Where affordable housing is scarce, these secondary homes may be the answer. Here’s what you need to know about today’s version of the mother-in-law apartment.

In carefully decorated model homes in Charlotte, N.C., Gina Anderson, sales manager for Adams Homes, sets out coloring books, board games, and Ping-Pong tables along with kid-friendly snacks like Goldfish crackers. The kids’ bedrooms are invitingly decorated with sporting themes for boys, “flowers and flounce” for girls, and musical themes for tweens. The staging...