I saw some red flags and some complaints about how they handled some construction with asbestos in the house.

Since I'm not an attorney and can't give legal advice, I did a little more digging and found the passage below.

I know in California it is legal and the state acknowledges it however it does restrict the finder to basically introducing the parties and they can not participate in negotiations, etc.

Without proper permits, there’s a chance the city may request you to revert any non-compliant additions (like the extra bathroom) to align with code standards.When it comes to refinancing, banks tend to look closely at the property’s legal compliance.

Out of the 20 letters I've sent, I've had 1 guy get back to me and we were in discussions but weren't able to find a price that worked for the both of us.The issue is that I only have a few houses on my mailing list so far (just places I've walked or driven by that caught my attention), and blindly searching every street and address # to see which houses are actually legal multifamily will take forever and doesn't seem efficient.I know the data is publicly available, I just don't know how to aggregate it in a way that would be efficient and useful for blasting out marketing mailers.Are there any websites that can pull massive lists of all multifamily houses in my area?

Excessive complaints or three violations in three years could lead to losing the ability to operate​.Impact on Neighborhoods: Non-owner-occupied STRs (Type 2) may cause friction in residential neighborhoods, as they often function like businesses, which can disrupt the local community atmosphere.Recent Regulations (2024 Updates):1) STR operators must obtain a permit for either Type 1 or Type 2 properties, depending on whether the owner lives on-site.2) Platforms like Airbnb are required to remove any listings that lack a valid permit.3) New noise and occupancy regulations apply, with stricter enforcement through permit revocations and finesThese regulations are part of the city's efforts to balance the benefits of STRs with the preservation of neighborhood quality and local safety standards.4o

May not be legal, but doesn't seem like it impacts your situation here. 

For both short- and mid-term investors, the market's diversity and relatively lower entry costs make it an attractive option.FYI - Philadelphia has a tenant-friendly legal environment, which can present certain challenges for landlords

4) Legal Options: who told you that you can't sue the LLC owner?

I would suggest to check with your local laws first.Most likely you will need permit ( I guess you can get away without, obviously not suggesting that :D)) but possible) , also check your zoning.If not possible to create separate unit legally, I suggest not putting an oven in the basement, and rent it without, or live in it yourself