We built an ADU and just started renting it through AirBnB in July. We had to switch our entire policy from PEMCO to Allstate to be able to insure the ADU as a short-term rental. It only costs about $300 for a 2-bedroom, 800 SF ADU on top of the cost for our main house. It is so cheap because it is treated like a secondary policy because you have a certain amount of insurance through AirBnB first. I'd recommend you talk to an Allstate broker. All of the other short-term specific insurance companies I talked to had ridiculously expensive policies.