Exactly what size of office are you building?   Why are building control even involved?   You can build a building up to about 100 square metres and my making it "portable" it becomes exempt from building regulations (it is treated for BC as a "caravan" and does not need to be on wheels for that)   I have wired many such portable buildings, real sturdy well insulated houses but built with a few rules in mind that makes them "portable" even though they are intended a