Sub base over soil

[davemc]

Hi all
Some help with my sub base query would be great thank you.
I have removed an area of decking approximately 6m x 5m, and am planning to lay a new patio.
Now here's my problem, my Mrs wants it to remain at the old decking finish height as it's nice to step up from the lawn. Unfortunately under the decking is 400mm of top soil before I hit clay, if I dig out the top soil to reach solid ground I will have to build back up around 450mm with hard-core to then bed my slabs on.
The top soil is well compacted, the previous bodger laid slabs on top with a little sand, which someone then decked over.
Here is my possible solution ???
Can I remove the top say 50+mm of soil, then lay a 4" cast slab of concrete with 10mm reinforcement mesh? Then bed slabs on that?
Just to add it's a small patio area that we sit on every now again by the pond.
Many Thanks for any input
Dave

[Tony McC]

Nope. You have to get rid of *all* the topsoil.

It's an organic material. It *will* decompose over time, settling downwards, and taking your paving down with it.

See sub-grade page