Arranging sensors to protect asset(s) just isn't as simple as looking at set of property plans and sprinkling a pepper shaker over it and placing sensors where the pepper falls. The most likely impact is budgetary - these things cost real money. Next is the unlikeliness that the pepper shaker has such mystical powers as to predict an intruder's path. Lastly, there are some design considerations you might want to entertain that affect the usability of the system. For instance, you might want to be able to use the restroom in the middle of the night without summon the local SWAT team for assistance.
Placing sensors in your home, business or other facility must work within your financial constraints, protect the asset(s), and facilitate your use of the space. We'll work with a home for now as an example.
It is important to detect the intrusion as early as possible. The farther away that the attack is detected and assessed the greater the opportunity to prevent them from being successful - regardless of their intent. It is essential to keep in mind that simply detecting activity is not sufficient. It must be assessed to ensure the detection is legitimate and not an error. There is a point between when a attack begins and when they are successful called the Critical Detection Point. It is that point after which a response will not be quick enough to thwart the success of the attack. With home burglaries it is an unfortunate fact that a response is not likely to arrive very quickly. Why is this you ask. Police departments are overwhelmed with service calls, most alarm activations are false alarms, and a burglar doesn't need to spend very much time in a home to get some good stuff and escape. This may not be as true if you live in a very large house that resembles a museum. In that case there are other concerns. The average home burglar will either be sent away when a loud siren activates or they will not. We'll address the bad guy a little later.
Early detection and assessment. In some areas the alarm must be assessed by phone contact by the central station, or by remotely viewing closed-circuit television (CCTV) or microphones, or possibly just through multiple sensor activations. Let's assume you do not want to have any CCTV systems in or around your home. It may just be enough to arrange the sensors to demonstrate the intent of an intruder and decrease the opportunities for unnecessary police dispatches. This is simply done with layers of sensors.
Many security professionals discuss "Concentric Circles." This is just as it sounds. Layers of protection wrapped around the asset being protected. This is also called "Defense in Depth" as well. Unfortunately this is not so easy in a typical residential structure. There simply aren't enough worthwhile layers. The wall of the house is typically the first line of physical defense (excluding the deterrent value of lighting and other features) and maybe there is a sturdy bedroom door after that - unless the bad guys uses a window.
Think in terms of occupied and unoccupied conditions. Will you be setting the alarm in the evenings while you rest or just when the home is vacant? If you plan on arming the system while you are inside the structure, try to created "protected corridors." These should let you move where you need to while wrapping the adjacent areas with reliable detection zones. In addition to the restroom and childrens' rooms, allow yourself enough movement to assess any odd noises or activities while keeping the alarm armed!
Next we can consider sensor types and locations...