Well, YMMV. I looked into permit-free structures in one California county. 1. 120 ft^2 max, outside dimensions. 2. No electricity > 30VDC. 3. No plumbing. 4. No wood-burning appliances.
You could still squeeze an office or studio into something like that. But it is easy to trip over a code-requirement that needs a permit.
You could still squeeze an office or studio into something like that. But it is easy to trip over a code-requirement that needs a permit.