Fireplace - wall cavity?

Morning

I am in the process of overhauling my kitchen, knocking some walls down, etc. My structural engineer and building inspector recommended removal of an old chimney breast in my kitchen (already removed upstairs) to create extra space, so I set about it yesterday.

From what I have done so far, it looks as though when the breast is removed completely, I will be down to only a singly layer of bricks - i.e. no wall cavity. What do you do in this situation? Do you build an inner skin to create a cavity? I was thinking of building one with thermalite blokcs or re-using the bricks from the chimney breast. Is it usual not to have a cavity behind a fireplace?

Sorry if I sound a bit sketchy, but rest assured I have got the professionals in on this one; I just want to know a little more before I go back to my SE and Building Inspector with the findings.

Thanks

Either form a cavity, or a make solid wall. It depends what the other external walls are

Older houses may have solid chimney walls. More modern ones will have cavity walls around the chimney or be a combination of cavity/solid.