I added on a chimney to my home 20 years after I bought it. The chimney is a part of the home with a hole cut out in the side of the wall, and the fireplace front and back are of course one part.
The contractor made one mistake that is costing me now. In Texas with our grounds shifting all the time, he did not make the base deep enough and thick enough to become somewhat stable. There is a trememdous amount of weight on this base and it needs to be very deep, like 24 to 30 inches deep or thick. You need a good earthen base for this to sit on also. Just keep this in the back of your mind.
My fireplace took around 800 brick and 30 to 40 feet high. Consider this weight.
Do some homework on this foundation before any work is done.
Just wanted to add some fuel to your decision making process.