Part of making a great landscape design includes changing up the terrain. A flat yard is boring. Nature is never flat. So, no thanks to the builder who graded your lot for you. You want hills!
You’ll be surprised how much dirt it takes to make a bump that is noticeable from the street. Especially after you have covered or surrounded it with plants. And moving dirt is hard work! Man, the stuff is heavy! I got lucky when a neighbor had a hole excavated in his back yard for a pool and I was able to have the dump truck give his load of dirt to me instead of hauling it off to wherever extra fill dirt goes.
Don’t forget valleys. If you are going to make a faux river through your yard, the rock can’t be lying on top of the surrounding dirt. It needs to be depressed. You might get enough dirt out of your river valley to make one hill. If you make some rivers, consider where water naturally flows in your yard. Use your rivers to help direct runoff toward the low spot where water collects. I’ve also seen faux waterfalls made with rock.
You do have to be careful when changing up your terrain that you don’t do enough to interfere with your yard’s drainage. There are laws against that or something.
So, if you choose to make hills and valleys in your yard, good luck. It will add to your overall landscape design and make your yard look more like a professionally landscaped space. If your yard is already partially landscaped, it may be hard to add hills at this point. You can, but you have to scrape aside the rock.