Your children are growing and they want a space of their own. You may need a separate room for certain projects. The solution is very easy. Just add a room over the garage or extend the garage by constructing a room.
The living space will be increased and thus you will be able to give the space your child requires, or you, or for guests or live in parents. The space can be used for so many things.
You can shift your office to home and save on paying rent on the rented property. The space thus made will be for your own use and nobody can disturb you there.
The other idea is if you are interested in Music, then you can convert the extra room for a music studio where you or any person of the family can enjoy uninterrupted music for hours on end.
If you are keen on adding an extra space into your home, first make a budget and hire the person who can guide you in proper way and can construct the place for you.