Be prepared to battle this plant with secateurs once it gets going! Its roots may find all the small cracks in the concrete and then go down and expand, so be careful.

Depends what you mean by laurel, to some extent - there are 3 plants commonly called laurel - the spotted laurel (Aucuba japonica, 8 feet by 8 feet), Prunus laurocerasus and Bay (12/25 feet). Whatever you do, don't plant the Prunus variety, it gets 25 feet, although using Prunus Otto Luyken would be okay - that's a much smaller plant, reaching only about 3 feet with a similar spread, but even so would need 2/3 feet of soil beneath. The other fact is, if its over concrete, then that means there's no drainage - all plants require drainage, so unless you mean there's a concrete lip at the base of the wall, with soil in front of that, then you can't really plant there at all.