Foul sewer capacity: making a pre-development enquiry

A pre-development enquiry to your water and sewerage undertaker tells you whether the foul sewer near your site can take the flow from your development, where you would be allowed to connect, and whether any reinforcement is needed first. Submit it before you submit the planning application, not after. The enquiry is made to the undertaker's developer services team, it carries a published fee, and the response arrives on the undertaker's own timetable rather than within your eight or thirteen weeks.

For a single plot the answer is usually simple. For anything from five units upwards on an older network, it frequently is not.

What the enquiry returns

A pre-development enquiry response normally contains four things. Extracts of the asset records showing public sewers, rising mains and water mains near the site. An assessment of whether the foul network has capacity for the flow you have described. An assessment of surface water, which in most cases is a refusal to accept it. And an indication of the point at which you would be permitted to connect.

The quality of the answer depends entirely on the quality of your question. Give the undertaker the number and type of units, the estimated foul flow, the site boundary, the levels if you have them, and the phasing if the scheme is phased. A vague enquiry produces a vague answer that a case officer will not accept as evidence.

Asset records are indicative rather than surveyed. Before you fix a layout around a sewer position, confirm it on site with a CCTV survey or a manhole survey, because sewers are not always where the plan says. Private drains serving neighbouring properties are frequently not shown at all.

Reading the response on foul and surface water

Three outcomes are common, and each leads somewhere different.

Surface water is a separate answer and usually a negative one. Undertakers resist new surface water connections to combined sewers, and the drainage hierarchy in national guidance points to infiltration and watercourses first in any event. Plan on managing surface water on site and treat any sewer connection as the last option.

Do not read a capacity answer as permanent. It describes the network as assessed on a date, and another development connecting first can consume the headroom. Note the date on the response, and re-run the enquiry if the scheme sits for more than a year.

The right to connect under section 106

Section 106 of the Water Industry Act 1991 gives the owner or occupier of premises the right to connect a drain or private sewer to the public sewer, by notice to the undertaker. The undertaker may refuse only on limited grounds, essentially that the mode of construction or condition of the drain is defective or that the connection would be prejudicial to the sewerage system.

That right is stronger than most applicants realise, and weaker than it sounds. It secures the physical connection. It does not oblige the undertaker to create capacity that does not exist, and "prejudicial to the sewerage system" is the ground on which capacity objections are expressed.

The practical sequence is the pre-development enquiry, then the section 106 connection application once the design is fixed, then the connection itself carried out by the undertaker or an approved contractor. Where the development creates new sewers that will serve more than one property and you want them adopted, that is a separate agreement under section 104, built to the current design and construction standards, with its own inspection regime and bond.

Requisitioning a new sewer under section 98

Where there is no public sewer within a reasonable distance, section 98 of the 1991 Act imposes a duty on the undertaker to provide one on requisition, subject to financial conditions.

The financial part is what people discover late. The requisitioner pays the cost of the works less the value of the income the undertaker expects to receive over a defined period, and provides security for the balance. On a scheme of a few houses a long way from the network, the resulting figure can be larger than the land value.

Get an indicative requisition cost before you buy the site or fix the price. It is a quotation exercise the undertaker is used to doing, and it belongs in the appraisal alongside the section 278 highway costs and the connection charges. Where the numbers do not work, a package treatment plant with a discharge permit may be the alternative, and that route has its own permitting and maintenance obligations.

Phasing conditions where capacity is constrained

Where reinforcement is programmed but not yet delivered, the usual planning solution is a condition tying occupation to the availability of capacity rather than refusing the application.

Two forms are common. A phasing condition permits a stated number of units to be occupied, with further occupation dependent on the works being completed. A negative condition prevents occupation of any dwelling until the identified network improvement is complete or an alternative agreed with the undertaker.

Both depend on the undertaker's investment programme, and a condition tied to a programme the undertaker has not committed to will be challenged. Ask the undertaker for written confirmation of the scheme, the trigger and the anticipated date, and put that letter in the application.

Grampian conditions and how they are worded

A Grampian condition prevents development from starting or being occupied until something happens on land outside the applicant's control. The test the courts and national guidance apply is whether there are reasonable prospects of the action being performed within the lifetime of the permission.

Good wording is specific and negative. It names the works, identifies the mechanism by which they will be delivered, and ties the restriction to a clear trigger such as first occupation. Bad wording asks for the works to be "agreed with the water company", which is unenforceable because it identifies no standard.

Draft the condition yourself and offer it. Officers under time pressure generally accept a well-drafted suggested condition, and you keep control of the trigger. Get the trigger right in particular: a condition biting at commencement rather than occupation can stop you building at all while the network works are programmed.

Evidence to submit with the application

Put the drainage case in one place so the officer and the undertaker's consultee both find it.

1. The pre-development enquiry response, in full, with its date.
2. A foul drainage assessment describing the flow, the connection point and the route, or a foul drainage assessment form where the proposal is not connecting to mains.
3. A drainage strategy dealing with surface water through the hierarchy, with calculations.
4. Any correspondence from the undertaker confirming reinforcement works or phasing.
5. A suggested condition where capacity is constrained.

A Foul Sewage Assessment covers the first two, and the surface water side belongs in a Drainage Strategy. The lead local flood authority will review the surface water case independently of the undertaker, so it helps to know what it checks in a drainage strategy and who will own the assets afterwards, which is a question about SuDS adoption and maintenance.

Enter your postcode on PlanScope to see which drainage documents your scheme is likely to need.

This is general guidance rather than legal advice. Check the position with your local planning authority, your water and sewerage undertaker 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.