How do you know if you'll be legally allowed to convert a detached garage into living space or create any other type of square footage increase: attached garage conversion, basement or attic conversion, ADU, etc.

Also, aim to get similar bed bath count and square footage and make adjustments if necessary.

I'm suspicious it was a PM problem and not so much a sq footage problem.

There are so many dynamics like specific location, size, square footage, type of home, ARV, utility hooks and a dozen other things.  

Then, I can add value to it by rehabbing and perhaps even adding square footage.

In my case the rental unit is approx 33% of the homes square footage so I can only write off 33% of those items.

Square footage and number of interior and exterior levels is also very important for rentals, you have to look at the comps.

Consider factors like square footage, number of bedrooms and bathrooms, and any unique features or amenities that may impact these costs.Overall, while calculating rental income for two houses being sold together may require some additional considerations, breaking down the costs and revenue for each house individually can help you make more informed decisions and projections.

At least the legal square footage is there for you to use...

There is a good amount of square footage and it is a single family house.