In many cases it is fully legal so long as it is your primary home and you occupy one of the units but definitely make sure you're in compliance before you move forward.

If you move your primary residence, he will lose yous tax advantage on the investment property. 

If family lives in the main home and friends stay for free in the garage, the property remains a primary residence, but there’s no rental income or tax benefits.

Then I search the county records by their name to see if I can find their primary residence which is where you can send a letter.

I am new to real estate investment and planning to start ramping up long term rental properties in 2025, however my first two primary residences were both converted into MIL's and rented within months after closing which effectively followed the House-Hack tactic (we currently rent our furnished basement to primarily traveling nurses for 3-6 months at a time which covers 70% of our mortgage).

If you don't already have a primary residence, could you buy a property and house hack?  

There are another 20-30 questions I could ask before I could even begin to advise.EDIT - It looks like in Aurora, it needs to be your primary residence if you want to STR the property.https://www.auroragov.org/resi...

Convert the Property to a Primary Residence: If you live in the property for at least two of the five years before selling, you may qualify for the primary residence exclusion, which allows you to exclude up to $250,000 of capital gains ($500,000 for married couples) from taxation.4.

For investment properties sometimes you can do this if there is a history or using Airdna but never for primary residence purchases.

Do you take out a second mortgage on an Airbnb that you never wish to sell that’s almost paid off  or do you take out 100 grand against your primary residence.