from your description it appears to be internal work to a non listed building that is in the curtilage of a listed building but not in a conservation area. if that's the case then no permission will be required for internal works.
However, as hinted at by other posters i suggest you double check that the circumstances you describe are right, and i'd do it yourself not trust your solicitor, the planning dept should help as they will have time on their hands as no-ones building much at the moment. But don't panic as if its a conversion then internal changes have already been allowed so chances are you'll get whatever permission you need.
For any external works your permitted development rights will be reduced (again see your local planning dept) because of the listed building for a start so a pump track and radio mast will have no chance ;-).
On the question of value, it certainly won't add to it. In this country normal houses are largely valued on bedrooms rather than floorspace (the approach on much of the continent) which is a dumb approach but is the approach. However, yours sounds like an unusual or 'character' property so valuers may have had to take a different approach on it and actually done some research. My guess is hold price or drop 5-10%.
