They are a bit undersized but perfectly suitable for 1 bedroom apartments and the slight increase in costs is far less than the costs associated with venting the units properly.

Capital gains are calculated as:Sale Price - (Original Purchase Price + Capital Improvements + Selling Costs) - depreciation (if applicable) = Capital GainYour mortgage or HELOC balance does not affect this calculation—it only determines how much cash you take home after the sale.In Massachusetts, if the home was your primary residence for at least 2 out of the last 5 years, you may qualify for the Section 121 Exclusion, allowing you to exclude up to $250,000 (single) or $500,000 (married filing jointly) of the gain from federal capital gains tax.

Pay the mortgage.

Converting your current primary into a house hack by adding a studio helps you offset your current mortgage while learning how to be a landlord.

That would be a separate mortgage and not what amounts to a second loan on your personal residence, which if you have spouses, they might not be that into. 

At the end of 2023 I purchased an old 4plex out of state (MD) for $180k (it appraised for this amount), 20% down with 7.99% rate, mortgage is $1364 with piti.

Secondly, condo, house, anything with a mortgage...

You can also do a 10% second home mortgage to buy another place and later turn it into a rental. 

LLCs are ineligible for FHA loans and other government-sponsored mortgage programs

@Dillon Clark-don't pay extra down on your mortgage-don't use a HELOC-save up as much as you can-hope this helps