Technical Booklet N Drainage·Page 24·3.9
Drain protection under buildings – granular filling
A drain running under a building must be surrounded by at least 100 mm of granular or other flexible filling to protect it from settlement.
A drain which runs under a building should be surrounded by at least 100 mm of granular or other flexible filling.
Source — /Users/richardhill/Documents/planning-arch-project/data/documents/regional/Technical Booklet N - Drainage.pdf