4 Unit Multifamily to 6 Units

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I'm looking to buy a multi family unit (MFU) with 4 units in PA with FHA. The seller indicated that the attic also could be made into two additional units to make it into 6 total units. Buying the currently MFU with 4 units will qualify it to 6. The local borough says I have to go through a zoning permit for the 6 units if I decide to renovate the attic. Does this mean I have to meet commercial property requirements? Does changing from 4 units with a FHA change my terms when it goes to 6 units (I'm having my realtor check this out with my lender)?