Raising subbase levels over existing concrete base

[Boardman]

I have an patio area, of which I will be taking out the old concrete base due to DPC issues, and making to a new lower level. However a quarter sector had a greenhouse which was on a concrete base 200mm lower than the rest of the area. As I plan to make the whole area the same level, would it be best to break up this lower area as well, or can a subbase be laid direct onto this smaller area? The concrete in this area is 170mm thick.
Also I will be leveling out the garden which currently meets the patio edge about 200mm below the new patio level, I was wondering what sort of support structure around the edge of the new patio area would be required before the new topsoil is put in place

[Tony McC]

The problem with having a sub-base that's partly laid over a flexible sub-grade and partly over a rigid base is that you could get differential settlement. This isn't a major worry for a patio area, but would be imore of a concern if this was a driveway or other pavement subject to heavy traffic.

So, in theory, you could leave the GH base in place and build up the levels with sub-base material, but make sure everything is as well compacted as is possible. Personally, I'd prefer to break out the concrete, smah it up or crush it, and incorporate the rubble with the sub-base material, but I appreciate that can be a lot of work for a DIY project.

As for a retaining edge, if there's a 200mm level diff between patio and garden then you need something like a flag-on-edge retainer. A "normal" edging kerb wouldn't provide sufficient lift.

[Boardman]

Thanks for speedy reply. I think I will go down your preferred route of breaking up the whole area. I would prefer to have a solid uniform base as there will be a grands worth of slabs on top of it!