Foul drainage capacity and pre-development enquiries

A foul drainage capacity enquiry is the lowest cost thing you can do early on a small residential site, and the one most often left until the drainage engineer is drawing connections. Sewerage undertakers run pre-development enquiry services that tell you where their assets are, whether the network can take the flow, and what reinforcement might be needed. Those answers take weeks, sometimes longer, and they run on the undertaker's timetable rather than the council's.

Start them before the layout is fixed, not after the application is validated.

Why capacity is checked before the layout is fixed

Foul flows have to go somewhere by gravity if at all possible, and gravity depends on levels. Where the connection point moves 80 metres up the road because the nearer sewer is at capacity, the finished floor levels, the plot layout and sometimes the number of units move with it.

A pumping station changes the scheme again. It needs a site, a standby generator or storage, an access for a tanker, and an odour separation distance from the nearest habitable window. Discovering that at technical approval stage after permission has been granted means a section 73 application or a non-material amendment, and sometimes neither works.

There is a policy dimension too. National practice guidance on water supply and wastewater expects the adequacy of infrastructure to be considered as part of the planning process, and authorities increasingly ask for evidence rather than an assertion that a connection is available.

Who to ask and what to ask for

The sewerage undertaker for the area is the starting point. In most of England that is the regional water and sewerage company, and each runs a developer services function with a published enquiry process and fee scale.

Ask for four things, and ask for them together:

1. A copy of the public sewer records for the site and the surrounding streets, with pipe sizes, invert levels, manhole references and, where available, the direction of flow.
2. Confirmation of the point of connection the undertaker will accept for the proposed number of units.
3. A capacity assessment of the receiving network for the estimated foul flow.
4. Confirmation of any planned or committed reinforcement works and their programme.

Where the site is large enough or the network is known to be constrained, ask whether the undertaker will provide a statement suitable for submission with the planning application. Most will, and a letter from the undertaker answers a consultee objection before it is made.

The Development Management Procedure Order 2015 sets out when a sewerage undertaker must be consulted on an application, and many authorities consult more widely than that as a matter of practice. Either way, the undertaker's response to the council will repeat what it has already told you, so it is better to have asked first.

Ordering sewer records and reading them

Every sewerage undertaker maintains a public sewer map under the Water Industry Act 1991 and will supply an extract. What arrives is a plan with lines on it, and reading it properly takes a little care.

Public sewers, lateral drains and private drains are shown differently and the distinction decides who is responsible for what. Many sewers that look private were transferred into public ownership in 2011, so an old survey or an architect's memory of the estate is not reliable. Combined sewers carry both foul and surface water, and connecting new surface water to one will usually be refused outright.

Check the invert levels against your site levels before you assume a gravity connection exists. Check whether a sewer crosses the site itself, because that changes the developable area and brings the build-over question into play. And check the date of the record, which is often older than people expect.

Capacity assessments and their timescales

A capacity assessment models the receiving network with your flow added. The output is one of three answers: capacity exists, capacity exists subject to conditions or a restricted discharge rate, or reinforcement is required.

Timescales vary by undertaker and by the complexity of the enquiry. Treat several weeks as the minimum for a straightforward response and allow considerably longer where modelling of a wider catchment is involved. Fees are published by each undertaker and are modest against the cost of redesigning a layout.

Connection applications and the consents that follow

The right to connect to a public sewer arises under section 106 of the Water Industry Act 1991, and it is exercised by giving notice to the undertaker. The undertaker can refuse in defined circumstances, including where the mode of construction is unsatisfactory or where the connection would be prejudicial to its system.

Where there is no public sewer within a reasonable distance, section 98 allows a requisition, under which the undertaker provides a sewer subject to a financial arrangement with the developer. That is a longer and more expensive route, and it is worth knowing it exists before concluding that a site cannot be served.

New sewers intended for adoption are built under an agreement with the undertaker and to its design and construction standards. Adoption is a separate process from planning and from building control, and a scheme can hold a planning permission, a building regulations approval and still not have an adoptable drainage design.

Infrastructure charges are levied by the undertaker on new connections under its published charging arrangements. They are not a planning matter and they are not part of the Community Infrastructure Levy, so budget for them separately.

Building over or near a public sewer

Where a public sewer crosses the site, building over or within the protected distance of it requires the undertaker's agreement. Each undertaker publishes its own requirements, and the usual outcomes are a build-over agreement with conditions on foundation design, a diversion under section 185, or a redesign that keeps the structure clear.

Diversions take time and money and need the undertaker's consent to the new alignment. They also need land. Where a diversion is the only answer, price it and programme it before the land deal completes rather than afterwards.

A council will not normally refuse permission because a sewer crosses the site, but it may condition the development so that no work starts until the undertaker's agreement is in place. That condition is entirely reasonable and it is also entirely capable of stalling a site for months.

Phasing where reinforcement is needed

Where the capacity assessment shows that reinforcement is required, the usual planning solution is a condition preventing occupation of any dwelling until the necessary works are complete, or a phased trigger tied to the number of units connected.

Draft the trigger with the undertaker's programme in front of you. A condition requiring the whole of a network upgrade before the first occupation, where the upgrade is scheduled for a later investment period, is a condition that stops the site. A condition allowing a first phase within the existing spare capacity, with later phases tied to the upgrade, keeps it moving.

Set the position out clearly in the Drainage Strategy, with the undertaker's correspondence appended. Where foul flows are the main issue rather than surface water, a Foul Sewage Assessment is the document the validation checklist is likely to name.

What to put in the application

Include the undertaker's written response, the point of connection, the estimated foul flow and how it was calculated, the proposed route, and the levels that make it work. Where a pumping station is proposed, show it on the layout with its access and its separation from habitable windows.

Where reinforcement or a build-over agreement is outstanding, say so and propose the condition yourself. Officers respond well to an applicant who has identified the constraint and offered the mechanism. What draws an objection is a drainage note that says connection to the public sewer with no evidence behind it. The points a consultee looks for are set out in what a consultee checks in a drainage model output, and the same scrutiny applies to foul flows.

For a larger scheme, the wider picture belongs in a Utilities Assessment covering water, power, gas and telecoms alongside drainage, because the constraints tend to arrive together.

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. To see which drainage documents your site is likely to need, enter your postcode on PlanScope.