Hey @Bpm22. When a keypad is connected to a Base Station, it controls turning all sensors on and off and into a certain mode. In the modes Home and Away, the sensors that are monitored in this mode will depend on what you select. Therefore, if you put yourself into Home mode, you can have this be your Annex only sensors. You would go into Settings > Modes > Home > Sensors enabled in this mode and only select the ones for the Annex. Therefore, when you want only the Annex monitored, you will be in home mode. Then, in away mode, you can have it be only your main house’s sensors.
This may not be the perfect configuration for you though, as there are only two modes to change sensors monitored in, and may want to look into have a Base Station unit for each part of your house. I did answer a question similar to this in this Community post here if you wanted to check it out and see if it helps!