Thanks for the tag @Shane Cloutier!
@Ruth Moynihan it sounds like you're jumping into a big project... congratulations for taking the first step! There is a LOT of work that goes into a conversion project from an existing commercial building (and what type of building we're talking about makes a huge impact... office, retail, warehouse, etc.) into an apartment building. While I don't have personal experience running a project such as this, our company, Elm Grove Companies, has completed several large adaptive reuse projects converting old, run-down, mixed-use buildings, and creating high end apartments with a light retail component attached. A couple of things to keep in mind for any project:
1) You need to be purchasing this property for a substantial discount. If you're paying full market value for a 10,000 SF commercial building that can be used functionally for it's intended purpose (in NH that's probably between $80 - $120 PSF), your numbers will probably never work to build apartments. While I don't know what type of building it is, if you're paying more than $30 or $40 PSF, you're probably paying too much.
2) It costs a lot to build apartments from the shell of a building. You can almost never maintain any of the existing infrastructure within a building because it is not suitable for residential use. Expect to spend anywhere from $80 to $150 PSF depending on the scope of the project, the type of apartments you're building, and the amount of infrastructure work that has be done. A project this scale has to be done with a professional construction company... especially with your first project, you can't try and bootstrap this yourself.
3) A part of your due diligence should be city approvals. Just because a building isn't zoned to support residential doesn't mean the city won't grant a variance. It makes it significantly more challenging to get approvals if the "spirit" of the zoning code doesn't support high density residential, but it still can be done. Make sure you hire the right people to get through the city approval process and have a working idea and a level of comfort with your project before you close on the property.
I'm sure I could go on and on about different pitfalls and points to remember but I'll wrap it up here. Conversion projects can have big upsides but also big risks. If you're willing to take them on and either have experience, or have a partner with experience in this area... I definitely think you can move forward and successfully tackle a project. If you want to talk more offline on specifics, happy to weigh in with what I can.