***** NOT A TEACHER *****
1. Sometimes it is easier to analyze a complete sentence. May I please add a few words?
2. "The [post] office near where he works [is 100 years old]."
a. I THINK that "Where he works" is a noun clause because it is the object of the preposition "near." (And, I believe, "near where he works" is classified as a prepositional phrase, which modifies "[post] office."
b. I THINK that "where" is an indefinite relative adverb -- indefinite because it does NOT have an expressed antecedent. (Compare: "The post office near the bank where [in which] he works is 100 years old." My books call "where" in that sentence a definite relative adverb because it has an expressed antecedent ("the bank").
i. IF I understand my books, your sentence means something like: "The post office near the place in which he works is 100 years old".
3. Do you agree with my opinion that "where" in your sentence does NOT refer to the [post] office?
P.S. If I am correct, then all credit goes to R.W. Pence and D.W. Emery in their A Grammar of Present-Day English (1963), pages 164 -165.
Student or Learner