Private drainage still needs an appropriate building regulations design. [1] [2] [3]
Establish what the pipe serves and who owns it
Do not assume a drain is private because it is in your garden. Thames Water's responsibility guidance distinguishes private drains from public sewers and lateral drains; the properties served and the pipe's location are relevant. Confirm responsibility for the actual asset. [4]
Obtain the available drainage records and survey the existing arrangement. A design needs the pipe route, depth and connections rather than a roughly positioned circle labelled “manhole”. Thames Water's technical specification requires the position and invert levels to be verified before construction. [5] [2]
When is a build-over agreement relevant?
Thames Water says an application is likely to be needed where building is within 3m of a public sewer or within 1m of a public lateral drain. Planning permission does not grant permission to build over the asset. Contact the actual sewerage provider where Thames Water is not responsible. [1]
An agreement is not just a payment. The provider considers the structure, pipe protection and future access, and may require the design to change. Its application process explains the technical information and checks involved. [6]
Can the manhole remain under the kitchen floor?
Thames Water's current build-over technical specification says that manholes and inspection chambers must not be built over or located inside the proposed structure. It expressly includes new double-sealed, bolt-down covers in that restriction. A sealed internal cover is therefore not a general workaround for a public chamber. [2]
The design may need a chamber outside the building, an altered pipe arrangement or a different footprint. The acceptable solution must be agreed for the actual network; do not instruct a contractor simply to cover the existing chamber and deal with paperwork later. [2] [6]
Coordinate the structural and drainage drawings
Ask the drawings to show how foundations relate to the pipe, where maintenance access remains and where the kitchen and other new drains connect. The engineer and drainage designer should use the same surveyed information. Foundation design can be affected by underground drainage and may require further investigation. [7]
Building control should also be involved at the relevant stages. Drainage and foundation work can need inspection before it is concealed, so include those checks in the construction programme. [8]
Before work starts
Keep the surveyed drainage plan, agreement where required and final technical drawings together. Identify any conditions or inspections needed before covering the pipe. If an unexpected drain is found during excavation, obtain advice before building across or altering it.
Thames Water warns against starting before obtaining the necessary agreement because design changes can then create additional cost and delay. Completing the planning application first does not remove that risk. [1]