I am adding an extension to the side of my house. The existing wall has a sink drain coming through it with down pipe going into a 6 inch square drain open topped drain with a grill on it. As things stand this drain will end up approx 6 in below floor level inside the extension with downpipe on the surface of the wall. Can I hide the pipe in the wall - it would have to pass through the DPC? I guess I could knock out a vertical series of half-bricks above and below the DPC but what happens to the drain?