I have a space of around 500mm between house wall and boundary to install both surface and foul 110mm drains at side of new build. The foul drain is deeper than the surface water drain. As the foul drain has more take offs for toilets etc I was planning on putting it closest to the house. Is there a minimum distance that the two runs need to be apart. I reckon there will be approx. 300mm of difference between the invert levels and assume the surface cannot go directly above the foul pipe. I am not allowed to use a combined system.
Any help or advice much appreciated.