Also, usually, historic landmark status must be applied for. If it never was (and, presumably, the owner here would not want such attention drawn to the structure), there may not be any such issue.
I’m not clear if non-owners can force buildings to be declared historic landmarks…
Use of gargoyles in buildings would predate the United States. Any decrepit building will work for your storyline as long as you describe it as such. I would go with Art Deco as something believable and yet having enough excess lines that could be described in a crumbling creepy manor.
Small towns and middle-class suburbs will sometimes require owner consent for landmarking, but most cities have ordinances that does not require it. After all, nothing of real importance would ever be saved by such an approach. If the owner wants it preserved, he preserves it. The general pattern is that the landmarks agency identifies the building as significant (sometimes in response to a perceived threat), makes judicially defensible findings of historic significance, then recommends a landmark designation to the city council. After that, the wishes of the owner are irrelevant.
Don’t confuse landmarking under a municipal ordinance with listing on the National Register. That’s often done by the owner—but provides no protection for the structure unless someone plans to use federal funds to tear it down.
Oh, I LOVE this!! (BTW, all… this is definitely an old creepy HOUSE by now. The characters have spoken. 
The enclosed stairways are great! They SEEM to lead nowhere… unless you know the secret. Then, they’re portals to hell.
I’m glad this thread wasn’t ultimately moved to IMHO… it’s really about specific information and details like this.