I was a MIET for many years. I assume you are refering to this document. https://electrical.theiet.org/media...tdoors-a-supply-to-a-detached-outbuilding.pdf If the cable from the dwelling to the outbuilding is not armoured then the RCD goes at the dwelling end. If the cable from the dwelling is armoured the RCD goes at the outbuilding. If you have PME at the dwelling and are going to TT the outbuilding there must be an RCD in the outbuilding, rendering the RCD in the dwelling as much use as an ashtray on a motorbike.