If all your doing is putting flags down I'm not sure i would bother taking too much grass off, just enough to get your levels right, you want the wendyhouse to be above or level with the ground, the grass that's left will die back, If your going to dig it out, go 3-5inch, no plastic,you don't want to create a pond underneath, ram hardcore in to near surface level then lay the flags as Fairygirl says with blobs of mortar in each corner & the middle or a layer of sharp sand thats well trodden in & scraped flat with a level & a piece of wood.
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If all your doing is putting flags down I'm not sure i would bother taking too much grass off, just enough to get your levels right, you want the wendyhouse to be above or level with the ground, the grass that's left will die back, If your going to dig it out, go 3-5inch, no plastic,you don't want to create a pond underneath, ram hardcore in to near surface level then lay the flags as Fairygirl says with blobs of mortar in each corner & the middle or a layer of sharp sand thats well trodden in & scraped flat with a level & a piece of wood.