I have been to Japan.
In this sentence, 私 is the topic of the entire sentence, and it's not in any relative clause. (Very literally: "As for me, a having-gone-to-Japan [fact] exists.") Structurally, this is yet another "double-subject" sentence (Aは + Bが + predicate) such as 彼は背が高い, 今日は仕事がある or このレストランは料理が美味しい. Naturally, 私 ...
How will the following sentence be written using intransitive verb.
The sentence literally means "(Someone) built a tall building in front of my house." It's missing the subject.
To use an intransitive verb, you can say:
"A tall building was built in front of my house."
建てる is transitive, and 建つ is ...