Turning a previously paved garden

[gardenwoe]

Not sure if this should be in here or Building/DIY thread so I'll give it a shot here first!
We have a smallish garden which was completely paved with crazy paving. It’s pretty old and starting to crack and look bad so we are going about getting rid of it and want to put a turf lawn down.

DH has got all the paving up and we are down to rubble/hardcore. We had forgotten there is a 1.5m diameter soakaway slap bang in the middle. We had to have this put in when we had our small kitchen extension for the rain water guttering/downpour to go in. He’s got the concrete slab off the top and down to a sheet of marine ply. He’s taken the soakaway down by a couple of bricks, (so we can get more topsoil in before the turf) and we need to cover again - presumably with new slab of concrete.

Anyone know how much topsoil we need to get down before we turf, and anyone foresee any problems with the grass not growing because of the 1.5 diameter circle of concrete underneath? Is there any alternative to make this work?

My thought was to do away with the soakaway completely, full it in with rubble and install a water butt (it really is a small roof) but I believe it could come up on a survey if and when we sell the house....and they insist on reinstalling the soak away (although if we left all the pipework I guess we could - just wouldn’t be very effective if we had filled it all in!)

Sorry OP should read 'Turfing' over

Shameless bump for the early evening crowd

Well, in my experience, grass doesn't need too much to thrive, so what you're planning sounds fine - but I personally would not take out a soakaway, if somebody told you it was needed.

About 6 inches of topsoil is usual. You could probably get away with slightly less, but you might find that patch doesn't grow quite as well as the rest or gets waterlogged during periods of heavy rain. What about making a feature of that area with a container garden/water feature?

The area over the concrete will dry out faster than other deeper soiled areas. You could alleviate this by incorporating a more moisture retentive compost into this area. On going maintenance could include the use of wetting agent to retain more moisture. This needs to be applied before drought conditions start.
Seed will give a more consistent lawn than turf in many cases. Ground prep is critical in either. Seed is also far cheaper.