Hi Andy,
From my own experience this is a complete minefield! I'm sorry to sound so pessimistic. If your solicitor cannot provide the information that is needed then perhaps you should think carefully about proceeding. If your house is not registered as a dwelling then presumably it will be classed as an almacen, ie. agricultural building? If this is the case then you are probably entering the minefield!! when I recently amended my deeds to include the old casita the notary made a point of telling me twice that the building was not for habitation and is an agricultural building. I already knew this and am using the land for agricultural purposes and not living on it.  There is quite alot of information on the forum about this issue and I imagine you have read this? I'm sure you will get some more replies! Houses that are fully legal and classed as dwellings will obviously command a much higher price so this is something to be aware of also. Alot depends on your particular situation and what you want to use the land for.
Best of luck.