Here are the rules for registering a tiny house on wheels in CA. It is quite a process!

Take the following to a CA DMV office:

  1. A completed Application for Title or Registration (Form REG 343).
  2. A vehicle verification done by the California Highway Patrol (CHP). You must start your application process with the DMV prior to contacting the CHP for a vehicle verification. DMV verifies trailers with an unladen weight of 6,000 pounds or less.
  3. A completed Statement of Construction (Form REG 5036).
  4. Invoices, receipts, certificates of origin, bills of sale for major parts (to prove you own the vehicle).
  5. If you don’t have sufficient proof you own the vehicle and the parts, you will need to submit a motor vehicle bond (if your parts are worth $5,000 or more).
  6. Brake and light adjustment certificates OR a completed Statement of Facts (Form REG 256).
  7. A weight certificate for commercial vehicles weighing 10,000 pounds or less.

Below is the official link:
https://www.dmv.ca.gov/portal/dmv/detail/vr/spcnsreg