Best solution for Grid-based building system?

Hello,

so I did spend some time now on thinking about a game concept. What I am in need of right now, is a really good tutorial on a building system that uses a grid.

So I want for example a flat 100x100 plane that has a grid on it 50x50 so one little square in the grid is 2x2. - Buildings get placed from a top view. - Buildings should be shown in a lighter color before placed

What is the best solution for this? I tried 2500 cubes which i quickly realized is not a good way to do this, despite from performance also. The only solution I saw so far was someone who manually set up all this in a script with vertices etc. which seemed to be a lot of work. Is there a somewhat easier way?

Thanks in advance!