I would say "the living room also serves as a home office". Using "for" in this case wouldn't work. The use of prepositions can be very tricky. Even after 40 years of practice using my native language I sometimes still stumble over which one to use.

Yes, this is a bit tricky to explain. I think if you use "for", that means the living room and home office are definitely two separate things. Using "as" means they can be the same thing, sometimes. "I work for a lawyer" = I am not a lawyer; the lawyer is someone else who employs me (eg. I am the accountant). "I work as a lawyer" = I am a lawyer. Legal work is my job. Follow Jamie's suggestion: "the living room also serves as a home office."