Pet-friendly burglar alarms: how to stop your dog or cat setting it off

A pet-friendly burglar alarm uses motion detectors that ignore animals below a set size. Ajax's standard indoor PIRs are rated to ignore pets up to 50 cm tall and 20 kg, but only when mounted at 2.4 m at a suitable sensitivity. Outdoor models are rated for animals up to 70 cm, 80 cm or 1 m. Get the height, the setting and the furniture right, and your dog or cat can stay downstairs while the alarm is set.

UK Pet Food's 2026 survey puts pet ownership at 62% of UK households, with about 15.5 million dogs and 13 million cats.

Why do pets set off motion detectors?

A passive infrared (PIR) detector does not see shapes. It reacts to warm bodies moving across its zones, so a dog crossing the hall and a person crossing the hall are both moving heat. The difference is the size of the heat source and how it moves.

Ajax's pet immunity works in two layers. The optics are less sensitive near the floor, where an animal walks, so its heat reaches the sensor only weakly. SmartDetect, Ajax's analysis software, then checks whether the shape of the signal looks like a person moving.

Weight is the figure people quote, but Ajax treats it as a rough guide. The detector judges the size of the heat source, so check your pet's height: a tall, slim dog under 20 kg can still be too big.

How big an animal will an Ajax detector ignore?

These figures come from Ajax's specification sheets and assume the mounting height shown.

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On indoor models the sensitivity setting moves that limit. Ajax's rough guide:

• High: cats up to about 25 cm.
• Normal: small dogs up to about 35 cm.
• Low: animals up to about 50 cm.

Where should indoor detectors go if you have pets?

Start with height. Fit a MotionProtect U PIR below 2.4 m and your pet walks through the part of the pattern that reacts most strongly. Ajax's pet figures assume 2.4 m, so do not rely on them at other heights.

• Watch for perches. A cat on a sofa back, a shelf or the stairs is no longer a small, low heat source. Keep high furniture out of the detector's view.
• Corridors and windows. The MotionProtect Curtain (£66.92) has no weight rating. Ajax's advice for homes with pets is to mount it upside down, above the height your animal can reach.
• Second sensor. The MotionProtect Plus U dual-tech detector pairs the PIR with a K-band microwave sensor and keeps the same 50 cm rating.
• Glass as well. The CombiProtect U motion and glass-break detector (£105.00) has the same pet rating and also listens for breaking glass.

Can outdoor detectors ignore the dog in the garden?

The MotionProtect Outdoor detector and MotionCam Outdoor (PhOD) each have two PIR sensors, and both must see movement before the detector alarms. Ajax rates them for animals up to 80 cm when mounted at 0.8 to 1.3 m with the upper lens looking level with the ground. A slight tilt shifts the far end of the pattern, and a large dog that crosses both beams can still trigger it.

The Curtain Outdoor detector has a Pet position on its mode slider, and Pet mode must also be turned on in the Ajax app. It is then rated for animals up to 1 m with the lens 1 m above the ground. For a side passage, the Curtain Outdoor Mini covers up to 5 m and ignores animals up to 70 cm at 1 m, with no app setting needed. DualCurtain Outdoor (£158.07) is rated to 80 cm, but Ajax notes that its near-zone detection reduces pet immunity.

If an outdoor photo detector can see beyond your boundary, the ICO's guidance on home CCTV says data protection law applies, so aim it at your own property.

Which Ajax detectors have no pet immunity?

• Grade 3 (G3) detectors. Ajax's spec sheets say pet immunity is not available on models such as the Superior MotionProtect G3 (£77.69). Use them in rooms your animals cannot reach.
• HighMount outdoor detectors. MotionCam Outdoor HighMount and CurtainCam Outdoor HighMount sit at 2 to 4 m and are not pet immune.

If an installer or insurer asks for Grade 3, decide where the pets sleep before you choose detectors.

What does a false alarm cost in the UK?

If your alarm is monitored with police response, the NPCC's 2026 Police Requirements for Security Systems say you are warned in writing after two false calls in a rolling 12 months, and police response is withdrawn after three. An activation with no sign of entry or damage counts as false unless shown otherwise. Getting response back needs evidence that the cause was fixed, or three months without a false call.

In England, GOV.UK guidance says councils can act on an alarm that is a statutory nuisance, and can enter without force once they have tried a key holder and the alarm has rung for 20 minutes non-stop, or on and off for an hour, in a way likely to annoy people nearby.

A pet owner's checklist before you arm

• Measure your tallest pet standing and compare it with the detector's rating.
• Mount detectors at Ajax's stated height, level.
• Set sensitivity for that pet and run the detection zone test in the Ajax app with the pet in the room.
• Clear perches from the detector's view: cat trees, shelves, the top of the stairs.
• Leave the pet's room out with security groups; Ajax Hub 2 (£161.53) supports up to 9.
• Fit a MotionCam U photo detector where the pet roams, so an alarm arrives with up to five photos.

Our take

Pet immunity is a set of limits, not a promise. An Ajax PIR at 2.4 m on Low or Normal suits a cat or small dog, a curtain detector on Pet suits a garden, and a photo detector lets you check an alarm before you act. Large dogs and Grade 3 jobs need more planning. See the full Ajax wireless alarm range, or compare other brands in our alarm detectors range.

We are a supplier, not an installer, trading since 2012 from our Wolverhampton trade counter. We can recommend a qualified installer depending on the job and area.

Not sure which detector suits your pet? Call 01902 902251 or send us a message. We ship nationally to England, Scotland and Wales.

Quick questions

Will a pet-immune detector ignore a cat on the stairs?
Not reliably. Pet immunity assumes the animal is near the floor, and a cat that climbs closer to the lens can trigger it.

Does my pet's weight matter?
Less than its height. Ajax quotes 20 kg as a guide, but the detector judges the size of the heat source.

Are Ajax pet-immune detectors graded?
Ajax lists MotionProtect U, MotionProtect Plus U and MotionCam U as EN 50131 Grade 2. Its Grade 3 (G3) models are not pet immune.

Do outdoor detectors ignore foxes?
Ajax rates outdoor pet immunity by height, not species, so the same limit applies to any animal, provided the detector is mounted and levelled as instructed.

Do I need a particular hub?
They pair with an Ajax Jeweller hub. Photo detectors need a hub with photo verification, such as Hub 2.

Source: Ajax Systems blog, Why motion detectors react to animals and how to avoid it, and Ajax product specifications. UK sources: UK Pet Food, UK Pet Population 2026; NPCC Police Requirements for Security Systems 2026; GOV.UK, Noise nuisances: how councils deal with complaints; ICO, Home CCTV systems.