You have been given some quite misleading advice here. Never take advice about needing planning or building regulation consent for a job from a builder. Nine times out of ten they will tell you not, to save time/money. In this case you will need to check if you have permitted development rights from the original planning or whether they have been removed. Almost certainly for a detached garage you will need planning consent for change of use anyway and also definitely building regulation consent for the actual work, which will involve at least partial rebuilding of the walls.