[RBS] Anyone ever sell their rights to expand property to a neighbor?

[The Jerenbergs]

Hi,
We have a neighbor who wants to extend his livingroom, for which he needs to 
build a roof on our mirpeset, to be his floor. And he needs our "achuzim" - 
the 5 meters or so that every apt. has automatic permission for in the Iriya 
to add to his apt. What would usually happen is that if one person needs 
more than the achuzim he has, he asks neighbor/s to sell him their portion. 
He claims that since building a roof for our mirpeset improves our property 
a lot (because then it's a lot easier to enclose it - not that we want to), 
and all building expenses will be on him, it's fair if we just give the 
achuzim to him.

Has anyone had such an experience who can help us out? We are being asked to 
sign papers with a lawyer about giving up our rights to these achuzim, and I 
wanted to know if this is as straightforward as he makes it sound.

TIA!
Batya Jerenberg