How Many Security Sensors Does My Home Need?
Sensor count is decided by openings and interior paths, not by square footage. Work outward from the doors and you will land within one or two devices of the right answer.
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Details verified August 28, 2026
Step one: every exterior door
Entry doors are where the overwhelming majority of intrusions happen, and a contact sensor triggers the moment the door separates from the frame — before anyone is inside. Every exterior door gets one. That includes the side door nobody uses, the garage-to-house door, and the door from a conservatory or porch that people forget counts as exterior.
- Front door
- Back door
- Side or utility door
- Garage-to-house interior door — counts as exterior for this purpose
- Patio or French doors — one sensor per moving leaf
Step two: ground-floor windows that open
Fixed panes do not need contacts; they need glass-break coverage, which is step three. Windows that open get a contact each, prioritising the ones out of sight from the street — side returns and rear elevations. Upper-floor windows are usually skipped unless a flat roof, a porch roof or an extension makes one reachable, which is worth walking outside and checking rather than assuming.
Step three: glass-break for large fixed panes
A patio slider or a bay window presents a large area of glass that a contact sensor does nothing about, because the frame never moves. One acoustic glass-break detector covers a 25 ft radius and handles a whole room. Placement is by acoustic path rather than line of sight, which is unintuitive and the most common self-install error — it needs to hear the break, so soft furnishings and room shape matter more than where the glass is.
Step four: interior motion on the path, not in every room
Motion detectors are the second layer, catching anyone who defeated the perimeter. One covering the hallway or landing that connects the entry points does more than three covering individual rooms, because anyone moving through the house crosses it. Mount at 7-8 ft in a corner, facing across the likely path rather than along it, and away from windows and heat sources, which are what cause persistent false triggers.
Step five: life safety, which is the part people skip
A monitored smoke and heat detector reaches the monitoring centre when the house is empty. A hardware-store smoke alarm makes a noise for nobody. If the system is monitored at all, one monitored smoke detector in the main hallway is the highest-value single device on the list, and it is routinely the one left off to keep the device count down.
Typical totals
A two-bedroom apartment with one entry door: three to four devices. A three-bedroom house with two exterior doors: four to six. A four-bedroom with a patio slider, a side door and a detached garage: eight to twelve, plus a repeater if the garage is beyond roughly 100 ft from the panel. Beyond about twelve wireless devices, battery replacement becomes a staggered chore worth planning for.
Where This Leads
The specification pages below carry the full figures behind everything above — speeds, upload, latency, device lists, sensor counts and contract mechanics.
- Monitored security packages and equipment lists
- Equipment-only, no monitoring term
- Camera specifications and coverage
Availability and terms depend on the address, so both are confirmed on the call. The arithmetic above gets you to the right tier or the right device count; the call confirms what actually provisions where you live.
Common Questions
Is one motion detector really enough?
In most homes with a single central circulation path, yes. Add a second when the layout has two independent routes between the entry doors and the rest of the house — a rear stair, or an open-plan ground floor with two separate hallways.
Do I need sensors on upstairs windows?
Usually not, unless a window is reachable from a flat roof, a porch roof, an extension, or a substantial tree. Walk the outside of the house and look up; the reachable ones are obvious from the ground and easy to miss from inside.
What about the garage itself?
An attached garage gets a contact on the interior door at minimum. A tilt sensor on the garage door itself is worth adding if the garage holds anything valuable or connects to the house without a lockable door. A detached garage beyond about 100 ft needs a repeater to reach the panel.
Can I add sensors after the system is installed?
Yes, and it is a reasonable strategy. Wireless devices pair from the app in minutes. Starting with doors and one motion detector, then adding where you find gaps after a few weeks, often produces a better-placed system than specifying everything on day one.
Talk It Through for Your Own Home
The general arithmetic gets you close. The specifics — your floor plan, your address, what has actually been built on your street — are a five-minute conversation.
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