Many new single-family homes are built with a few bedrooms instead of just two, and if you're part of a smaller family unit -- maybe it's just you, or you and a spouse or maybe one child -- you're going to have some extra space to fill. Rather than just leaving the rooms empty or placing some generic guest furniture in there, put the space to good use and give the rooms specific purposes. Some of these options may require a little more work or attention than others, but if the purpose matches your goals in life, the work and attention will be worth it.
One of the more obvious options is to turn the rooms into home offices, workout rooms, craft rooms (for serious crafters, a dedicated room is a blessing), or even a library or entertainment room. If you like to meditate, turn the room into a peaceful space so that you don't have the get-up-and-go energy of your bedroom interfering with your meditations in the morning.
Renters and Visitors
Another option is to rent the room out, either to a long-term tenant or as part of a home-sharing service. If you live in an HOA-controlled neighborhood, double-check the regulations regarding short-term stays, but even if you can't take that route, you can still rent a room out to a student, for example. If you have two empty rooms, consider renovating one to hold a small kitchenette, and rent the two as a more independent unit for an adult. This is especially helpful if you're buying in an area known for high rents and tight rental housing supply.
Music Studio, With Some Renovation
If you practice an instrument, here's your chance to set up an insulated studio. Even if you play an unamplified instrument, your neighbors may still be able to hear it. The cheap way to do this is to get a bunch of thick wall tapestries and a thick rug, and add those layers to the room. (Add thick blackout curtains to the window on top of the blinds already there). Another option is to get strong shower stall tension rods and add blackout curtains all around the room (plus the rug). Or, you can have sound baffling material professionally installed on the walls, ceiling, and door. Don't forget the thick rug and curtains. By the way, it's still best if you avoid amplified instruments or use amps with headphone jacks -- houses in these developments tend to be close together, so noise will still be an issue.
Foster Pet Room
Finally, if you've wanted to foster pets, turn that room into the foster pet introduction room. When you bring a new pet home, especially one that's in an unstable situation like fostering, you want to place it somewhere that isn't frightening or cluttered. A plain room with a few toys, the food and water dishes, and some blankets would be perfect.
Enjoy having that extra space and putting it to good use. Your real estate agent can help you find a house that has enough rooms for all the options you want to pursue.