Why it is more than a room addition
The bathroom is what makes a master suite addition different, and what the Florida Building Code inspects hardest. New supply and drain lines have to reach the addition, and where the drain ties into the existing system, at what fall, decides a lot about where the bathroom can sit. That constraint drives the layout more than preference does.
A master suite addition downstairs, on purpose
A large share of these are built to move the primary suite to the ground floor of a two-storey house. If that is the reason, it is worth designing it with the next twenty years in mind: a curbless shower, blocking for grab bars, and a door wide enough to matter later. See aging-in-place remodeling.
What happens to the old one
Worth deciding at design time. The former primary usually becomes a guest room or an office, and occasionally its bathroom is worth reworking at the same time while the trades are already in the house.