Ask most New Zealand homeowners how many alarms their house should have and the answer is a shrug and a guess, usually low. The honest answer is that it depends on the layout, but the method for working it out takes about five minutes with a pen and a floor plan.

Here it is.

Start with the non-negotiable positions

Three categories of position are required before you consider anything else.

• Every hallway or landing that serves bedrooms. If bedrooms open off two separate hallways, that is two units, not one.

• Every level of a multi level house, including a basement, an attic conversion and a mid-level landing. Vertical separation is the most commonly underestimated distance in a house.

• Every self contained sleepout, cabin, minor dwelling or converted garage where someone sleeps. This is the single most commonly missed position in New Zealand and it is not optional.

For most standard three bedroom single level homes, that alone is one or two units. For a two storey house it is two or three. Already more than many houses have.

Then add the sleeping rooms

A hallway unit assumes bedroom doors are open. In a New Zealand winter they are usually shut, and a closed door blocks sound substantially while also holding smoke back.

A unit inside each bedroom removes the problem and keeps the door's protective benefit. It becomes close to essential where anyone in the house is a heavy sleeper, elderly, hearing impaired, medicated, or a child, since children frequently sleep through alarms that would wake an adult.

For a three bedroom home, that is three more units.

Then the living areas

Lounges and family rooms deserve detection, particularly where there is a fireplace, wood burner, heat pump or portable heater, or where someone naps in a chair. Furniture is a substantial fuel load and upholstery fires are the archetypal smouldering overnight fire.

One unit per main living space is the usual answer. Open plan areas may need the position considered carefully rather than simply adding more units.

Then the heat alarm rooms

Kitchens, laundries and garages need detection and must not have smoke detectors, because steam, cooking smoke, lint and vehicle exhaust will cause repeated nuisance activations until somebody disables the unit.

Heat alarms are the correct device for those rooms. They respond to rapid temperature rise rather than airborne particles, so they ignore steam and burnt toast while still reacting quickly to a genuine fire. They interconnect with smoke units within the same range, so the system remains one system.

Kitchen, laundry, garage. Usually three more units, and these are the rooms where fires most often start.

Then the optional additions

Worth considering rather than required. A unit in the roof space if the house has older wiring, a lot of recessed downlights or a history of rodent activity, since electrical faults develop there unseen. A unit in a workshop or shed. A carbon monoxide alarm in sleeping areas if the house has gas appliances, an unflued gas heater, a wood burner or an open fire, because smoke alarms cannot detect carbon monoxide at all.

Running the numbers

A typical three bedroom single level New Zealand home with a garage comes out at roughly eight to nine units. Hallway, three bedrooms, lounge, kitchen heat alarm, laundry heat alarm, garage heat alarm.

A two storey four bedroom house with a sleepout lands around twelve. Most houses have two or three.

That gap sounds expensive until you work out the cost per year. Sealed ten year lithium units cost nothing to maintain and are replaced once a decade, so the annual cost of properly covering a house is modest and the alternative is a set of gaps you have chosen without quite deciding to.

Why the count only works if they are linked

Here is the part that makes the whole exercise worthwhile. Twelve standalone alarms are twelve independent devices, each protecting only whoever can hear it. A fire in the garage sounds in the garage.

Interconnected alarms sound together, so every unit you add extends detection coverage for the entire household rather than just for that room. The garage heat alarm now wakes the upstairs bedrooms. The sleepout unit alerts the main house. The value of each additional unit compounds rather than sitting isolated.

This is also why interconnection is required for new builds and consented alterations under the updated Building Code Acceptable Solutions from November 2024, referencing NZS 4514:2021. Wireless linking satisfies it, which means retrofitting an existing house needs no cabling.

Placement, briefly

Ceiling mounted, at least 300mm clear of walls, beams and light fittings. At least three metres from cooking surfaces for any smoke unit. Clear of heat pump heads and ventilation outlets, where air movement can carry smoke past the sensor. Below the apex rather than at it on raked ceilings.

Buying the set

Buy the whole group at once from one manufacturer's range, because interconnection only works within a compatible family and mixed brands fail silently. That also means they expire together, which is tidier than replacing one at a time.

On Point Distribution supplies CAVIUS, Watchman and SmartSense through more than 700 retailers and electrical wholesalers across New Zealand, with smoke and heat alarms in matching ranges plus wireless accessories, so putting together an interconnected smoke alarm system sized to your actual floor plan is straightforward.

Five minutes with a floor plan

Hallways, levels, outbuildings. Then bedrooms. Then living areas. Then heat alarms in the kitchen, laundry and garage. Then the optional extras.

Count it properly once, and stop guessing.