Nest protect would probably do what you need. The crestron integration is a PITA, but if you install them in you main house and the back house they will notify through the detectors in the main house detectors that there is an issue in the guest house. They also support phone notifications.
I¡¯ve got them in my house and love the way it works. I put one in the attic and garage as well so if there is smoke or a fire, it will notify me where the issue is for example ¡°there is smoke in the kitchen¡± is announced through all of the other protects as well as lighting up a specific color based on the warning.
