Approved Document H4 (building over sewers)

Approved Document H4 sets out guidance on the design and approval of building works that pass over, alongside or close to a public sewer or lateral drain. The document is particularly relevant to extensions across London, where Victorian-era public sewers run beneath many gardens and shared accesses. Where works fall within 3 metres of a public sewer, a build-over agreement with the sewerage undertaker, typically Thames Water in Greater London, is generally required in addition to Building Control approval. Approved Document H4 sets out structural and access requirements including provision of inspection chambers, suitable foundation design and protective concrete encasement where needed. Practical implications for homeowners include early engagement with Thames Water before submission of planning or Building Control applications, and CCTV survey of the existing sewer to confirm condition prior to works.