Foul drainage capacity enquiries before you apply
You have a right to connect to a public sewer, so capacity problems arrive through the planning system as a condition. The pre-development enquiry is the evidence.
Before you submit a planning application for a new dwelling, make a pre-development enquiry to the sewerage undertaker for the area and get a written answer on foul drainage capacity. It is a separate exercise from the surface water design, it takes weeks rather than days, and the response is the evidence a validation officer and the drainage consultee will look for. The quirk that catches self-builders is that section 106 of the Water Industry Act 1991 gives you a right to connect to a public sewer, and lack of capacity is not generally a ground for the undertaker to refuse that connection. So the capacity problem arrives through the planning system instead, as a condition.
Make the enquiry early. A connection you have a right to make is not the same as a connection the council will let you occupy a house through.
Why capacity is assessed separately from surface water
Foul and surface water are different flows with different destinations, and modern drainage design keeps them apart. Surface water should be managed on site where possible, through infiltration or attenuation to a watercourse, with a connection to a combined sewer as a last resort.
Foul flow has nowhere else to go. It either reaches a public sewer and a treatment works, or it is dealt with on site by a package treatment plant or a septic tank. The capacity question is therefore about two things: whether the local network can carry the additional flow without causing flooding or overflow, and whether the receiving treatment works has headroom within its permit.
Those are the undertaker's questions, and the Environment Agency's where the permit is the constraint. Neither is answered by the drainage strategy you prepare for surface water.
The enquiry to make and what it returns
Every sewerage undertaker runs a developer services function with a pre-development enquiry process. The names differ by company, the substance does not.
A full enquiry response usually includes:
- Public sewer records for the area, showing position, size, depth and type
- Whether the network has capacity for the proposed foul flow
- An indicative point of connection
- Whether network reinforcement is required, and an indication of timing
- Comments on the receiving treatment works
- Whether any asset crosses the site, and the easement or diversion implications
Submit the enquiry with a site plan, the number and type of units, and the proposed foul flow in litres per second or litres per day. Vague enquiries get vague answers. If an asset crosses the site, the easement position needs resolving before the layout is fixed, because building over or near a public sewer requires the undertaker's agreement and the standoff distances will move your house.
Reading a capacity response
Responses are written carefully, and the phrasing matters.
| What the response says | What it means for the application |
|---|---|
| Capacity available at the identified point of connection | Submit the response as evidence and proceed |
| Capacity available subject to a connection application | Normal position, but confirm no conditions are attached |
| Reinforcement required before the development can be served | Expect a condition linking occupation to the upgrade |
| Capacity constrained at the treatment works | The Environment Agency's position becomes the issue |
| No public sewer within a reasonable distance | Non-mains drainage, with its own evidence requirements |
A response that says capacity is available today is not a guarantee for a scheme that will complete in three years, and the undertaker will say so. Where the programme is long, re-run the enquiry before submission and again before the connection application.
Where reinforcement is needed and who pays
Network reinforcement is the undertaker's responsibility, funded in part through the infrastructure charges developers pay on connection. That sounds reassuring and the practical experience is more complicated, because the work has to be programmed into the undertaker's capital schedule.
Three routes exist when the timing does not suit. Wait for the programmed scheme. Requisition a public sewer under the Water Industry Act 1991, which gives a mechanism for the undertaker to provide a sewer with the developer sharing cost. Or fund the works yourself under an agreement, recovering what you can through the charging arrangements.
Charges and the way they are calculated are published annually by each undertaker, so check the current charging arrangements rather than a figure from a previous project. For a single self-build plot the sums are usually modest; for ten units with a reinforcement requirement they are not.
Connection applications and their timescales
The connection application is a separate step from the enquiry and from the planning permission. It is made to the undertaker, with drawings of the proposed connection, and leads to an approval that governs how the connection is physically made, by whom and under what supervision.
Where new sewers are to be adopted, a separate agreement under the Water Industry Act 1991 covers design approval, inspection and adoption. On a single plot that rarely arises. On a small scheme with a shared drain serving several houses, it does, and the adoption question affects what the council will accept and what a purchaser's solicitor will want to see.
Allow months rather than weeks for all of this, and sequence it so the connection approval is in hand before the groundworks programme needs it.
Evidence to submit with the application
For a site served by a public sewer, the pack is short:
- The pre-development enquiry response, in full rather than quoted
- A drainage layout showing the foul route and the point of connection
- The foul flow calculation, with the assumptions stated
- Confirmation of the receiving treatment works where the undertaker has identified a constraint
- Where an asset crosses the site, the easement position and any build-over agreement
Where there is no public sewer, the evidence changes. Building Regulations set out a hierarchy that puts connection to a public sewer first, then a private sewer, then a package treatment plant, then a septic tank, and the application should show why each higher option was discounted. Percolation testing, the outfall position and compliance with the general binding rules for small sewage discharges all become relevant, and a foul sewage assessment is the document that pulls them together. If the discharge is to a watercourse, the outfall design is a separate problem covered in outfall design, bank stability and scour protection.
Nutrient sensitive catchments add another layer. Where the receiving works discharges into a catchment with a nutrient constraint, the foul drainage position becomes part of the habitats assessment, and that is a question to ask at enquiry stage rather than after validation.
Conditions tying occupation to capacity
Where reinforcement is needed, the usual mechanism is a condition preventing occupation of any dwelling until the identified works have been completed. It is a sensible condition and it transfers programme risk onto you.
Negotiate the detail. A condition tied to a specific named scheme is easier to discharge than one requiring confirmation that capacity exists. A condition allowing phased occupation as capacity becomes available is better than one blocking the whole site. And a condition that lets the undertaker confirm completion in writing is cleaner than one requiring the council to satisfy itself.
Ask for the wording before the officer drafts it. Once the decision notice is issued, changing it means a section 73 application and another eight weeks.
Enter your postcode on PlanScope to see which drainage documents your site is likely to need.
This is general guidance, not legal advice. Check the position with your local planning authority, the 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.
Frequently asked questions
Can a water company refuse a foul connection because the sewer is full?
Section 106 of the Water Industry Act 1991 gives an owner a right to connect to a public sewer, and lack of capacity is not generally a ground for refusal. The constraint is instead dealt with through planning, usually by a condition preventing occupation until identified network reinforcement has been completed.
What does a pre-development enquiry response contain?
Public sewer records showing position, size, depth and type, a view on whether the network has capacity for the proposed flow, an indicative point of connection, whether reinforcement is needed and when, comments on the receiving treatment works, and whether any asset crosses the site with easement implications.
Who pays for network reinforcement?
Reinforcement is the undertaker's responsibility, funded partly through the infrastructure charges paid on connection, but it has to be programmed into the capital schedule. Where timing does not suit, the options are to wait, to requisition a sewer under the Water Industry Act 1991, or to fund the works under an agreement.
What if there is no public sewer near the site?
Building Regulations set a hierarchy putting connection to a public sewer first, then a private sewer, then a package treatment plant, then a septic tank. The application should show why each higher option was discounted, supported by percolation testing, the outfall position and compliance with the rules for small sewage discharges.
Can I negotiate the wording of the occupation condition?
Yes, and it is worth doing before the officer drafts it. A condition tied to a specific named upgrade scheme is easier to discharge than one requiring proof that capacity exists, and phased occupation as capacity becomes available is better than a condition blocking the whole site.
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.