You need ideally to excavate it all out to atleast 6 to 12 inch or else the lawn will will grow but will dry out and show signs of drought far quicker than the rest. The deeper you can excavate the material that was under the paving, the better. Grass roots a good few inches down. I had a job where i had to turf an area that was just an old yard. We cleared loads of rubble and hardcore to atleast a foot, spread topsoil, firmed and laid turf and it is still slightly showing signs of not being happy. That was 2 years ago and its been well cared for throughout.