Identify the existing system
Before moving a kitchen, adding a bathroom or extending the building, establish the drainage route, connections, levels and accessible inspection points. Where the information is uncertain, arrange an appropriate survey rather than designing a connection from an assumed line on an old drawing. Thames Water recommends site investigation where needed to identify relevant pipes. [3] [4]
Distinguish private drainage from the sewer company's public assets. The building owner cannot treat alteration of a public sewer as an ordinary internal plumbing decision. [3] [5]
Foul drainage and ventilation
Approved Document H addresses the sanitary pipework and drains that carry wastewater away, including arrangements intended to prevent blockages and foul air entering the building. Pipe sizing, gradients, ventilation and access need to be selected for the actual layout. [2]
Ask the designer to show the proposed route before the new room layout is fixed. A bathroom location should be coordinated with the available drainage levels and structure rather than leave the installer to cut an unplanned route through joists or foundations. [2] [6]
Rainwater needs its own strategy
The guidance prioritises a suitable infiltration arrangement where practicable, then a watercourse and finally a sewer where earlier options are not reasonably practicable. The relevant pipe and discharge point must be identified; rainwater should not simply be added to an unidentified foul drain. [2]
National SuDS standards provide broader guidance on runoff, water quality and maintenance. Where the project has a planning drainage requirement, coordinate that submission with the building regulations drawings so the systems are consistent. [7]
Public sewers and chambers
For relevant work to existing properties in Thames Water's sewerage area, building within three metres of a public sewer or one metre of a public lateral drain engages its build-over process. A manhole affected by an extension must be addressed explicitly; Thames Water does not permit its public-sewer manholes to remain inside the building. [8] [3]
New development or a required sewer diversion may follow a different route. Obtain the necessary company approval before the relevant construction, rather than assuming the building control application covers it. [5]
Inspection and completion
Agree which drainage stages need inspection or testing before backfilling and covering. Keep the completed route, levels and access points recorded, together with relevant agreements and test evidence. [9] [10]
For basements, the drainage and flood-protection arrangements need particular design attention. Do not assume an ordinary gravity connection will suit every below-ground room; coordinate the solution with the basement and groundwater design. [11]
At handover, the owner should be able to understand where drainage runs and how it can be maintained, not just receive a certificate without the supporting layout.