Adding units on property with an existing duplex

[Mike Nobil]

Hi - I own about 1 acre of land in Mansfield, MA that currently has a fully occupied up/down duplex on it. The units are connected to public water and sewer. I think I have enough land to build a second structure with 2 or 3 more units on the property. My first step will be to go to town hall to learn more about the local zoning and building code that may or may not allow me to build a second structure. 

Assuming local zoning and building code allows it, what kind of cost would I be looking at to build a new 2-3 unit structure from the ground up? And if anyone has experience doing this what are some gotchas I should be on the look out for?