Adding balanced flue fire to extension

[jackboy29]

we are just finished off an extension that has cavity wall with 75mm gap between block and brick. We would like to add a balanced flue fire but the one we like is 36cm deep and so with a false chimney breast and the fireplace surround would stick out too far into our room. Does anybody know the regs regarding sinking the firebox into the inner leaf of the cavity wall. Removing the dot and dab plaster would gain 25mm. Then 100mm block then if we went back 25mm into the cavity it means we could gain 150mm meaning the false chimney breast would be much less imposing and boxy.

Thanks

 

[^woody^]

What about the damp? Fire risk to the cavity?

Above all, what does the manufacturer's guide say about it?