For HRE and OOTD, they can research the “Slate and Stone Construction” upgrade from their Keeps, which gives +5 fire armor to all buildings.
This also affects Farms, Wood Walls/Gates, and Stone Walls/Gates.
I don’t know if it’s intended that their Farms and Wood Walls/Gates are more fire resistant. It doesn’t quite make sense that those things are “better constructed”, as opposed to regular buildings.
For Stone Walls/Gates, that can only affect them if units are using torches against it during construction.
Anyway, I feel like these “buildings” did not get the extra fire armor before the DLC, and the change may not have been intended.
Build: 10.1.48.0