Nana Osaki in apartment 707 on the cover of Volume 1 of the manga.
Apartment 707 is the apartment that Nana Komatsu and Nana Osaki lived in. The apartment has a reputation among its neighbors of nothing good ever coming from it.
Bathroom[]
General view of the apartment's clawfoot tub.
Nana Osaki relaxing in the tub.
The apartment notably has a western style bathroom with a clawfoot bathtub in it. The clawfoot tub was the deciding factor for why Nana Komatsu wanted to move into the apartment. Nana Osaki often takes bubble baths here and would get disturbed by Nana Komatsu who would barge in to talk to her and even got in with her at one point. Mai Tsuzuki has taken a bath in it one time while staying the night.
Later on in the future after Nana Osaki leaves the apartment, the clawfoot bathtub was removed and replaced with a standard grounded tub with no explanation as of yet to why the tub got removed.
The bathtub had a dedicated page to it in the Nana 7.8 fanbook as part of it's "A to Z of Nana" segment. It's significance was detailed through the recurring theme of characters taking baths together throughout the series as an environment where it becomes easier for them to discuss delicate topics with each other.