Technically you must build out your planning permission exactly as per the plans which were submitted to the planning authority as part of your planning application. While it does not expressly say this in the Planning Act, there is some case law on this fact. Although this doesn't make practical sence (recognised by the judge in his ruling), the upshot of has been that the exempted development provisions set out in the planning regulations are only applicable after the development has been built out.
I suggest you speak with the planning case officer dealing with your case and ask can you submit revised plans to be agreed with the council by way of compliance. They may (and legally should) ask you to submit a modification application, but in practical terms once the council agree to the changes most people take this as being good enough and proceed to build with the modifications included.