I made an alarm system for my garage/workshop.
Used magnetic contacts on up & over front door and side door, with foil strip on the 2 small d. glazed windows.
Connected together with a loop connector, meaning that all the time the ‘loop’ is connected the alarm is healthy, as soon as the loop is broken, by a magnetic switch releasing on door opening, or a foil tearing if the glass get broken then the alarm is tripped on.
2 110db sounders inside, 1 motorbike horn outside with a strobe light.
Put a delay timer in for exit after setting, and another in for a delay of 15 secs to enter and switch off.
Controlled by a key switch, with a 12v battery on float charge, all mounted in the loft, out of reach of scrote.
Cost <£50