I want to build a deck overtop of an existing 8x10 concrete patio. The
main problem is that the existing patio sits about 3 or 4 inches below
the door sill. I don't see a way without removing the existing patio
and lowering the grade of the yard for the horizontal support joists to
fit without creating a step-up deck. The yard was raised for the patio
so the yard grade will not need to be lowered at the edges of the deck.

Do I just need to remove the existing patio?

Thanks.