Zoning - What to look for? Where to get started

Hi,

I'm interested in a single family home in my area that has great potential to be converted into a duplex and definitely be something I'm interested in.  I'm hoping for some advice on where to start with seeing if the zoning can be changed?  If so, how long does it usually take and how hard is it to do these things?  Any other advice or tips?

Thanks!

Taylor