Loan affected by adding unit to a quad?

[Frankie Lotrec]

Hello, please bear with me as I'm a relatively new investor. I have a quadplex it has a basement we roughed in to add a unit one day....well, we're thinking it's "one day".  My question is, since we may not need to borrow anything to finance this,
 will the mortgage payment be affected?  I have no idea how that works.....

thanks so much