Hello, I have been told by Building Control that I have to run my sink drain in 110mm if it is under the floor, but can run 40 or 50mm if above the floor. I have a cloakroom sink which is in 32mm, and runs under the floor in 32mm to a stack. Is the Building Control man correct or has he misunderstood what I am proposing, and if so, would it be ok to run a sink waste under floor level for about 3.5m in 50mm to an outside stack (with his permission)? Thanks.