Who prepares a foul sewage assessment

There is no licence for writing a foul sewage assessment and no chartered title attached to it. In practice the foul sewage assessment author is one of three people: the drainage engineer on a scheme with any complexity, the architect or agent on a single dwelling connecting to a public sewer, or a specialist non-mains drainage contractor where a package treatment plant and a drainage field are proposed. What matters is not the job title. It is whether whoever writes it can produce the three pieces of evidence a council actually reads: the sewerage undertaker's capacity position, the percolation test result, and the permitting position for any discharge.

Councils expect a suitably qualified author for the technical content, and professional review before submission is strongly advised. On a single plot that review may be twenty minutes of a drainage engineer's time. On a 30 dwelling scheme discharging to a small works it is a proper appointment.

Mains connection against non-mains solutions

National guidance sets a drainage hierarchy and councils apply it strictly. Connection to the public foul sewer comes first. A package treatment plant discharging to a watercourse or to ground comes second. A septic tank with a drainage field comes last, and only where the others are not reasonably available.

So the first job is to establish whether a mains connection is available, and "available" is a legal and practical question rather than a matter of distance. Section 106 of the Water Industry Act 1991 gives an owner the right to connect to a public sewer. Section 98 allows a sewer to be requisitioned where one does not reach the site. Either route can be expensive, and cost alone does not usually justify dropping down the hierarchy, though a disproportionate cost properly evidenced sometimes does.

The assessment has to show the enquiry was made and record the answer. An assertion that no sewer is nearby, with no undertaker correspondence attached, is the most common reason a foul drainage condition gets imposed rather than discharged.

Capacity enquiries and who makes them

Any competent agent can make a pre-development enquiry to the sewerage undertaker. Doing it early, and asking the right question, is what distinguishes a useful answer from a useless one.

Ask three things in the same enquiry.

• Whether a public foul sewer is available, with its location, size and invert level.
• Whether there is capacity in the network and at the receiving treatment works for the proposed number of units or the proposed trade effluent load.
• What upgrades, if any, would be required and on what programme.

The answer comes back as a letter or a standard response form, and it goes in the application as an appendix. Where the undertaker identifies a capacity constraint with a delivery date, the usual outcome is a Grampian condition preventing occupation until the upgrade is complete. That is manageable if you know about it before you buy the site, and painful if you discover it at week six of a thirteen week determination.

Package treatment plants and septic tanks

The distinction matters because the regulatory treatment differs. A septic tank gives primary settlement only and may discharge to ground through a drainage field, never directly to a watercourse. A package treatment plant provides secondary biological treatment and may discharge to a watercourse where the rules allow.

For planning purposes the assessment needs the plant's population equivalent, its treated effluent quality, the discharge point, and the maintenance regime with who is responsible for it. For multiple dwellings sharing a plant, say who will own and maintain it, because an unadopted shared plant with no management company is a condition waiting to be imposed.

Sizing follows the number of bedrooms or the population equivalent rather than floor area. A five bedroom house is not the same load as a three bedroom one, and undersizing is the failure mode that shows up two years after occupation.

General binding rules and permits

This regime sits outside planning entirely and the assessment should say so plainly, because applicants routinely assume planning permission authorises the discharge. It does not.

Small sewage discharges in England are controlled by the general binding rules, published on gov.uk, made under the Environmental Permitting Regulations 2016. Where the discharge meets the rules, no permit is needed but the rules must be complied with, including limits on volume, requirements about the type of system and restrictions in sensitive locations such as groundwater source protection zones. Where the discharge falls outside the rules, by volume or by location, an environmental permit is required from the Environment Agency.

Two practical points. The rules and the volume thresholds are revised periodically, so cite the current version rather than a figure remembered from an earlier project. And a site in a source protection zone, or within a given distance of a designated site, can be taken out of the general binding rules altogether, which changes the programme because a permit application takes months.

Percolation testing and who carries it out

Where the discharge is to ground, the drainage field has to be sized from a percolation test. The test method is set out in British Standard guidance referenced by Approved Document H of the Building Regulations, and it involves trial holes, pre-soaking and timed water level drops repeated across the proposed field area.

This is site work, not desk work. A ground investigation contractor or an engineer with the right equipment carries it out, and the results need the test pit positions plotted on a plan with the water table depth recorded. A single test in the driest week of August is not evidence; the standard expects repeat measurements and the winter groundwater position matters.

Two outcomes stop a scheme. Percolation too slow, so the ground will not accept the discharge. Or a water table too high, so the required separation between the base of the drainage field and groundwater cannot be achieved. Both are better discovered before the land is bought.

An illustrative example

Illustrative only: a two dwelling infill plot in a village with no public foul sewer within 300 metres. The agent makes a pre-development enquiry and receives written confirmation that no sewer is available and requisitioning would require 280 metres of new sewer through third party land. A contractor carries out three percolation tests, returning an acceptable rate and a water table at 2.1 metres. The scheme adopts a shared package treatment plant discharging to a drainage field, with a management company named. The assessment attaches the undertaker's letter, the test results, the plant specification and a statement that the discharge falls within the general binding rules. Four appendices, one page of narrative, and no outstanding drainage objection.

Who writes the assessment

Match the author to the complexity.

For a single dwelling connecting to a public sewer, the agent can write it, with the undertaker's response attached. For a single dwelling on non-mains drainage, the agent can still write it provided the percolation results and the plant specification come from people qualified to produce them. For anything over a handful of units, for a commercial process generating trade effluent, or for any site in a source protection zone or a nutrient neutrality catchment, appoint a drainage engineer.

A Foul Sewage Assessment and a Drainage Strategy are different documents answering different questions, and which of the two a site needs depends on whether surface water is also in issue. Where there is no mains sewer at all, what to show in the submission is a longer list. A reviewer from the professional directory can sign off the technical judgements on an AI-assisted draft.

Securing upgrades by condition

Where capacity is the constraint rather than availability, the usual mechanism is a condition preventing occupation until a specified upgrade is delivered or until a drainage scheme has been approved. Negotiate the wording. A condition tied to a named upgrade with a date is manageable. One requiring the applicant to demonstrate capacity to the undertaker's satisfaction, with no mechanism, is a trap.

This is general guidance, not legal advice; 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. For specialist technical reports, qualified professional input may be required.