soil depth
 

I've realised that my previous question wasn't clear, but thanks
Dwayne for your reply. My paving slabs cover an area about 10 feet by
6 feet and I'm not sure what's under them but I feel it's probably not
soil. It may be rubble. I want to dig it out and have a planting area
but don't know what to fill it with. I'll have to buy some topsoil but
I assume I don't want to completely fill it with bought good quality,
topsoil as this will be expensive and a waste. It will be about 3 feet
deep. Do I want a foot of loose rubble and then manure and soil?
Should I fill it and then water it thoroughly until it sinks and then
top it up again? I don't mind spending a bit of money because I want
to end up with an area of good soil unlike the heavy stuff in the rest
of the garden.