Building over a public sewer under permitted development

Building over sewer permitted development is one of the most common misunderstandings in householder work. Permitted development rights under the General Permitted Development Order remove the need for planning permission. They do nothing at all about the sewerage undertaker, whose powers come from the Water Industry Act 1991 and whose consent you still need before a rear extension goes over a public sewer. Homeowners discover this when the building inspector asks for the build over agreement, usually after the foundations have been dug.

Why permitted development does not displace the undertaker's rights

The Order is planning legislation. It tells you when a local planning authority's permission is not required for a particular class of development, and its limits are about size, position and materials.

The sewerage undertaker's position rests on different statutes. Public sewers vest in the undertaker, which has statutory rights of access to maintain and repair them, and separate powers to require alteration or diversion. A private extension built over a public sewer interferes with those rights whether or not planning permission was needed.

Building control is the third strand. Approved Document H to the Building Regulations deals with building over or close to a drain or sewer, and the building control body has to satisfy itself that the arrangement is acceptable, which in practice means seeing the undertaker's agreement.

Three regimes, three timetables, none of them automatic because of the others. The same pattern appears with wildlife law, as our note on protected species and permitted development explains.

Identifying a public sewer beneath the proposal

Start with the records, then verify them on site.

• Order a drainage and water search, or request the sewer records directly from the undertaker. Both show mapped public sewers with approximate positions, sizes and manhole locations.
• Lift the manhole covers in and around the garden. The direction of flow, the number of incoming branches and the invert depths tell you more than the plan does.
• Where a drain serves more than one property, treat it as public until proved otherwise.
• Where the records are ambiguous or the pipe is not shown, commission a short drainage survey with a camera and, if needed, dye testing from the neighbouring properties.

Mapped positions are indicative. Sewers were laid a century ago in some streets, and a pipe drawn straight down the middle of the garden on the plan is regularly found two metres from where it should be. Excavating a trial hole before the design is fixed costs very little and saves redesign.

The practical point is that you need the position, depth and diameter, not just the existence, because all three feed the foundation design.

Transferred private drains and who owns them

The single biggest source of confusion is the 2011 transfer. Private sewers and lateral drains that were connected to the public network before 1 July 2011 transferred into the ownership of the sewerage undertakers later that year.

The effect on a typical Victorian or interwar terrace is substantial. The shared drain running along the rear of the gardens, which everybody assumed was private and which appears on no deed plan, is very likely a public sewer now. So is the lateral drain from your inspection chamber to the sewer in the street, beyond the boundary of the property.

Older records do not always reflect the transfer. If the run serves more than one property, or crosses a boundary before reaching the main, assume the undertaker has an interest and ask.

The ownership question also affects who pays for repairs and who is liable for a blockage, which is worth knowing before you build a kitchen over it.

Build over and build near agreements

Undertakers operate a consent process for building over, or close to, a public sewer. The trigger is usually building within three metres of the pipe, and the process splits between a standard arrangement for simple cases and a bespoke one for anything unusual.

Standard consent typically depends on the sewer being of modest diameter and depth, in reasonable condition, with no manhole enclosed by the new structure, with a limited length of pipe covered, and with foundation details meeting the undertaker's requirements.

A bespoke agreement is needed where any of that fails: a large or deep sewer, a rising main, a manhole inside the footprint, a long length covered, or a pipe in poor condition. Expect a condition survey, sometimes a requirement to reline the pipe at your cost, and a formal deed.

Apply early. The undertakers publish service standards for these applications, and the time taken is measured in weeks rather than days even for a straightforward case.

Foundation design to satisfy the undertaker

The design principle is that no load from the new building should reach the pipe, and that the pipe must remain accessible for repair.

The usual solutions:

1. Take the foundations below the level of the pipe, deep enough that a line drawn upward at 45 degrees from the invert of the sewer passes clear of the underside of the foundation.
2. Where that is impractical, bridge the sewer with a reinforced concrete lintel spanning onto foundations either side, so the pipe sits in an unloaded void.
3. Maintain the minimum clearance the undertaker specifies between the foundation and the pipe, and keep concrete out of the trench around it.
4. Where the pipe is in poor condition, replace or reline the section before building over it, since access afterwards is expensive.
5. Never enclose a manhole inside the building. Where one falls within the footprint, it must be relocated or the sewer diverted.

Have the structural engineer draw the sewer on the foundation plan with its invert level, rather than noting it in text. Undertakers reject applications that do not show the relationship in section.

Programme effects of the agreement

Sequence matters more than cost here. The agreement has to be in place before the foundations are poured, the building control body will want to see it, and a warranty provider will want it too.

Build the sequence in from the start: drainage records and site verification during design, foundation design that responds to the survey, application to the undertaker as soon as the design is settled, and the start on site after the consent is issued. Where the works also affect surface water discharge, a short Drainage Strategy covering both foul and surface water keeps the conversation in one document.

If the extension is part of a larger scheme needing a planning application, the position of utilities and the capacity of the connections may need setting out in a Utilities Assessment, which the council will occasionally ask for on larger householder or infill proposals.

Where diversion is the only option

Diversion becomes necessary where a manhole cannot be avoided, where the sewer is too large or too deep to build over, or where the undertaker refuses consent on condition grounds.

The route is a requisition under section 185 of the Water Industry Act 1991, under which a person can require the undertaker to alter or remove a pipe, subject to conditions and to paying the cost. Design, approval, easements and construction all take time, and where the new line crosses a neighbour's land you need their agreement as well.

Budget for months rather than weeks, and get a cost estimate before committing to the layout. A redesign that moves the extension two metres is frequently the better answer, and the extent of the garden available for that move is a curtilage question, covered in our note on measuring curtilage and the enlarged garden.

Records to keep for a future sale

Keep the build over agreement with the deeds, alongside the building control completion certificate and the approved foundation drawings.

A buyer's drainage and water search will show the public sewer, and their conveyancer will ask whether the extension was built over it with consent. Without the paperwork you are into indemnity insurance or a retrospective application, and the undertaker is entitled to require works that were avoidable at the time.

Photograph the open trench showing the pipe, the clearance and the lintel or deepened foundation before it is backfilled. It takes two minutes and it has settled many arguments.

To see which supporting documents your project is likely to need before you commission drawings, enter your postcode on PlanScope and run the constraint check.

This is general guidance, not legal advice. Check the position with your sewerage undertaker, your building control body and a qualified professional before relying on it, and note that a Lawful Development Certificate is the only formal way to confirm that works are lawful for planning purposes. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.