If the chimney stack is to be removed to below the roof, the roof timbers will need to be extended to fill the gap, and matching roof covering used to fill the gap.
If the chimney stack is to be left in place and only part of the chimney breast below is to be removed, the brickwork above must be supported with a Rolled Steel Joist (RSJ) or other suitable support. What will be required will depend upon the size of masonry above and the adjoining walls which will need to bear the load.
If any unused chimney breast is retained, it will need to be ventilated at both the top and bottom.
We recently removed these chimney stacks from a house in Enfield where the residents of two adjoining houses both wanted to remove the breasts inside. We discovered, however, that in one house a chimney breast had previously been removed with no supporting lintel being added. Therefore, the chimney stacks had been compromised and were potentially very dangerous. We carefully removed the pot, stacks and all chimney breasts in both properties leaving both home owners happy with new, bigger rooms and no concerns about the wobbly stack!