Please read in regards to parents property and building g

Hi Yall!

My grandparents (63) have a 30×30 garage with an apartment above it that has never been built out. They offered me (23) the apartment space to live I just have to build it out myself.

I expect to get the realestate when they pass one day, not the point.

I want to take this opportunity, however I want them to sign a document that says if I build it put then I can't be evicted or charged rent.

Is there a document available that I could have them sign?