Question about land ownership

[John Marcal]

Okay so this is my situation :

My father bought a home 10 years ago, on the map of our land it shows that a chunk of the culdesac is ours and I'm wondering what we have to do to take it and turn it into an addition to our yard. 

Red boxed in area is approximately our total land area(my father owns both units). Green is what we are using. It's a small piece of area but it is ours and I do not want to lose it.

This is on Martha's Vineyard, MA