4 unit to a 5 unit, mortgage

[Rigo V.]

I am buying a 4 unit property. It has space to make a 5th unit. The town needs rentals so the town seemed ok to approve if proper avenues are followed. I don't plan to finish the 5th unit for a few years but my question is if I get a residential loan, will the mortgage be affected in any way by finishing the 5th unit? I know 5 unit properties normally require commercial mortgages.