Hello. Im a time served plasterer. I've been asked to build a 6m x4 m concrete block shed with a double apex roof. There's an existing concrete slab which the original sheds on around 4m x 3m. I'm looking to dig out and extend the slab. My question is can I lay the bricks off the slab or I's it better to lay off a footing ? I'd have to dig a small trench around the existing slab. I'm not using concrete blocks below dpc. Only above then the blocks are being rendered. Tiled roof felt and battens first.