I just unbricked a bricked-up fireplace, and intend to turn it
into an alcove which could take a shelf or cupboard door on the
front. The floor of the fireplace was just tiles on earth.
Now I want to run whatever flooring (carpet or wood) I eventually
use into this alcove, so I'm thinking of excavating this enough
to pour in a solid floor, preferably with a dampproof layer and
possibly with thermal insulation too (although we're only talking
of an area of 2-3 square feet). One other factor is that I don't
want to rule out the possibility of using it as a fireplace sometime
in the future, so I guess a layer of expanded polystyrene under
the screed is ruled out. Is there anything similar suitable for
high temperatures?

Any suggestions on construction methods, including suggestions
for the mix and depth of the floor?

--
Andrew Gabriel