In general, an ADU (accessory dwelling unit) is a permitted structure that has become increasingly easier to build in California from a regulatory standpoint. An ADU is recognized by the building department, has a building permit, permanent foundation, and permanent connection to utilities. It is thus considered “real property” and cannot be sold separate from the main house. In contrast, in general a “tiny house” is a small mobile structure, not recognized as a land use by the building department, not permitted, not complying with codes, and not permanently connected to utilities. (Is there separate utility connections?)
When you say the renter would remove the tiny house then that makes me quite confident that it is a tiny house and not an ADU. There is a bit of confusion over the technical differences so that’s why I was mentioning that I saw it as a ground lease of the space and that the tiny house was probably illegal. And I’m not putting any judgment on it to say it’s illegal, just that it’s not permitted and of course the building department could tell you to remove it.
You do not want to ask the building department about a tiny house because it could invite scrutiny and a citation. But, virtually no city is encouraging tiny houses to cure the housing shortage, that’s more of people promoting tiny houses saying such things. One of the main problems is that tiny houses are not built to specific codes nor inspected so there are life safety issues (electricity, sewage, smoke & C02 detectors, dual fire exits, etc.).
In contrast, many cities are promoting permitted ADUs and are required to do so by state law. There has been a boom in illegal rentals being converted to permitted ADUs, mostly in Los Angeles. Tiny houses, for the most part, remain illegal when occupied long term.
Hopefully that’s food for thought and you can conduct further research if your offer is accepted. Good luck!