Allowing build buildings on designated areas of terrain?

Unity Engine
1

Got another subject I need to start thinking about. Creating designated areas of terrain only on which player can build buildings. As far as I can work this out I need to introduce grid system for terrain so that only in specific grid player could build a building. Anyone can point me in right direction of creating grid system and applying it to unity.

2

I’ve got a bookmark about turn based rpg that uses a hex grid:

3

Thanks for the link