A soakaway must be far enough from buildings that infiltrating water cannot soften the ground under foundations or cause damp, and it must also be kept clear of boundaries, wastewater percolation areas and wells. The separation distance is set by the drainage designer using the guidance in Technical Guidance Document H and the site conditions.
Infiltrating water spreads laterally as well as downwards. Too close to a building, it can saturate the ground beneath foundations, contribute to settlement in susceptible soils, or cause damp in basements and floor slabs. Guidance therefore sets minimum separation distances from buildings, and steeper or clay sites can require more.
Other separations matter too. A soakaway placed near an on-site wastewater percolation area can interfere with its operation, and one placed near a well can compromise water quality. On sloping sites, downslope structures also need to be considered.