Bonus rooms are typically found tucked away over garages or in the attic space on the second floor. The reason they are called "bonus rooms" is twofold. First and foremost, the square footage of that area is typically not used in the calculation of the total living space. As a result, it does not count on your property tax purposes which means you can enjoy a 'bonus' living area without the financial costs.
Secondly, the bonus room is just that, an extra room that is not designated as anything specific in the plan. You can utilize that space for an extra guest suite, a media room, a gym, a home office, or you can just use it for extra storage.
Additionally, the bonus room is an excellent option to have for future expansion. While you might not have any specific use for the space today, it gives you an easy way to expand your home in the future. The cost to expand your home with a new addition can sometimes be prohibitive. Not to mention the time and inconvenience. But converting a bonus room is usually a simple and inexpensive project. One that many owners can do themselves.