Protecting outbuildings

You have multiple choices.

1) With the system disarmed, have the motion sensors in the garage on Secret Alert, which will send you a text message.  

2) Arm the system, but have all the in-house sensors set to Disabled, while the garage is armed.  Lousy solution though.

3) Buy a 2nd Simplisafe system just for the garage.  

Note that cameras don't trigger alarms.  Cameras have motion sensitivity and start recording, but you just get an alert, they don't trigger an alarm.  

Keep in mind that the outbuilding's sensors must be in range of the Simplisafe base station, and the cameras must be able to access your WiFi network.

I have a detached garage and have the doors and windows armed with entry sensors tied to the main alarm system in the house.  My garage is 35' from the house and it works just fine.  The furthest sensor from the base is about 100 feet and I've never had issues.  I opted to NOT put motion sensors in the garage because garages get bugs.  I figured a spider walking a cross the sensor might set it off.  I had read that glass breaks can be triggered by loud automatic garage doors so you might want to test that before committing to it, I never tried it.  The cameras should work fine as long as you have decent wifi available out there.  If you don't, install a wireless repeater or run a cable to the garage and hook up a 2nd router for it.

But if you want the garage protected when the house is not (not sure why), then the post above gives you the only real options.

You can make the "home" mode have only the garage sensors active, and all sensors in "away" mode.  Not ideal, but if you don't have the home armed when you are there, it should do what you want.