Whilst it is true that listing applies to the house and the curtilage, my understanding is that it isn't required for alterations to structures that were built (with approval!) in the curtilage. Depending on what you are doing you may need planning permission and/or building control.
One option to be sure might be to get a "Certificate of Lawfulness of Proposed Works" from the council - I've heard this is less onerous than listed consent or planning and is a good thing to have around if you want to sell the property later on.
I'd probably recommend joining the Listed Property Owners Club as it's pretty cheap and they have a service where you can email and get opinions on what sort of approvals are needed for various things.