Office blocks all have raised floor tiles that are (from memory as a building site labourer) about 6" above the slab. Could they not be raised to 8" or 9" for wider sewage pipes?
The office buildings I've been in have also had really high ceilings. You could easily lose a foot to provide room for additional plumbing for the floor above.
Yeah, either do that or raise the floor a foot using the raised floor tiles. Just need to watch out for rodents. Mice love the raised floors.