I strongly suggest that if you do decide to finish it; get a Building Permit; even if you are doing the work yourself.
When you get the B.P. the County Recorder will be notified and therefore when you go to sell the house, it will show the additional 300 sqft.
You will also be able to show the permits to the Buyer.
The downside is, the assessor will also be notified.
Good luck and may God bless