The answer is this - you can and you can't. If you want each zone to have a unique house code, and you want more than 16 zones, you can't. But, the only true reason to have a unique house code (except for logical division for programming etc) for each zone is if you have ~16 devices in that zone. Most people do not. So, you can have 16 zones (A1-16 thru P1-16), or 32 (A1-8, A9-16 thru P1-8, P9-16) or even 64 (A1-4, A5-8, A9-12, A 13-16 thru P1-4, P5-8, P9-12, P13-16).
Logical division by house code is a good idea, but for most people, it is irrelevant. You can group two rooms together to use one house code. If your garage only has two devices, just tag it on to an existing house code which doesnt use all 16 device codes.
Or, make a specific house code for the 'odds and ends'. Random sensors and devices throughout the house.
hope that helps...