Question about building on top of grass

Hi! I’m sure this is answered somewhere, but I’m not using the right search words or something.

I want to build my catio on the side of my house. It’ll extend out of some windows, over grass.

How do I build using lumber on grass? Do I have to build like I would with fencing and go 18 inches deep with a 4x4 post? And for the bottom horizontal piece, do I just lay it on the grass?

I know to purchase cedar, I’m confident I can build the frame and shelves, but I’m just not sure about how to do the bottom.

(I don’t want to lay pavers down, nor do I want to pour a concrete slab).