Ross

Thanks for the helpful replies. Good to know what other people are doing. Another ignorant question: Can a water supply pipe go in the same trench under the slab as the soil drain pipe?

Thanks @Jeremy Harris for sharing details of your passive slab. One further detail I would like to know is how the trenches were done and made up. Unfortunately the building regs don't have clear guidance on how to lay drains under a passive slab, and I've got an engineer suggesting that the drain should be encased in concrete! (which I'm hoping isn't necessary). It was my impression that the drains and services should only need to be in a bed of gravel, in a trench dug after having compacted the sub-base (but perhaps what they do in other countries isn't appropriate here). Unfortunately @Nick1c I don't think my engineer is very familiar with the system (Jackon Atlas in my case), but I don't really want to be contacting an engineer in Ireland.