Dear Pigro as a resident & first time buyer of a property in Italy, I bought an old farmhouse & done it up and put in a more than 80sm pool I was very concerned about your questions. So I went to a local lawyer and got the definitive answer!. Not that anything is definitive.Anyway he said:
If you buy a house as prima casa say A2 (villa civile), then despite whatever changes you make to it it remains as A2, which sounds fair otherwise noone would make any improvements to their property. Many of A1's attributes (villa di lusso & therefore cant attract prima casa tax reduction), such as flocked wallpaper, handpainted floor tiles, solid wooden doors etc. you dont need permission for and therefore the council wouldnt /couldnt charge you more ICI. However for extension work and new pool you would need building permission, a geometra acting as direttore dei lavori, who would also register the finished work in relevant catastal value to the comune and get your certificate of living (certificato of agibilita') which you need when you come to sell. & you would get charged more ICI, but only what improvements youve made, not on a jump into A1 category. After five years you can sell the property (if you sell before you must pay the tax difference between prima casa & seconda casa). If the house (this bit am not sure who decides I presume the comune) is now a luxury home (villa di lusso A1) thanks to all the wonderful changes you have made to it, its the buyer who will pay extra costs & wont be able to purchase it as prima casa. Is that clear?

I have read that even though you are not resident full time in italy you can be classified as resident if you spend more than 183 days here & therefore take advantage of tax reductions.