My grandfather converted the original garage on his house into a family room, and then built a 2-car garage connected to the house by a breezeway. He also closed in the utility closet and turned it into a second bathroom, a half bath with a toilet and sink as well as the water heater - cramped, but needed in a 3-bedroom house with 1 bathroom on the other end of the house. He also closed in an adjacent area which became the pantry; the kitchen in that house was tiny, with very little storage space. Like Maggie's, it was lower than the rest of the house - there were three steps down from the level of the rest of the house - which was more of a problem in flood season, as the family room was low enough to flood, where the rest of the house was not. The family room was ducted when it was enclosed, and Grampa also built an indoor/outdoor fireplace - open to the breezeway on one side and the family room on the other. It was a great room.
I don't have any idea what the time or cost was as Grampa did this well before I was born - he and Grandma bought that house in 1950, and Grampa lived there until 2000, when he was 89.