Planning the floor slab
I imagine that the building the floor is one of the most complicated and one of the hardest jobs to be undertaken in the construction of the extension as there are so many layers to the job!
The drawing below shows what has to be laid down and in what order.
All the layers that form the floor
From the bottom up...
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The hardcore base that will need to be consolidated with a vibrator and be at least 100mm thick.
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A thin layer of sand to protect the damp proof membrane from being damaged
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The damp proof membrane usually 1200g polythene (shown thicker here so that it can be seen)
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The 80mm thick phenolic or polyurethane insulation layer
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The 100mm concrete slab that might need to be reinforced if the floor is large
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The final screed to provide a flat finish
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The final floor - tiles for me.
The damp proof membrane shown in red
The above diagram shows the damp proof membrane in red (and thicker than it is in real life) and shows how it folds over the DPC material in the wall to form a complete watertight seal.
The thicknesses of the individual layers
The above diagram shows the thicknesses of the individual layers (TBA) which might vary around the country so it's worth checking with your local planning control officer.
Note: The above theory is all wrong following a talk with the Building Control Surveyor - see here
You will see that during the build I found out that the slab needs to be keyed into the existing house and the slab rests on top of the inside block wall.
Source: Govnt. Planning Portal