Start With a Floor Plan
Effective security layout begins with a simple floor plan sketch — room dimensions don't need to be precise, but you need to identify all exterior walls, every door and window opening, stairway positions, and the location of the electrical panel and existing phone/internet entry points.
Mark the current entry points and label each exterior door and window. This becomes your sensor inventory baseline. Professional installers bring blank zone worksheets to site surveys; the exercise forces you to count every entry point rather than approximating.
The Three-Layer Model
Professional security design uses three concentric layers of protection. Each layer catches threats the one outside it misses.
Layer 1: Exterior Deterrence
This layer operates before an intruder attempts entry: exterior lighting with motion activation, visible camera housings, alarm system yard signs and window stickers, and reinforced entry hardware (deadbolts, door frames, window locks). Deterrence is the cheapest and most effective security measure — most opportunistic burglars choose a different target when they see visible security measures.
Layer 2: Perimeter Detection
This layer triggers when the exterior barrier is breached: door contacts, window sensors, and glass break detectors on all ground-floor and accessible openings. Perimeter sensors detect entry before an intruder is inside the living space, giving more time for police response and maximizing the deterrent effect of the alarm siren.
Layer 3: Interior Detection
This layer catches any intrusion that bypassed the perimeter without triggering a sensor — a window that was left unlocked, a door sensor that failed, or a technique that defeated the perimeter. Motion detectors, pressure mats under rugs at high-value areas, and interior vibration detectors form this layer. It provides redundancy, not primary protection.
Some DIY installations put motion detectors in key rooms but skip perimeter sensors entirely. This means an intruder can enter through a window, defeat the siren, and have free run of the house before a zone trips. Perimeter-first design changes the game.
Zone Planning
A zone is a logical grouping of sensors monitored together by the panel. How you zone a system affects how the panel responds and how useful the event log is during an incident investigation.
Entry/Exit Zones
Assign entry delay to the primary entry point only (usually the front door or garage entry). All other zones should be instant. Overly broad entry delays are a security weakness — they give an intruder more time to find and defeat the panel.
Perimeter vs. Interior Zones
Separate your door/window sensors from your motion detectors on different zones. This allows you to run in "perimeter only" mode when someone is home — exterior zones are active but interior motion detectors are bypassed. This is standard for households where people move around the house at night without wanting to trigger alarms.
Fire and CO Zones
Fire and carbon monoxide zones should always be active regardless of arm/disarm state. These devices are not security sensors — they protect life, and they should never be disarmed. Most panels have separate zone types for life-safety sensors that cannot be bypassed.
Sensor Count Estimates by Home Size
| Home Size | Door/Window Sensors | Motion Detectors | Smoke/CO | Total Zones |
|---|---|---|---|---|
| 800–1,200 sq ft (2-bed) | 6–10 | 2–3 | 2–3 | 10–16 |
| 1,200–2,000 sq ft (3-bed) | 10–16 | 3–4 | 3–4 | 16–24 |
| 2,000–3,500 sq ft (4-bed) | 14–22 | 4–6 | 4–5 | 22–33 |
| 3,500+ sq ft | 20–35+ | 6–10 | 5–8 | 31–53+ |
These are starting points. Actual sensor counts depend on window count, floor plan complexity, and the presence of attached garages, basements, and detached structures.
Special Considerations for St. Louis Homes
South city bungalows and two-flats: These typically have multiple basement windows at or near grade, a rear alley entry, and an external cellar or coal door. All of these need sensors — they're frequently overlooked entry points.
Older homes with balloon framing: Running wires through balloon-framed walls is more difficult than platform-framed construction. Wireless sensors are usually the right choice.
Ranch homes: Long perimeter with many ground-level windows. Motion coverage at the hallway intersection (usually center of the home) can cover the entire interior with one or two detectors.
Multi-story colonials: Second-floor windows above a porch or garage roof require sensors. The most common missed entry point in St. Louis colonial homes is the second-floor bedroom window above the attached garage — easily reached with no ladder required.
The planning process described here is exactly what Philibert Security does at every free site survey — floor plan review, entry point inventory, zone assignment, and sensor selection. There is no obligation, and the survey produces a written proposal with exact sensor counts, placement, and pricing.