I'm in the process of converting a garage into a studio apartment. You can follow the journey here for inspo! https://bachelorettepadflip.com/category/airbnb-guest-house-garage/
You need to check local building/zoning codes before you start work.
Is your building at least 20 foot tall? Need to get prof plans drawn up then get permits.
Plans, permits and inspections will be necessary. You will need electric, plumbing , heat, etc. for a living space.
I'm in the process of converting a detached storage garage into a studio apartment. Here's my progress: https://bachelorettepadflip.com/category/airbnb-guest-house-garage/