Without downloading your file i can suggest: either draw "grass" room AROUND the house (NOT UNDER it), or raise the house level a little bit so it's no longer at the same elevation as grass level.

Usually I use sattelite or cadastre map to find needed land area, screenshot and import this picture as a background image, scale it to real dimension and draw "grassed" room over it, to discuss where to place building, moving it around.
It's possible to draw room around house, as a last resort, thank you.

If house elevation will be hier then ground level, no grass will be seen on the plan at all.