Add 2 apartments to an existing 4plex?

I found a house that looks good on paper so far in regards to the numbers with the house unchanged. A bonus is that the ad says this: “The attic is finished. Attic could potentially be converted into two very large one-bedroom units.” Now, what needs to be done with the town records if this were to happen? Does this change anything going from 4-6 families, is this now considered commericial and need different zoning or something like that? Also, do you think this would be something that is doable soon after aquisition? Would a lender write a construction loan for a major project like this not too long after I started making payments? Financially it seems like it would make sense, the added per month costs on the loan don’t seem like they would come close to the added income so it looks like it would help cashflow.

Also, anyone have any clue (real rough estimates guys, I don’t need precise costs) how much something like that might cost to put in a few walls, a kitchen sink, bathroom, etc? Maybe $50-60k?