Foul vs surface water
Building regs require all foul water (toilets, kitchens, bathrooms) to discharge to mains sewer. Surface water (rain from roofs and patios) can use soakaways, attenuation crates, permeable paving, rain gardens or mains. Combined sewers in most older London streets accept both but boroughs increasingly push for source-control SuDS.
When soakaways work
Adequate ground percolation (gravel, sandy clay) confirmed by BRE Digest 365 percolation test. Sufficient garden depth — soakaway pit typically 1.5m³ to 4m³, sited 5m+ from any building, 2.5m+ from boundary. London Clay sites often fail percolation; attenuation crates with restricted outflow used instead.
SuDS requirements
London Plan and most borough policies require SuDS for new impermeable area >100m² (often lower thresholds). Hierarchy: rainwater harvesting, green roofs, soakaways, attenuation, then mains last resort. Specifying SuDS at design stage avoids planning condition discharge delays.
