Build over agreements where a public sewer crosses a site

If a public sewer runs under your garden, you need a build over public sewer agreement from the water company before you build on or close to it, and that consent is nothing to do with your planning permission. They are three separate permissions: planning from the council, building control approval under the Building Regulations, and the water company's own agreement as owner of the pipe. People discover the third one at the worst possible moment, which is when the groundworker's digger finds a brick-built sewer under the line of the new foundations.

The cost of finding out late is measured in weeks and in rebuilt foundations. The cost of finding out early is a sewer map search, and most undertakers provide one for a modest fee or free of charge.

Finding out whether a sewer is public and where it runs

Start with the sewerage undertaker's statutory sewer map. Every undertaker in England and Wales must keep a map of the public sewers, drains and disposal mains in its area under the Water Industry Act 1991, and it must be available for public inspection free of charge. In practice you order a drainage and water search or an asset location search online and get a plan back.

Two things to understand about what comes back.

First, the mapped position is indicative. Plans show the approximate line, often drawn from historic records, and the pipe can sit a metre or more from where the plan puts it. Where the sewer determines whether your extension fits, a CCTV survey or a trial hole to confirm the actual line and depth is money well spent.

Second, the map may not show everything. Private sewers and lateral drains serving more than one property transferred to the undertakers on 1 October 2011 under regulations made at the time, and a great many of those transferred assets have never been surveyed or added to the map. A shared foul drain running from your neighbour's house across your garden is very likely a public sewer now, even though no plan shows it. If a drain serves any property other than yours, treat it as public until the undertaker tells you otherwise.

Lift the manhole covers in your garden. A chamber with flows arriving from more than one direction is the clearest sign you are looking at a shared, and therefore probably public, drain.

When a build over or build near agreement is required

The trigger is proximity, not just position. Approved Document H to the Building Regulations requires the sewerage undertaker to be consulted where building work is to be carried out over or within 3 metres of a public sewer or disposal main shown on the statutory map, and the undertakers apply the same 3 metre test in their own procedures.

So you need an agreement if the new structure sits directly over the pipe, and you also need one if it sits anywhere within 3 metres of it. A single-storey rear extension that stops 2 metres short of the sewer is still caught.

Most undertakers operate two routes. Straightforward cases, typically a small domestic extension over a shallow sewer of modest diameter with no manhole involved, can go through a self-certification process handled by your building control body against the undertaker's published standard details. Anything else, and above all anything involving a manhole, a deep sewer, a large diameter pipe or piled foundations, needs a full build over application with structural details, foundation design and a method statement.

Expect the undertaker to refuse outright where the proposal would enclose a manhole inside a building. Access for maintenance is the whole point of the chamber, and the answer is relocation or diversion rather than negotiation.

Easement widths and the no build zone

The protection zone either side of a sewer is set by the undertaker and scales with the pipe. The common pattern is 3 metres either side of the centreline for smaller sewers, with wider zones, often 6 metres or more, for larger diameters and for deep or strategic assets. Check the figure your own undertaker publishes rather than assuming the 3 metre default.

Inside that zone the undertaker controls what can be built, how foundations are formed and what loading can be imposed on the pipe. Typical requirements include a foundation designed to span or bridge the sewer so that no load transfers to it, a minimum clearance between the underside of the foundation and the crown of the pipe, lintels or a reinforced raft where the pipe is shallow, and a prohibition on piling within a stated distance without a specific piling risk assessment.

Where there is a registered easement in the deeds the position is stricter still, because the easement is a property right. A deed easement with a no-build covenant can prevent development that the undertaker would otherwise have been willing to permit, and no planning permission overrides it.

Diversion as an alternative and its cost

Where building over is impossible, moving the sewer is the alternative. Section 185 of the Water Industry Act 1991 lets an owner or occupier of premises require the undertaker to move a relevant pipe, and the undertaker must comply where it is reasonably practicable to do so and the requirement does not compromise the sewerage system.

The applicant pays. A domestic diversion of a short length of shallow foul sewer is usually the cheapest option on the undertaker's menu. Deeper sewers, combined sewers, pipes crossing a highway and anything requiring a new manhole become expensive quickly, and the design and legal process commonly runs to several months.

Budget the time, not just the money. The diversion has to be designed, agreed, programmed and built, and the undertaker's own works programme governs when that happens. Starting the section 185 conversation after planning permission is granted is a reliable way to lose a construction season.

How the requirement interacts with planning permission

The council will not refuse your application because you have not got a build over agreement. It is not a planning matter, and the officer has no power over the water company's asset.

What the council can do is condition drainage. Where foul or surface water disposal is uncertain, expect a condition requiring details of the drainage scheme to be submitted and approved before construction, and that condition can only be discharged once the sewer position is resolved. On larger schemes the position belongs in a Drainage Strategy, and where foul capacity rather than pipe position is the issue, in a Foul Sewage Assessment.

The sewer also shapes the application itself. If the pipe forces the extension 3 metres away from the boundary, that is the scheme you apply for, and there is no point in submitting a layout you already know the undertaker will reject. Show the sewer line on the submitted block plan. An officer who can see that the drainage constraint has been designed around is far less likely to attach an open-ended pre-commencement condition.

Other below-ground consents follow the same pattern of sitting alongside planning rather than inside it, as set out in construction dewatering: the consents you actually need.

Timing: before design, not during excavation

The order of operations that works is short.

1. Order the sewer map and a drainage and water search before the architect starts drawing.
2. Lift every manhole on the plot and photograph what is inside, noting flow directions.
3. Where the mapped line matters to the footprint, confirm it physically with a trial hole or a CCTV survey.
4. Decide between building over, building near with a designed foundation, or diversion, and get the undertaker's view in principle in writing.
5. Only then fix the design and submit the planning application.
6. Submit the build over application or the section 185 diversion request in parallel with planning, not after it.

Running steps five and six together is the single change that saves the most time, because the two processes have no dependency on each other.

Evidence to submit with a householder application

Keep it proportionate. For a householder extension the council wants to see a block plan with the sewer line and manholes marked, a note confirming that an asset location search has been obtained and that the undertaker has been or will be approached, and a drainage layout showing where foul and surface water will go.

Adding the undertaker's in-principle response, where you have it, is worth the effort. It answers the drainage question before the officer asks it and often avoids a pre-commencement condition entirely. Your Planning Statement can deal with the point in two paragraphs.

What you should not do is leave the sewer off the drawings and hope. Councils share constraint data with the undertakers, and an officer who spots an unmarked public sewer under the proposed footprint will ask about it.

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.

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