hi atom,
Under carpet pressure mats are simple to interface and cheap.
Battery back up in the final unit is a good idea.
Make it a dual purpose system, 'low level' and 'high level' alarm modes.
Low level, turns on the hallway lights during the night, when the alarms is triggered and sounds a 'silent' alarm.
[for your ears only]
High level, lights on, 'sound bomb' activated.!!!
Dont forget to include exit and entry delays to the alarms, so you can leave the house before the system becomes active , likewise the entry delay, give you time to disable the system.
Remember the idea is to frighten away any intended intruder before he enters the house.
Deterrent not confrontation is best, let the bas**ds hear the alarm from outside, not when they are inside your house...