I would bid based on SFH and consider it a mother in law bonus space.

The building is a 1925 - built two flat that I am entertaining the idea of adding living space upwards.

looks like a good use of the space to me.it's already commercially zoned and would be it's intended purpose. 

Have you ever heard of people renting out garage space for storage?

I would start by talking to the renters to gauge interest before crunching the numbers and moving forward.You could also convert the space into indoor storage.

Since COVID restrictions are starting to relax are there any local meet ups in the NH area? 

Economists credit a mix of factors including ultra-low mortgage rates, increasing demand for space, and an emerging home-buying demographic: millennials.

There is definitely enough space to build something larger (multifamily), but it probably isn't zone for that.

I've been following commercial real estate for the past few years and I've developed a personal preference for the STNL space.

Looking for advice in regards to planning, design, financing (Boston offers loans you do not need to pay back unless you sell the property - if you qualify), construction etc.If you have been through the process and successfully converted extra space into an ADU I would love to chat.