The insurance companies will always check for causes not only of the start of the fire, but things that are conducive to spreading or allowing the spread the fire. Some fires are caused by accidents but they consider building alterations as deliberate actions and not accidents.
If someone installs or alters anything that could possibly have required a permit and a permit was not issued, and it can be traced to the start of a fire, they will not cover you. If the same item was conducive to the fire spreading, they will not cover you. In this instance the breach in the firewall caused by installing the stairs would give them an out if the fire started in the garage.
Insurance underwriters and investigators are there to protect the assets of the insurance company, not the insured. If you ever have a fire, I would highly recommend the hiring of a private adjuster. You would be surprised how much more they come up with even after you pay their fee. They come up with things like that half bottle of mustard or pickles in the fridge that still have value. One of my better friends and the head of my credit union received an additional $46k after her private adjuster was done.
How many of us get the terms of our policies with every renewal, stuffed in the envelope with the privacy notices and printed in those little tissue paper pamphlets that are 40 pages long, in size 1 font, and either file them or put them in the recycle bin without reading them? I have to plead guilty.