Letting outbuilding annex of prima casa

If you turned your outbuilding into a habitable space suitable for letting it would have an A classification in the catasto which means it would effectively be another house and so your seconda casa. You would not be letting your prima casa at all.

Hi, we are in a similar situatio, with an outbuilding used as a guest room and possibly in future for lettings. if it is a seperate building, and is furnished, it counts as seconda casa for IMU.  I did make the point that if our guest room had been upstairs in the same building, it would still be part of prima casa, but to no avail!