Buenchico. Land development use categories in the UK (England & Wales, at least), are driven by the parcel of land, not the building(s) on it. So a parcel of land with consent for a dwelling is use C3. This entitles the owner to have a dwelling on the parcel of land (of a size, style etc as permitted by the consent), and use the rest of the land parcel as a domestic garden. It also entitles the owner to erect permitted developments allowed within that C3 land use, as permitted within GPDOs, passed by Goverment. This explains why one can generally erect a shed in a garden without asking for consent, but stops one doing the same in an agricultural field. The answer you originally gave would thus not seem relevant to the question. The planners are already questioning the right of the OP to undertake uses in the buildings on the land that are greater than those of an agricultural use. Agricultural use no number attached to it - like A1, A2 etc, for retail, B codes for business use, and C for residencies, of which C3 are dwellings.
OP - you might like to look at the recent relaxation put in place from April 2014, which permitted greater flexibility including from agricultural to dwellings, including a new MB class. Not sure if you could try to use that. It involves something referred to as a prior application process to the planning authority.