Going to add a patio to step out onto from new big doors. I need to (I think) build a 2 course retaining wall for patio to suit levels I have. I need about 4mt long wall close and parallel to wall below door sill. And a short wall forming L shape from that to act as step to patio. As I won't see the 4mt wall, I hope a cheap concrete block solution will suffice. I was going to leave the gap between retaining wall and house empty, it shouldn't need clearing often and the door sill should form a lip over the gap. Any comments on this, spec for wall foundation etc? Thanks in advance.