Planning an extension near a sewer or drain

Locate drainage before fixing the footprint

A drain cover in the garden can materially affect an extension. Establish the route, depth, condition and ownership of the relevant pipes before assuming the new wall or foundation can go where the concept plan shows it. Records can help, but a site investigation may still be needed.

Do not assume that a pipe is private because it runs through your garden. Equally, not every pipe needs the same consent. The designer and drainage specialist should establish what is present and which organisation needs to be consulted.

Planning permission is only one approval

Water-company requirements are separate from planning consent and Building Control. Thames Water provides a checking process for work over or near its sewers and public drains. Use the current guidance for the actual proposal rather than assuming that an existing extension nearby demonstrates acceptance.

The appropriate solution might involve keeping clear, changing the foundation arrangement or considering a diversion. A new building and an extension are not necessarily treated alike. Ask the relevant water company before relying on a particular construction approach.

Coordinate access and maintenance

The layout must consider future access as well as the day the concrete is poured. Discuss inspection chambers, rodding access and the consequences of moving a drain. An apparently small adjustment can affect kitchen drainage, garden levels and neighbouring connections.

Record the agreed route and any approval conditions in the technical information. The contractor should understand what can be built and what needs inspection before it is concealed. Keep completion evidence with the house records for future maintenance and sale enquiries.

Information to collect

• The property address and proposed extension outline.
• Drainage records and visible chamber locations.
• Survey findings, including depths and pipe condition where investigated.
• The proposed foundation and drainage design.
• Any correspondence with the water company.
• Access limitations affecting investigation or construction.

Contact us while the scheme is still flexible. Moving a wall on a feasibility drawing is usually easier than redesigning around a drainage issue once a contractor is mobilised. We can coordinate the architectural layout with your drainage and structural advisers and identify the approvals that need to be programmed.