Start with the drainage route
Identify the existing soil stack and how the WC, basin, bath or shower will connect. Approved Document H requires adequate drainage capacity, ventilation and access for clearing blockages. Pipe size, fall, length and the arrangement of connections need to work as a system. [2]
Drawing the bathroom above existing services is a useful option to test, but it does not prove that the connection is satisfactory. Ask for the actual route, including boxed-in sections and access points, to be shown before fixing the layout. Any new external soil or vent pipe should also be included in the planning assessment: Class B roof-development rights do not themselves cover those pipes, although a separate permitted-development class may apply. [2] [5]
Check loads and space together
The engineer needs to know the positions of sanitary fittings and any water-storage equipment when designing the floor. The existing ceiling joists should not simply be treated as a ready-made bathroom floor. [4]
Check standing and circulation space beneath the finished roof, including the approach to a shower and the ability to use fittings comfortably. Show the floor build-up and roof lining in section. These design checks sit alongside the stair-headroom requirements; a stair dimension is not a universal bathroom ceiling-height rule. [6]
Do not leave pipe penetrations to be improvised on site. Restrictions on holes and notches depend on the structural member, and generic guidance for solid timber joists is not a drilling schedule for engineered joists or steel. Coordinate routes with the structural design. [7]
Provide proper extraction
Under the current 2021 Approved Document F guidance, an intermittent bathroom extract system has a minimum extract rate of 15 litres per second. Continuous systems use a different design approach. The fan needs a suitable duct and discharge outside; the manufacturer's headline capacity alone does not demonstrate the installed airflow. [3]
The ventilation design should also address controls, replacement air and commissioning. A bathroom window is not a reason to omit the mechanical extraction required by the selected compliant arrangement. [3]
Agree the water and electrical work
Ask the services designer or installer to check the available supply and the proposed hot-water system against the new demand. Hot-water safety and sanitary facilities fall within Part G; electrical installations must comply with Part P. Agree the required checks and certificates as part of the loft package. [1] [8]
Work planned for 2027
The 2026 ventilation guidance accompanies regulations taking effect from 24 March 2027 for ordinary non-higher-risk work, subject to transitional provisions. It includes guidance for work to existing dwellings. The new-home Future Homes Standard should not be confused with a requirement to rebuild an existing house to new-home specifications, but the applicable ventilation edition still needs checking. [9] [10]