Separating an existing building from a new addition by a firewall always (it seems) has unanticipated consequences. This refers to a true firewall rather than a fire separation wall. No matter how hard you try, there is always a surprise waiting for you.
Technically a firewall must be able to remain standing after the collapse of the structure on either side. This means that the wall cannot help support that structure. A favored way around this is to use two walls, each supporting the structure that is on its side. This approach triggers the need for two fire doors at each opening, one in each wall. Two doors creates all kind of collateral issues, and usually results in a vestibule to separate the doors, which in turn is fire-rated construction.
The alternative is to use a free-standing firewall, which is basically a cantilevered wall anchored in its footing. Engineers tend to over-react to this situation. Maybe they should.
The two drawings below show how complicated the situation can become. Click the images to download a PDF for easier viewing; or download Plans here and Sections here.
We used single, self-supporting firewalls where we had to pass through the firewall. Elsewhere we resorted to the double-wall solution to avoid the underpinning required by the single wall.
The problem with a single wall that is braced by the construction on each side is that a collapsing building creates a wind load on the wall. There are ways to anchor the wall to resist the wind load; and there are ways to let the structure fall away. There just aren't any good ways of doing both on both sides of the same firewall. If your code official knows this, ...