Opening up the chimney - exposing the builders opening

Hi,

Decided to take on a project whilst in lockdown: remove a gas fireplace, mantel, surround and hearth (the easy bit) and fit a cast iron fireplace (decorative only, not going to burn anything), a new mantel and new hearth. We live in a terrace house id say is well over 100 years old.

On the face of it it seemed fairly simple given the fact that the new fireplace is going to be non functional but I do have a query with regard to widening and heightening the current fireplace. As you can see when the gas fire was installed the original fireplace was reduced in size so it would be a snug fit. I need to increase the height by 100mm and width by 185mm on each side.

I think I have located the original lintel which sits about 1200 mm off the ground but there also appears to be a metal rod/support of some kind a little bit below it and above the concrete. You can see theothe concrete on the outside of the breast (pictured) - not sure if this is structural. I'm guessing so long as any heightening does not extend beyond the stone lintel (not sure how I'd even do that, it's a big bit of stone!) I am ok?

With regard to widening, I'm assuming the bricks/cement that form the current opening are not structural and I can just break this away by my desired amount (180mm each side)? I know there are limits to how far out I can go but given the width of the chimney breast I'm guessing I'll be fine?

Is there anything else I should be considering? Any advice/tips would be welcome. Thanks

Crack on.

Or... Google "builders opening", and see myriad before and afters. Providing (and I've never met it) the original lintel is not gone you should be fine with whatever is placed below that, support wise. Sides- you are looking for intact whole brick vertical joints- people hardly ever tie in the reducing bricks.

If in doubt/ fear, go slow to strip all the plaster or render off until it jumps out at you- the layout, I mean.