The hierarchy

Soakaway first, then a watercourse, then a surface water sewer, and only as a last resort the combined or foul system. Building Control will ask you to evidence which level you have used and why.

Soakaways in practice

Soakaway design depends on soil percolation, water table, and rainfall return periods. A trial pit and percolation test should inform the design — not the other way round.

What goes wrong

Undersized soakaways, soakaways too close to the house or boundary, and rainwater quietly piped into the foul system because the soakaway 'did not work'.