Not only but also : complex or compound

“Not only did George buy the house, but he also remodeled it.”

George did buy the house - George is the subject, did buy is the verb [and helping verb I believe], house is the direct object. There is one subject, George. But George did two things. Not only did he buy but he also remodeled .

I'm no expert, but I would definitely not consider the first segment of this sentence to modify remodel. Adverbs modify verbs. There could easily be two separate sentences - both having subjects and verbs, indicating neither is dependent; however, the two actions DO go together in that he did both things. I'd say complex is a good word to use. I Googled complex sentence and read this:

Two clauses connected with a coordinating conjunction is a compound sentence.
Two clauses connected with a subordinating conjunction is a complex sentence.

So I Googled "subordinating conjunction" and found:

Subordinating conjunctions are conjunctions that are used at the beginning of subordinate clauses. Some examples of these conjunctions are; although, after, before, because, how, if, once, since, so that, until, unless, when etc.

"Not only ... but also" is a correlative conjunction. In further research, I found sentence examples as to when a comma is needed. Because the second clause is also a complete sentence, the sentence would be grammatically incorrect without the comma before “but.”

I'm curious to see what other comments are made on this.

To address your concern with interrogative vs declarative, the sentence COULD read as follows:

George not only bought the house, but he also remodeled it.