Send a written notice (yes, paper) to the tenants with proof of delivery required (the postal carriers have this time to an art form; it's inexpensive and you get confirmation via email or receipt very quickly; it does not require the tenant to sign - it just confirms that the notice was delivered.A paying, respectful tenant is golden.

After looking at your post, I realized it's probably necessary to have a dedicated real estate attorney handle this issue for me since it's serious and I might make a mistake big enough that probably will cost me too much money to fix.

Would it make sense to form this into an ADU or leave it as is?

As someone actively investing in STRs and looking to scale, this raises some concerns for me about the long-term viability of this strategy.I understand the tension between hotels and STRs, especially in popular tourist destinations, but I’m curious:How real is this threat?

The real property investment can be repositioned if it proves disappointing in its original form; it can be refinanced or recapitalized if it has too much or too e pensive debt; inflation over time bales out many bad purchases, and finally the investors knowledge, experience and abilities can be leveraged to turn around a cash flow negative property.I’ll provide an exampleThree years ago we purchased a note from a bank secured by two auto repair facilities with a large amount of excess land.  

Initially with the 3 rents it looks like it would be a go for me.

On the plus side now I will have a recorded Certificate of Occupancy with the city, as this property will be a long term hold for me most likely.

Looking ahead, once you secure a tenant, perhaps this is something that should be explored to make the next lease up easier.Also, @Caria Mitchell raised an interesting point in saying: We are experiencing a shortage of qualified applicants, I’ve noticed non-conventional applicants vs qualified applicants that don’t have a voucher or some form of subsidy. 

Mainly the Brokerage is for me just to sell the properties that we fix and flip and save the commission.

My track record makes buying SUbject viable for me.