Hello Dpg: Good question. So, you paid $340K and added about $17K since to it. That's $357K total, 3 1/2 years ago. I'd say you're right in the ballpark at it being worth in reality about $300K in today's market. That would be down about 15-16% since the high. That's about right for most of our area.
If its a basement, how are you adding 500 SF of Living Area above grade? Basements are defined by being below grade. The only way around this is if it is a walk-out basement; then you may be able to call "some" of your finished area there "above grade" to include it within a new GLA.
I assume the Condo Board approved you doing this internal work? Whne you go to sell it, the new buyer or their bank will want to know that the improvement was done in compliance with the condo bylaws and also the town's codes. I'm sure you know that...
Please contact me if I can be of any help. Good luck,