FYI, you do not (normally) need permission to demolish a building / structure. As freddymurcerystwin pointed out, you do need permission to build a new one.
So, you can demolish a garage, but would need permission to construct a new one in its place.
However, you do have an extant planning permission which, i assume, shows approved drawings which indicate your garage and your side extension. With this in mind if you continue building and the result looks exactly like the approved plans then you will have built in accordance with the plans.
