Intruder Alarm Systems Explained: Types, Costs and Benefits

What Is an Intruder Alarm?

An intruder alarm — also called a burglar alarm or security alarm — is a system designed to detect unauthorised entry into a building and alert the occupants, a monitoring centre, or the police. Most systems combine door and window sensors, passive infrared (PIR) motion detectors, a control panel, and an external sounder.

A well-installed intruder alarm is one of the most effective deterrents against burglary. Studies show that homes with visible alarm systems are significantly less likely to be targeted.

Bells-Only vs Monitored Alarm Systems

The most important distinction when choosing an intruder alarm is whether you want bells-only or monitored protection:

• Bells-only alarms — When triggered, a loud external sounder (and optional internal siren) activates. The alarm relies on neighbours, passers-by, or you hearing it and calling the police. These are lower cost and simpler to install. However, if no one is nearby or if the sounder isn't heard, there's no automatic response.
• Monitored alarm systems — The alarm is connected (via mobile network, broadband, or dedicated line) to a 24/7 Alarm Receiving Centre (ARC). When triggered, the ARC contacts key holders and, if appropriate, dispatches a security guard or calls the police. Monitored systems give the fastest, most reliable response — particularly important for businesses or second homes.
• Smart / app-connected alarms — Increasingly popular for homes, these send push notifications directly to your phone when the alarm triggers. Not a substitute for professional monitoring, but a useful addition.

Wired vs Wireless Intruder Alarms

Intruder alarm systems are installed in one of two ways:

• Wired systems — Detectors and devices are hardwired back to the control panel. More reliable long-term (no batteries to replace in detectors), harder to jam, and generally preferred for permanent commercial installations. However, installation is more disruptive as cables need to run through walls and floors.
• Wireless systems — Devices communicate with the panel via radio frequency. Much quicker and cleaner to install, with no chasing of cables — ideal for occupied homes, listed buildings, or any property where cable runs would be difficult. Modern wireless systems use encrypted, frequency-hopping signals that are highly resistant to jamming.

Graded Alarm Systems: What Does 'Grade' Mean?

In the UK, intruder alarms are graded 1–4 under European Standard EN 50131, based on the level of risk they're designed to protect against:

• Grade 1 — Low risk. Basic DIY-level systems.
• Grade 2 — Low-to-medium risk. The most common grade for UK homes and small businesses. Required by most insurers for residential policies.
• Grade 3 — Medium-to-high risk. Used in larger commercial premises or properties holding high-value goods. Requires keypad or token access and more robust tamper protection.
• Grade 4 — High risk. Banking, data centres, and similar high-security environments.

SSAIB Approved Intruder Alarm Installation

If you need a monitored intruder alarm — particularly if you want police response — your system must be installed by an SSAIB (Security Systems and Alarms Inspection Board) or NSI approved company. Security Red Alert is SSAIB approved for intruder alarm installation across London.

An SSAIB-approved system also means your insurer may reduce your home or business insurance premium, as it demonstrates your alarm meets recognised industry standards.

How Much Does an Intruder Alarm Cost in London?

Costs vary by system type and property size. Typical ranges:

• Bells-only wired system (4-zone home): from £400–£700 installed
• Wireless bells-only system: from £500–£800 installed
• Monitored Grade 2 system (home): from £600–£1,200 installed, plus monthly monitoring fee
• Commercial monitored system: quoted on survey based on zones and risk level