Extending over a public sewer in a back garden

If your extension will sit over a public sewer, or within 3 metres of one, you need a build over agreement from the sewerage undertaker as well as planning permission. The two consents are separate. Planning officers rarely mention the sewer at all, and the problem usually surfaces months later when building control asks for the agreement before signing off the foundations. Building over a sewer on an extension is a drainage and building regulations matter first, so the sooner you find the pipe, the less it costs to design around it.

Most back gardens in terraced and semi-detached streets have a shared drain running along the rear boundary or straight across the middle of the lawn. Since October 2011 those shared private drains and sewers transferred to the water companies, which is why a run that your neighbour still calls "our drain" is now a public sewer with a statutory undertaker attached to it.

Finding the sewer before you commit to a design

Order a sewer map from your local water company. Every undertaker holds a statutory public sewer map under section 199 of the Water Industry Act 1991 and sells extracts, usually as an asset location search returning a plan of public foul, surface water and combined sewers with approximate depths and manhole positions.

The map is approximate. Treat the plotted line as a search area rather than a survey line, because the record was inherited from the 2011 transfer and much of it was never surveyed to the centimetre. If the pipe runs anywhere near your proposed footprint, lift the covers in your garden and the gardens either side, measure the invert depths and the direction of flow, and mark the real line on your drawings.

A drainage survey with a camera costs a fraction of the cost of moving a foundation later. And it tells you something the map cannot: whether the pipe is clay, pitch fibre or plastic, and whether it is already cracked.

What a build over agreement actually covers

The agreement is the undertaker's permission for a structure to sit over or close to its asset, granted on conditions that protect access and the pipe itself. Requirement H4 of the Building Regulations sits behind it, and Approved Document H sets out what building control expects: the sewer must not be damaged, access for maintenance must be preserved, and the risk of the building settling onto the pipe must be dealt with by the foundation design.

Undertakers split applications into two routes. A standard or self-certified route covers small domestic extensions over a single shallow pipe of modest diameter, with no manhole involved and a straightforward foundation. Anything else goes to a full technical assessment by the undertaker's engineers, who may ask for a structural detail, a CCTV survey of the pipe before and after the works, and a condition requiring the sewer to be re-surveyed once the build is finished.

Fees are set by each undertaker and published on its own website, and they are revised periodically, so check the current schedule rather than a figure quoted on a forum. Determination usually takes a few weeks for the simple route and longer where engineers must review a design.

The 3 metre rule and inspection chambers

The 3 metre figure is the distance from the outside edge of the pipe at which most undertakers want to be consulted, and it is the trigger used in Approved Document H for a building near a drain or sewer. Inside that zone the questions are about load: whether your foundation will transfer weight onto the pipe, and whether the pipe would be reachable if it failed.

Manholes are the sticking point. An undertaker will not normally agree to a habitable room built over an inspection chamber, because a jetting crew cannot work inside someone's kitchen. Some will accept a chamber under a non-habitable space with a double sealed cover and independent access, but that is a case-by-case judgement rather than a right.

Diverting the sewer instead

Where the pipe cuts straight through the middle of your proposed extension, a diversion is often the cleaner answer. Section 185 of the Water Industry Act 1991 lets an owner or occupier require the undertaker to move a pipe, at the owner's cost, provided the alteration is reasonably practicable and does not prejudice the undertaker's ability to run its network.

Diversions are slower and more expensive than build over agreements. You are paying for design, a road or garden excavation, new pipe, connection works and reinstatement, and the undertaker will want a deposit before it starts. Budget months rather than weeks. Some schemes work better if the extension is redesigned around the run, particularly on a side return infill where a metre of plan width is worth less than the delay.

If you are already reshaping the rear of the house, look at how a side return infill on a Victorian terrace handles the same constraint, because the drainage run and the party wall usually appear together.

Foundations near a pipe

The engineering point is simple. Your foundation must not load the sewer, and the sewer must not undermine the foundation if it leaks.

Three details do most of the work on a domestic job:

1. A deepened trench fill foundation taken below the invert of the pipe, so load is carried past it rather than onto it.
2. A reinforced concrete lintel spanning the pipe, bearing on undisturbed ground either side, where digging deeper is impractical.
3. Flexible joints and a protective surround where the new structure crosses the line, with granular fill that lets the pipe move slightly rather than fracture.

Your building control body checks these details, and the undertaker's engineer may want to see them too. Get the structural engineer involved at design stage rather than after the trenches are open, because a redesign at that point stops the job.

Sequencing the agreement against the planning application

Planning permission and the build over agreement run on separate tracks and neither waits for the other. You can apply for both at once. What you should avoid is submitting a planning application with a footprint you already know the undertaker will refuse, because you will then need a fresh application or an amendment for the revised layout.

A sensible order for a householder scheme:

1. Sewer map and a lift-the-covers check, before the design is fixed.
2. Outline discussion with the undertaker where a manhole or a deep pipe is involved.
3. Planning application, with the drainage constraint shown on the site plan.
4. Build over application with the foundation details, alongside the building regulations submission.
5. Post-construction CCTV survey if the agreement requires one.

Where the council asks for a drainage narrative, a Drainage Strategy covers surface water and foul connections in one document, and the supporting case for the works usually sits in the Planning Statement. For the mechanics of the agreement itself, including what an undertaker asks for on larger sites, see our note on building over or near a public sewer.

One honest uncertainty: undertakers differ in how they treat shallow pipes serving only two or three properties, and a run that one company waves through under a self-certification form will get a full assessment from another. The map extract costs very little and the phone call is free, so ask before you draw.

This is general guidance, not legal advice. Check the position with your local planning authority, your building control body and your water company before relying on it.

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