The main clause says "The house is still standing." This is in the present tense. Therefore, something that happened before now can be in the simple past.
Past perfect is used to indicate something that happened before something in the past.
You might argue that the author had to depict the house before the novel could be published, and therefore the choice should be "had depicted." I don't think this is correct, because the verb "depict" should take its form from the time relative to the main verb, "is." The phrase "published in 1856" is a participial phrase modifying "the novel," and for that reason, I don't believe that it should control the tense of "depict." The verb "depicted" belongs to a phrase that modifies house. The sentence is equivalent to: The house that he depicted in the novel, published in 1856, is still standing firm.