Converting Sloping Driveway to Garage Base

Hi All,

I'm looking at having a lean to garage installed down the side of my house.

I've laid a concrete slab before, but the querie is - can I leave the existing sloping concrete drive in place and simply lay a level concrete slab on top? I assume the driveway was intended for cars as it leads to an existing garage, so it should be fairly substantial.

The other question is - if it is OK, as the driveway slopes away it will need to be built up in height to make the base level. What kind of infill should I use to acheive this?

Thanks in advance for any advice.