Thanks Alex. I should probably have stated the usual disclaimer of I am not a qualified electrician (I know enough to be dangerous!

). I was just thinking that you don't want it low enough it could get wet should you get a little flooding.
My 5x5 man-cave (3/5ths office, 2/5ths shed) came under permitted development rights. Although I did get planning permission and I tried to involve building control but they weren't really interested. I think that it isn't classified as habitable unless you have plumbing of some kind. If you have a toilet you'll have both kinds of plumbing and you may find your council tax goes up as an additional bedroom or something - I am not a bureaucrat either

I think I remember reading that if you put a TV aerial up that makes your life complicated too - but that could just be requiring an additional TV license for 'separate' premises. Mine's wired from the house but it's mainly because it was easier for me and I can then run what I want down the cable.