NSW smoke alarm requirements, explained

Where they go. On every storey, and in the path of travel occupants would use to leave — in practice, hallways outside bedrooms. A single alarm in a central hallway does not cover a two-storey home, and often doesn't adequately cover a long single-storey one either.

Hardwired or battery. Homes built or significantly renovated since 2006 generally require mains-powered alarms with battery backup. Older homes may lawfully have battery-only alarms, though hardwired and interconnected is substantially better.

The ten-year rule. This is the one most often missed. Smoke alarms have a manufacture date printed on them and must be replaced within ten years of it, regardless of whether they still respond to the test button. The sensing element degrades over time; the test button only confirms the horn and the battery.

Rental properties. Landlords have specific obligations: alarms must be working, checked annually, and repaired or replaced within set timeframes. Records matter, because the obligation is on you to demonstrate it was done.

Photoelectric over ionisation. Photoelectric alarms respond considerably faster to smouldering fires — the type most likely to develop overnight in a home — and they're less prone to nuisance alarms from cooking, which is what causes people to take batteries out.

Interconnection. Interconnected alarms all sound when any one detects smoke. In the large, spread-out homes common across the Hills, this is a meaningful difference: an alarm sounding in the kitchen at 3am may simply not wake someone in a bedroom 20 metres and two closed doors away.

Interconnection can be hardwired where cabling allows, or wireless, which is far less disruptive in an existing home without roof access above every hallway.