Building over or near a public sewer: the agreement

If you want to build over or within three metres of a public sewer, you need a build over agreement from the water and sewerage undertaker for your area, and that is a separate consent from both planning permission and building control approval. It is a private agreement with the undertaker, made under its statutory powers, and it is the one most extension projects discover late. Nothing in a planning permission authorises building over a sewer, and a building control body will not sign the work off without the agreement in place.

The starting point is finding out whether there is a public sewer under your garden at all, and there is a good chance there is one you never knew about.

How private drains became public sewers

On 1 October 2011 most private sewers and lateral drains in England and Wales that connected to the public network transferred into the ownership of the sewerage undertakers. Hundreds of thousands of shared drains beneath gardens, driveways and side passages became public sewers overnight.

The practical consequence is that the pipe running from your neighbour's house across the back of your garden to the street is very likely a public sewer now, even though no map showed it before 2011 and no easement appears on your title. Undertakers have been updating their records ever since, and the records are still incomplete in places.

So a clear sewer map is reassuring rather than conclusive. If a drain serves more than one property and it reaches the public network, treat it as public until the undertaker says otherwise.

Finding the sewer on the undertaker's record

Request a sewer map, sometimes called an asset location search or a public sewer plan, from the undertaker for your area. Most publish an online request form and return a plan showing sewers, manholes, pipe sizes and invert levels where they hold them.

Read three things off it. The alignment of the pipe relative to your proposed footprint. The invert level, which is the level of the inside bottom of the pipe and the number your foundation design has to respond to. And the position of manholes, because building over a manhole is refused almost without exception.

Where the plan shows nothing but you suspect a shared drain, lift the covers and trace the run. A drainage survey with a camera costs a fraction of a redesign, and the report becomes part of your application to the undertaker.

What a build over agreement permits

The agreement is the undertaker's consent to construct over or close to its apparatus, on conditions that protect the pipe and preserve access to it for repair. It is granted under the undertaker's powers in the Water Industry Act 1991 and it runs with the property, so it will appear in the pack when you sell.

Building Regulations requirement H4 covers building over or near existing drains and sewers, and the supporting guidance in Approved Document H is what most building control bodies apply. The undertaker's own conditions are usually stricter, and where the two differ, you satisfy both.

The application asks for the site plan, the proposed foundation design, the drainage layout, and a statement of the distance from the sewer to the nearest part of the new structure. Expect the undertaker to want structural detail rather than an outline.

Standard conditions on foundations and access

The conditions vary between undertakers and the themes are consistent.

• Foundations taken down to a level below the invert of the sewer, commonly assessed on a 45-degree line from the pipe, so that the new load is not transferred onto it
• A reinforced concrete lintel or bridging slab where the structure crosses the pipe, spanning clear of it onto undisturbed ground either side
• A minimum clear distance, often expressed as a horizontal offset from the outside of the pipe, with no part of the structure bearing on the pipe surround
• No building over a manhole, and no covering of an inspection chamber
• Retained access for maintenance, with a stated route and sometimes a requirement to reinstate at your cost if the undertaker has to excavate
• Use of flexible joints near the structure, and protection of the pipe during construction

Where the pipe is shallow, brick-jointed or already deformed, the undertaker may require replacement of a length in your ownership as a condition of agreement. That is a common and legitimate ask, and it is worth pricing before you commit to the layout.

Diversion under section 185 of the Water Industry Act 1991

Where building over is impractical, the alternative is to move the pipe. Section 185 of the Water Industry Act 1991 gives an owner or occupier the right to require the undertaker to alter or remove a relevant pipe, subject to the undertaker being satisfied that the work is reasonably practicable and that the alternative alignment is acceptable.

The cost falls on the person requesting the diversion, and it includes design, supervision, the works themselves and often a contribution to future maintenance. The undertaker will normally quote after a feasibility stage, and you pay for the feasibility work too.

Diversion is the right answer more often than people expect, particularly where the sewer crosses the middle of a proposed extension or where a manhole sits inside the footprint. Building over a sewer you cannot reach is a problem you hand to the next owner.

Timescales and how they affect a build programme

Treat the undertaker's process as a parallel programme, not a post-planning tidy-up. An asset location search comes back quickly. A build over agreement takes longer, because it is a technical assessment of your foundation design, and a diversion under section 185 takes considerably longer again because it involves design, land access and a works programme.

Start the sewer search at the same time as the measured survey, before the drawings are fixed. The two decisions that depend on it, foundation depth and footprint position, are the two that are most expensive to change later.

If surface water drainage is also being altered, the same programme logic applies to the lead local flood authority, and our note on what the lead local flood authority checks in a drainage strategy sets out what it wants to see.

Where building control and planning each fit

Planning permission deals with land use, scale, appearance and amenity. A case officer will not check the sewer position and a planning permission grants nothing in relation to the undertaker's apparatus. On a householder extension the drawings may show the pipe purely for information.

Building control checks compliance with requirement H4 and will want evidence that the undertaker has agreed. On a larger scheme the council will usually ask for a foul drainage position in the application itself, and a Foul Sewage Assessment or a Drainage Strategy is the normal place for it.

The sequence that works is: sewer search, then design, then planning application and build over application in parallel, then building control. Doing it the other way round is how a foundation gets dug twice. Long-term responsibility is a separate question again, covered in our note on who holds SuDS adoption and maintenance.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

If drainage is shaping your extension, enter your postcode on PlanScope to see which reports the site is likely to need. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.