Evidence base for development near a public sewer

Building near a public sewer is a two-track problem. The planning application is one track and the agreement with the water company is the other, and the second one is the one that delays projects. A permission granted in March is worth nothing if the build over agreement takes until September, and the local planning authority has no power to speed it up.

Start with the assumption that any drain running through the garden of a house built before the 1930s may be a public sewer. Large numbers of private sewers transferred to the water companies in 2011, which means pipes that appeared on nobody's plan became public assets with statutory protection.

Finding the sewer and reading the asset records

Order the sewer records first, before any design work. The water company's asset plan shows recorded public sewers with manhole references, and a drainage and water enquiry gives the same information in a conveyancing format with additional detail on connections and water mains.

Read them sceptically. Asset plans are indicative. They show alignments schematically, they often omit laterals and transferred sewers entirely, and the recorded invert levels can be decades old. Two site checks turn an indicative plan into usable information: a manhole survey lifting covers to record cover and invert levels, diameter, direction of flow and condition, and a CCTV survey where the alignment between manholes is uncertain or where a pipe appears to run under the footprint.

Where no record exists but an old drain is suspected, a dye or tracer test from the nearest gully or inspection chamber will usually establish where the flow goes. Do it before you fix the layout, not after the foundations are marked out.

The build over or build near agreement

The requirement comes from Part H of the Building Regulations, which deals with building over or close to existing drains and sewers and requires that access for maintenance is preserved, that the pipe is not overloaded and that the building is protected if the pipe fails. Building control will ask for evidence that the water company has agreed to the arrangement.

Each undertaker publishes its own criteria, and the usual limits are consistent in shape. Most will consider a build over for smaller diameter sewers at modest depth, will refuse to allow building over a manhole, and will set conditions on foundation design so that no load is transferred to the pipe. Common solutions are a reinforced concrete lintel spanning the pipe, deepened foundations taken below the invert with a defined clearance, a raft or, on constrained sites, piles positioned outside the protection zone.

Three points that catch self-builders out.

An agreement is needed for building near a sewer as well as over it, and the trigger distance is usually measured from the outside of the pipe rather than the centreline.

A conservatory, an extension, a garage and a raised deck can all require agreement, and so can a soakaway or an attenuation tank placed too close.

The agreement is separate from the planning permission and from the right to connect. Having one does not give you the others.

Standard easement widths and their effect on layout

Undertakers protect their assets with easement or protection widths measured from the pipe, and those widths increase with diameter and depth. For a small residential sewer the protected strip is commonly three metres either side of the centreline, and it widens considerably for larger or deeper pipes. Each company publishes its own standards, so use the one that applies to your area rather than a figure remembered from another job.

Treat the protected strip as undevelopable when you lay the site out. It can be garden, parking, access road or landscaping, but not a building. On a small infill plot a sewer crossing the middle of the site can reduce the developable area enough to change the number of units, and finding that out at reserved matters stage is expensive.

Where the strip makes the scheme unviable, there are two options: design around it, or divert the sewer.

Diverting a sewer under section 185

Section 185 of the Water Industry Act 1991 gives the owner or occupier of premises the right to require the undertaker to move a public sewer where it is reasonably practicable to do so, at the applicant's expense. The undertaker can refuse only on defined grounds, and it can require security for the cost.

What this means in practice: a diversion is available, it costs money, and it takes time. The sequence is a technical enquiry, a design, a quotation, a legal agreement including new easements over the diverted route, and then the works. Several months is normal and a year is not unusual on larger diameters or where third party land is involved.

Two points to check before relying on a diversion. Whether the new route can be kept entirely within land you control, because a diversion across a neighbour's land needs their agreement and new rights. And whether the diversion has to be built before development starts, which affects the programme and sometimes the discharge of pre-commencement conditions.

Capacity information for foul and surface water

Separate from the physical position of the pipe is the question of whether the network can take the flows. Ask the undertaker's developer services team for a capacity assessment for foul and for surface water, giving the expected population and peak flows.

Two rules that shape the answer. Surface water must not be connected to a foul sewer, and undertakers routinely refuse it. And discharge to a public sewer for surface water sits at the bottom of the drainage hierarchy, below infiltration and discharge to a watercourse, so the drainage strategy has to show why the higher options were rejected.

A Foul Sewage Assessment is the document that records the connection point, the receiving works, the flows and the undertaker's response on capacity. The Drainage Strategy deals with the surface water side and the discharge rate. Where several utilities cross the site, a Utilities Assessment pulls the constraints onto one drawing, which is worth doing on any plot with more than one buried service.

Programming the agreement against the planning timetable

Run the two processes in parallel from the outset.

1. Order sewer records and utility searches at the same time as the topographical survey.
2. Carry out the manhole and CCTV survey before the layout is fixed.
3. Submit the build over enquiry, or the diversion enquiry, as soon as the footprint is stable, without waiting for planning permission.
4. Submit the planning application with the sewer shown on the layout and the protection strip annotated.
5. Resolve the agreement while the application is being determined.

Applicants who wait for permission before contacting the water company add the full agreement period to the end of the programme rather than overlapping it. On a single plot that might be six weeks. On a scheme needing a diversion it can be most of a year.

What the council will and will not condition

The local planning authority will condition foul and surface water drainage details, usually requiring a scheme to be approved before development starts and implemented before occupation. It will consult the sewerage undertaker on larger applications, and it will take an objection from the undertaker seriously.

What it will not do is grant the build over agreement, enforce it, or accelerate it. A condition requiring you to obtain the undertaker's agreement is of limited use, because conditions should not depend on the action of a third party where there is no reasonable prospect of that action being taken, and in any case the obligation already exists under the Building Regulations.

So the sewer needs to appear on the application drawings with its protection strip shown, and the planning statement needs to say what the arrangement will be. Officers do not expect you to have the agreement in hand at submission. They do expect the layout to be capable of being built.

The detail of the agreement itself is set out in the note on building over or near a public sewer, how to obtain the records is covered in the note on ordering sewer records and pre-development enquiries, and the householder case is dealt with in the note on extending over a public sewer in a back garden.

This is general guidance, not legal advice; check the position with your local planning authority, your 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.