If it is zoned for a duplex or AUD (Accessory  Dwelling Unit, i.e. mother-in-law unit) You will have to get Permits and bring it up to todays codes.  

One thing to check though is see what the house is zone for or if the allow an accessory dwelling unit if your going to use it as a duplex  or multifamily. 

NH is passing a new law next year, in June of 2017, making it legal to rent accessory apartments in your home to people other than family members.

We're planning to add an accessory dwelling unit (ADU) to the existing duplex by merging the two parcels together. 

Zoning is always a key factor too.If "storage" is allowed as a primary use on that third parcel, it may be as an accessory use to the MF.

We've missed a few deals because lenders don't see our work history or current employment as reliable.We are going to house-hack a multifamily or SFR with accessory apartment for 200k or less.We have a combined monthly income of $2500-$3000 and 15k in liquid assets.

The other units will pay your mortgage so you can spend your money on more important things like musical instruments and accessories. 

Light (paint, drywall, some flooring, accessories) - $7.50/sq ftMedium (one bathroom or vanities only, a kitchen only and the light stuff) - $15/sq ftHeavy (all baths and kitchen and flooring) - $30-$35 sq/ft (depends on area and finishes)I hope this helps.  

I am in Portland, Oregon and have been planning on building an Accessory Dwelling Unit (ADU) on our current primary property for the past 6 months or so.

My wife and I currently live in a 2 bed/1 bath ADU (Accessory Dwelling Unit) on Kauai and pay $1500/month.