Capital Gains after property split?

[Elvis T Santana]

Hello BP Community!

I purchased a property for $220k 6 years ago in Fall River, MA. Two houses on one lot/deed/title - a 3-family and a single family. It was mortgaged as commercial since I was not a MA resident at the time. I live in the 3-family now and still have the commercial loan.

The city approved a property split. I now have two deeds and can sell off the single family. Lender will facilitate partial release. I assume/expect the single family to be listed for 220-240k

Do I have a capital gains issue?

One person said no because of overall cost basis. Someone else said the cost basis may be 25% of the purchase price because I am selling one of four rental units.

All feedback is greatly appreciated!