Package treatment plants and permitting
A package plant is a legitimate answer where mains connection really is not available, and it needs four things evidenced.
- The plant specification, sized on stated flows and loads, certified to the relevant product standard.
- The discharge route, whether to ground through a drainage field or to a watercourse, with the receiving water named.
- The permitting position, either compliance with the general binding rules or an application under the Environmental Permitting Regulations 2016.
- The maintenance and desludging arrangement, with the responsible party named.
That fourth item is the one applicants skip and officers notice. A treatment plant serving five houses with no management company and no maintenance obligation is a future enforcement problem, and councils increasingly condition it.
Check the discharge volumes against the binding rules thresholds before designing around them. A scheme that grows from four to six dwellings during the planning process can cross a threshold and turn a binding-rules discharge into a permitted one, which changes the programme by months.
Pumping stations, adoption and the section 104 route
Pumping stations are accepted routinely and resisted almost as often, because adoption is not automatic. If the station is to be adopted, it has to be designed to the undertaker's current standards, sited with adoptable access for a tanker, set back from habitable rooms by the undertaker's required distance, and covered by an agreement under section 104 of the Water Industry Act 1991, usually with a bond.
Show the station on the layout at the right size, not as a symbol. Wet well, kiosk, hardstanding, turning area for a tanker, and the odour control if required. Retro-fitting the real footprint after layout approval loses a plot more often than people expect.
Where a station will stay in private ownership, say who owns and maintains it, and how that obligation binds successors. The same discipline applies to private drainage runs serving more than one property.
Phasing and connection triggers
The council cannot approve a scheme that has nowhere to drain, but it can approve one where the drainage arrives before occupation. That is what a Grampian style condition does.
Set out the programme: which works have to be complete before the first connection, which before first occupation, and which before the final phase. Where reinforcement by the undertaker is on a published programme, name it and give the date. Where it is not, propose the condition wording yourself, because a condition you have drafted is easier to discharge than one written defensively by a case officer.
Keep the foul and surface water stories separate but consistent. Combining them in one paragraph is how misunderstandings start, and the surface water side belongs in the Drainage Strategy with its own hierarchy and its own consultee.
Who decides what
Four bodies have a say and none of them decides for the others.
The local planning authority decides the planning application and imposes conditions. The sewerage undertaker decides connection, capacity and adoption under the Water Industry Act 1991. The Environment Agency decides permits for discharges outside the binding rules. Building control decides compliance with Approved Document H at construction stage.
An approval from one is not an approval from another, and the Foul Sewage Assessment should say which consents are still outstanding and who will obtain them. Where water supply is also constrained, the same evidence pack usually feeds a Water Supply Assessment, and the two read better prepared together.
Drafting a technical report to a fixed section structure is the same exercise whichever discipline it sits in, and the approach mirrors the one set out for Overheating Risk Assessment inputs and outputs: name the input, name its source, and let the conclusion follow.
To see which drainage and water documents your site is likely to need, enter your postcode on PlanScope and answer the guided questions.
This is general guidance, not legal advice, so check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
What is the foul drainage hierarchy?
National guidance sets an order of preference: connection to the public foul sewer first, then a package sewage treatment plant where mains connection is not reasonably available, then a septic tank with a drainage field. Each step down the order has to be justified with evidence rather than asserted, and cost alone rarely makes a sewer unavailable.
What evidence do councils want about sewer capacity?
A written pre-development enquiry or capacity response from the sewerage undertaker, quoting the number of units, the calculated foul flow and the proposed point of connection. Include the calculation itself: population equivalent, per-head daily flow, peaking factor and the source of each figure, with non-residential flows shown separately.
Can a septic tank discharge to a stream?
No. Septic tanks discharging directly to a surface water body are not permitted under the general binding rules for small sewage discharges. The options are to replace the tank with a package treatment plant, to connect to the mains, or to discharge to ground through a drainage field. Discharges outside the binding rules need an environmental permit.
Does a pumping station have to be adopted?
Not necessarily, but adoption is usually preferred. An adopted station must meet the undertaker's current design standards, have tanker access and the required separation from habitable rooms, and be covered by an agreement under section 104 of the Water Industry Act 1991, normally with a bond. A private station needs a named owner and a maintenance obligation binding successors.
Who decides foul drainage, the council or the water company?
Both, on different questions. The local planning authority decides the application and imposes conditions. The sewerage undertaker decides connection, capacity and adoption under the Water Industry Act 1991. The Environment Agency decides permits for discharges outside the binding rules, and building control checks Approved Document H at construction stage.
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.