Land piece not belonging to me but accessible only to me

I got a property registered to me in a closed community layout. 31*34. There is a triangle piece of land behind my plot which the builder did not registered (may be as per law he cannot register). Behind that, it is 14 feet  layout boundary  The triangle is accessible to only me. 

I want to know If I can use the triangle for my construction legally, Is there any way I can get my planned approved which uses the plot. 

Or shall I completely avoid using for my construction.

Same type of owner has paid 50K for similar triangle and got it mentioned in sales agreement, not on sale deed. they have done construction on it

please see the image attached.