Make sure your lease lists the number of consecutive an annual days they can have overnight guests (daytime is none of your business unless they are causing disturbances in the neighborhood or damaging the property), make sure it lists the individuals permitted to live there, etc.On this renter, you can tell her you know he's staying there, he's not on the lease, and him living there is a violation of the lease.

Some cities are banning glamping/tiny homes unless its permitted by the city.

For example, we are limited here to the exact amount we pay for the background check; no mark ups permitted.  

This immediately led us down the rabbit hole of needing engineered plans, a ton of additional permitting, and having to bring the entire renovation up to 2022 building code with the city inspectors checking every little detail on a 95 year old house. $40,000 unexpected dollars  later, we had new windows (worst decision ever!!

If I'm handed a permitted set, I will work up a budget for free, but that's it.... 

I don't pull permit nor would I have need to. 

Has anyone successfully permitted existing ADUs and garage conversions in Riverside or surrounding areas? 

They met and when he was finished looking he asked them for their permits.  

@Jacob Jenkot if it’s going to be a permitted ADU, the city may have a laundry list of requirements beyond plumbing.

If you can meet these conditions, then you should apply for a building permit.