we are converting a small hose in a

Presumably your ground floor is classed a deposit/magazzino/stalle or such like - and therefore isnt recognised as habitable.   For it to be habitable it needs light, height and satsify all the requirements for dwelling.   Making it habitable obvioulsy increases your living space - and as such the square metrage of your home, with increases in taxes and imposte.    To make non habitable areas habitable  the comune will ask for projects, building concessions etc - and they mount up.  The actual costs at the catasto and comune for a change of use arent that great, but the comune decides on its fees.   

Unfortunately the time to look into this is before you buy, but many go by what it would cost in their home country or what they are told by the people selling e.g. agents. It would be an ideal world if we could get quotes from builders for what we want to do before we buy. Better still if those quotes included everything! Hopefully others will read your post and answers and benefit from it.

First of all, a question arises spontaneously - If I understand you well, Stuart, you are not a major agricultural entrepreneur, that is, you do not get 75% of your economic means of subsistence from the cultivation of the land. So how did the notary register the deed of purchase, the non-agricultural portion of the house, in favor of a non-agricultural buyer? - The deed of sale, in this case is < nullo >  ( = not valid ) ! - The Italian law establishes that in order to sell an agricultural property, to a buyer who is not a farmer, it is necessary to transform the agricultural property into an urban building (that is to say, urbanize it) !

Falling into traps is relative.   As a buyer you have the right to ask for information - and any property that is registered as an A at the catasto is a house.  If its a C or D then its not.  If you choose to sleep in a stalle or deposito then its your choice, nobody can stop you, but you cant say that your converted cantina is a 'room',  because it isnt.   A 2 bed house that has 3 bedrooms in the old barn (which is still registered as a barn) cannot be sold as a 5 bed house.  Your agent has a legal requirement not to tell you a pack of lies, as has the seller.   The visure for a property are public and you can see them - it will tell you exactly what you are buying.  

Modi ,  sorry if I correct you:1> you base your reasoning on the belief that a non-Italian buyer, can know how to read and, above all understand the real meaning, of the Italian land registry (which in truth are also obscure to many Italians born in Italy), Read a cadastral survey, for a non-employee (for example, you or me), it may be similar to reading the inscriptions of an Egyptian tomb, without having the Rosetta stone. 2> Stuart, can safely sleep in any stable, but can NOT buy a STABLE, if it is not a farmer3> no Italian notary, can register the sale of a STABLE, to a buyer who is not a farmer (according to the Italian notary law)

Sorry .. in italian language only ,  From  Italian Notary website https://www.notariato.it/sites/default/files/Le_guide_per_il_cittadino_Gararanzia_preliminare_set_14.pdf

I think there is some confusion here.  Ugo is referring to a fabbricato rurale FR - which cannot be sold to non farmers since 2012.  However if the FR was converted to an ente urbano and is accatastato - as a C/2 or a C/6 then it can be sold to anyone.    

Ugo, its not really the rosetta stone - You want to buy a property, when it's described in the proposta d'acquisto it will say A/4 or C/6 (for example) - if its A then its a house, if its C then its not.