Approved Document H3 (rainwater drainage)

Approved Document H3 provides statutory guidance on the design and installation of rainwater and surface water drainage from buildings, paved areas and SuDS schemes in England and Wales. The document covers gutter and downpipe sizing, gully selection, soakaway design under BRE Digest 365 and the use of sustainable drainage systems. It applies to all new builds, extensions and material alterations across Greater London, where the London Plan further sets a sustainable drainage hierarchy. Enforcement is by Local Authority Building Control or Approved Inspectors, often in consultation with the lead local flood authority. Practical implications for homeowners include the need to manage surface water on site where feasible, choice of permeable paving for driveways and assessment of soil infiltration capacity. The formal reference document is Approved Document H, Volume 3: Rainwater drainage.