Buying the "Grunt" in a mieszkanie in Poland - How does that work?

I'm looking at buying a small underground parking garage, and part of it has already had the 'grunt' (ground) purchased from the Urzad. The other part doesn't?

Anybody have any experience with this? The seller explained that I didn't have to buy this grunt but can 'rent' it from the city indefinitely

This is quite common situation in Poland (buying a property on the city ground)
I have just one hint. People sometimes purchasing a property without any notarial act. Remember that such agreement is not valid.
So the garage will be yours, but for the ground you will pay the rent.