Sanitary facilities and drainage 

Any WC in the new storey must also have a washbasin in the same room, or in a room giving direct access to the sanitary accommodation (i.e., in the outer room to an en-suite WC). It must have a supply of hot and cold water.

Any new WC, washbasin, bath or shower must be connected to the drainage system. The position of the existing drainage stack will dictate the position of the fittings.

  • If the WC branch exceeds 6m, or if it is not a straight connection to the existing stack, then additional ventilation may be required to prevent water being sucked out of the trap. An air admittance valve is usually acceptable, but check with Building Control.
  • Bath or shower waste pipes must be 40mm diameter minimum and should not exceed 3m for a 40mm diameter pipe, or 4m for a 50mm diameter pipe.
  • Washbasin waste pipes must be 32mm diameter minimum and should not exceed 1.7m for a 32mm diameter pipe.


A ventilation stack should discharge at least 900mm above any opening window (if less than 3m away). The existing stack may need to be raised above any new windows or rooflights.  An air admittance valve may be acceptable, but you are advised consult Building Control.

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