TW builds a bike shed

What are the dimensions?

The shed bit itself is 2065 wide, 1200 deep, 1780 height rear , 1950 height front (all internal measurements)

The roof extends about 1000mm further

How deep is the concrete?

There's 50mm stone slabs on a 50 - 100 mm concrete & hardcore base, on top of another 50-100mm concrete slab (all pre-existing)

The top layer would have been fine by itself, and in any case, it is all resting on two 100mm skids