Foul Sewage Assessment outline and the capacity question

A foul sewage assessment outline has one question at its centre: can the receiving system take the flow, and what is the evidence. Everything else in the document supports that. The national application form asks how foul sewage will be disposed of, and on anything larger than a single house the council will want more than a tick in the mains sewer box.

Write the assessment in seven sections, in the order below. It mirrors the way a case officer and a sewerage undertaker read it, and it keeps the capacity evidence at the front where it belongs.

Section one: the drainage hierarchy for foul flows

Open by working down the hierarchy and showing where your scheme lands. Connection to the public foul sewer comes first. Where that is not reasonably available, a package treatment plant is next. A septic tank discharging to a drainage field sits below that, and a cesspool, which stores rather than treats, is a last resort that councils and undertakers both dislike.

State the distance to the nearest public foul sewer, measured along a practicable route rather than as the crow flies, and say who owns the land in between. "No mains available" is not an argument on its own. Section 98 of the Water Industry Act 1991 gives a right to requisition a new public sewer, and an undertaker will point that out if your assessment implies the mains option was impossible when it was merely inconvenient.

Where the hierarchy takes you off mains, give the reason in one sentence and move on. A short honest explanation reads better than three paragraphs of justification.

Section two: confirming public sewer capacity

This is the section that decides the application. Record four things: the point of connection, the flow you propose to add, the undertaker's response on capacity, and the date of that response.

Foul flow should be calculated rather than asserted. Set out the population equivalent, the litres per head per day figure used, the resulting dry weather flow, and any peak factor applied. For a small residential scheme that is a five-line table. For a mixed scheme, show the commercial element separately, since trade effluent has its own consent route and different flow characteristics.

Do not conflate foul and surface water here. Separation is expected, surface water to a foul sewer is resisted, and keeping the two calculations in separate documents avoids the confusion that leads to a query. The boundary between the two documents is worth understanding, and the difference between a Foul Sewage Assessment and a Drainage Strategy is more than presentational.

Section three: the pre-development enquiry evidence

The undertaker's own words carry the weight, so reproduce them.

Submit a pre-development enquiry or developer services enquiry response as an appendix, and quote the material sentences in the body. What you want from that response is a statement on network capacity at the proposed connection point, any reinforcement required, an indication of timescale, and whether the undertaker would seek a condition. A response that says capacity is available without qualification effectively closes the issue.

Three points on handling it. Make the enquiry early, because responses take weeks and a stale one covering a different unit count invites a fresh consultation. Describe the scheme accurately in the enquiry, including phasing, since an undertaker assessing 40 units at once gives a different answer from one assessing 40 units over four years. And if the response is unfavourable, deal with it head-on in this section rather than hoping nobody reads the appendix.

Section four: non-mains options and their consenting

Where the scheme is off mains, the assessment has to show the discharge itself is lawful as well as showing that a plant will be installed.

A discharge to surface water from a new septic tank is no longer acceptable, so the off-mains answer is either a package treatment plant or a drainage field. Discharges may be able to proceed under the general binding rules made under the environmental permitting regime, subject to volume limits and location conditions, with a permit required where those rules are not met. The volume limits are lower for discharges to ground than to surface water, and sensitive locations remove the general rules option altogether. The detail of package treatment plants and the general binding rules is worth reading before you specify a unit.

For a drainage field, the assessment needs percolation test results and a field sized from them, with the separation distances set out in Building Regulations Approved Document H: at least 10 metres from a watercourse, 50 metres from any water abstraction point, and 15 metres from a building. Show the field on the layout plan with those distances annotated. A soakage test done in August on a clay site is the single most common weakness in an off-mains submission, so state the date and the ground conditions at the time.

Section five: pumped connections and their risks

Gravity is preferred and pumping is often unavoidable. Where a pumping station is proposed, the assessment should cover the station's location, the rising main route, storage volume for a pump failure, the alarm and telemetry arrangement, odour control, and who will own and maintain it.

Adoption is the part that changes the planning outcome. An unadopted pumping station left to a management company is a long-term liability that officers reasonably question, and the answer needs to be in the document rather than left to a condition.

Section six: phasing where reinforcement is needed

Where the undertaker identifies that network reinforcement is required, the assessment should propose the mechanism rather than waiting for the council to invent one.

1. State the reinforcement needed and the undertaker's indicative programme.
2. Identify the scheme capacity that can connect without reinforcement, if any.
3. Propose a condition preventing occupation of the relevant phase until the reinforcement is complete and confirmed in writing by the undertaker.
4. Confirm whether a requisition under section 98 has been made or will be, and by whom.
5. Set out the fallback if reinforcement is delayed, and be honest where there is none.

A well-drafted Grampian condition offered by the applicant is almost always better than one written by an officer, because it will be tied to the right phase and the right trigger. Occupation is the usual trigger, and commencement of development is the one to resist, since it stops the scheme starting for reasons unrelated to construction.

Section seven: the conditions and agreements the undertaker asks for

Close the assessment with the list, so the officer can lift it into the decision notice.

Expect a requirement to separate foul and surface water, a prohibition on surface water entering the foul system, easements kept clear either side of any public sewer crossing the site, a build-over agreement where a structure sits over or close to a public sewer, and a condition tying occupation to any reinforcement. Where the site is in a nutrient neutrality catchment, the foul drainage route affects the nutrient budget, and the assessment should cross-refer to that work rather than duplicating it.

Finish with a short statement of what remains to be confirmed and when. A document that says clearly which figure is provisional is more use to an officer than one that presents an estimate as a measurement.

Draft your Foul Sewage Assessment with the enquiry response in hand, and see what each section has to prove before you start writing. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.