Building over a public sewer: what to know before you extend

Can I build over a public sewer?

Usually yes, but not without approval. If your extension, conservatory or outbuilding will be built over a public sewer, or within 3 m of one, you need the sewer authority's agreement before you build. Across most of London that means Thames Water. Building Control also checks this under the drainage requirements of the Building Regulations, and will not normally sign off the work until the sewer authority is satisfied.

Approval is not guaranteed. Large sewers, deep sewers, sewers in poor condition and manholes are often difficult or impossible to build over, so it pays to check early, before the design is fixed.

Build the approval into your programme from the start. It runs alongside planning and Building Control rather than after them, and the water company's own decision time is outside your control. Finding a sewer once the foundations are being dug is the expensive way to discover it, because the trench may have to be redesigned on the spot.

Why so many London homes have a public sewer in the garden

In October 2011, private sewers and lateral drains that served more than one property and connected to the public network were transferred to the water companies. Overnight, the shared drain running along the backs of a terrace became, in most cases, a public sewer.

This catches out a lot of homeowners planning a rear extension. The run behind a Victorian or Edwardian terrace, in places such as Lambeth or Hammersmith and Fulham, typically collects from several houses before heading to the street. A single-storey extension across the back of the house can easily sit on top of it, or within 3 m.

A drain that serves only your own house is usually still private. You can generally build near it without the water company's agreement, though Building Control will still want it protected and accessible.

How to find out where the sewer is

Start with the sewer records. Thames Water keeps asset maps, and a drainage search often shows the public network near your property. These records are a guide, not a survey. Shared drains transferred in 2011 are frequently missing or drawn in the wrong place.

The reliable answer comes from lifting the inspection chamber covers and running a camera through the pipes. A CCTV drainage survey records the route and condition of each run and shows which neighbours connect into it. Many build-over applications need pre-construction footage anyway, so it is rarely wasted. The survey should establish:

• The route of the pipe in relation to the proposed foundations
• Depth to the invert (the bottom of the inside of the pipe)
• Pipe diameter and material, such as clay or pitch fibre
• Condition, including cracks, root ingress and displaced joints
• The position of any manholes or inspection chambers

What Thames Water looks for

The water company's concern is simple. It needs to be able to reach, maintain and if necessary replace the sewer, and it does not want your foundations to damage it. Its approval process therefore looks at the size and depth of the pipe, its condition, how close the foundations come and whether access points remain usable.

Smaller sewers in good condition at modest depth are the most straightforward, and some proposals may qualify for a simpler route. Larger pipes, deeper sewers, pipes in poor condition and anything covering a manhole usually need a full application with detailed drawings, and some are refused. The criteria and fees are set by the water company and change from time to time, so check the current position at the start of your project.

Manholes are the usual sticking point. Building over an access chamber is generally not allowed, so the design may need to relocate it, replace it with a different type of access fitting or reroute part of the drain, all subject to the water company's agreement.

How foundations are designed around a sewer

The aim is to keep the weight of the extension off the pipe, and to make sure future movement of the foundations cannot crack it. The common solutions are summarised at the end of this section.

Which option suits depends on the depth of the pipe, where it crosses the footprint and what the ground is like. A shallow sewer under the middle of a kitchen extension might be bridged by the floor slab and a pair of lintels, while a deep one along a flank wall may push the foundation down or call for piles. The water company and Building Control both review the same drawings, so the details need to satisfy each of them.

A foundation running alongside a sewer generally needs to go deep enough that it does not bear on the backfilled pipe trench or press on the pipe from the side. On London Clay that requirement sits on top of the tree and shrinkage rules that already govern depth, so the design has to satisfy both.

• Taking foundations down below the pipe invert so load spreads beneath it, not onto it
• Bridging the pipe with a reinforced concrete lintel where a wall crosses it
• Using piles and ground beams to span over the sewer entirely
• Wrapping the pipe in flexible material where it passes through a foundation
• Replacing a damaged section of old pipe before building over it

The application, the build and the paperwork

Our build over or near sewer technical package brings the steps together. We review the records and survey, design the foundation and pipe protection, prepare the application information and deal with the water company for you. It starts from £850 + VAT, with the water company's own fees charged separately.

During the build, the contractor works to the approved details and keeps debris out of the sewer. Many approvals require a post-construction CCTV survey to show the pipe is undamaged. Keep the approval, drawings and footage with your property documents. A missing build-over agreement can hold up a sale, and in many cases a retrospective application is possible if one was never obtained.