It's a romantic comedy movie about a woman who wants her ex-boyfriend back. So she started staying opposite to his house in an old building. Where she met a guy who helped her in her purpose. They both started living there and kept eye on her ex-boyfriend. In one scene they paid a street artist and his monkey to create problems between her ex and his girlfriend. The monkey kissed her ex, on which his current girlfriend over-reacts. In the end she got successful in breaking her ex's relationship, but she realizes that she doesn't want him anymore and goes back to the guy who helped her. The building she used to keep an eye on her ex-boyfriend is in very bad condition. And it appears to be late 90's or early 2000's Hollywood film. And it has famous cast.
It sounds like Addicted to Love, (1997):
Matthew Broderick and Meg Ryan are trying to break up Kelly Preston and Tchéky Karyo. Matthew Broderick sets up a "Camera Obscura" to spy on the other couple. Meg Ryan puts moldy strawberries in Tchéky Karyo's pillow, knowing he is allergic.