Using a "Solid Wall Porch" as a Master Suite

[Rob Scarborough]

Good afternoon all!  

I looked at a 2-1, 750 sq' property.  However, when I visited the property with my GC there was an addition on the back of the house that had plumbing (for washer), electric and heat.  It looked like it had been there for a very long time (30+ years).  We devised a plan to turn this 250 sq' area into a master bed and bath making the home a 3-2.  However, when I checked the tax assessor's website, this addition is listed as a "solid wall porch."  The 250 sq' was not part of the 750 sq' listed.  

What pitfalls do I have in front of me if I move forward with this? From what it looked like, there were footings poured just like the rest of the house.  Would I be able to market this as a 3-2?  What would I need to do to make get this 250 sq added to the 750 sq'?

Thanks in advance!