My neighbor had a similar sized garage. He wanted more space. So he bought a pole barn kit and built it directly in front of the existing building. Tied it in real nice. He left the overhead door on the original building, so they can shut it when his carpenter kid does woodworking in the back half. But that only required three of the four sides that came with the pole barn kit, so he used the 4th side to build a small garden tool shed on the back-side of the existing garage for his wife to store garden tools. Worked out really well.