You can apply to change the use of agricultural land to residential curtilage (garden), yes. What that piece of marketing you linked to doesn't say is that it is usually quite difficult to get approved.
Council's generally like to keep the countryside looking like countryside and see the change in appearance associated with such changes as detrimental (think open field compared to manicured lawn, shed, green house, swimming pool, tennis court, flower beds etc). There can also be implications in terms of wildlife and habitat.
Even if you were to get it, there are often restrictions preventing you from building outbuildings etc without gaining further planning permission.
I would have a chat to the planning officer at the Council concerned to see if the y have a general approach to such applications. You could also look at the relevant local plan online to see if there is a specific policy relating to such applications - there often is. Also check the online applications on the Council website to see if there have been any similar applications nearby to see what their outcome was.