Thanks for all your advice.

As has been mentioned planning permission would more than likely be
required,
I'd be wary though, if you stride into the planning office asking for
permission to build an office in the back garden then the answer will
probably be no. A garden room, studio or the like would probably be better
received. Try talking to a planning officer about their policy in this area
in general without giving away any details.

Also what is your location, rural or urban?

My location is rural. What difference may this make planning permission?