Our two-storey Sydney house has bedrooms upstairs and the living areas downstairs. The certifier wants an extra smoke alarm on the lower floor even though there are no bedrooms there. Where do smoke alarms have to go in a new Class 1a house, and do they need to be interconnected?

The certifier is right. The location rule for a Class 1a house has two limbs, and the second one catches your ground floor: a smoke alarm is required in each other storey not containing bedrooms. [2] Two storeys means at least two alarms, and more than one alarm means they must be interconnected. [1]

Where the alarms go. In a Class 1a building, smoke alarms must be located in: [2]

• any storey containing bedrooms: every corridor or hallway associated with a bedroom, or, if there is no corridor or hallway, an area between the bedrooms and the remainder of the building; and
• each other storey not containing bedrooms.

"No bedrooms, no alarm" is the misconception. The first limb is about protecting the sleeping area, the second is about every other level of the house, and they apply together.

What that means for your house.

• Upstairs: an alarm in the hallway serving the bedrooms. If the bedrooms sit in separate parts of the floor with separate hallways, each of those hallways needs one, because the rule says every corridor or hallway associated with a bedroom. [2]
• Downstairs: at least one alarm, even though it is all living space. [2]

The other three requirements. In NSW the national clause is replaced by NSW 9.5.1, which keeps the same rules for a Class 1a house. Smoke alarms must: [1]

• comply with AS 3786;
• be powered from the consumer mains source where a consumer mains source is supplied to the building; and
• be interconnected where there is more than one alarm.

Interconnection means an alarm going off downstairs sets off the one outside the bedrooms too. The figure notes to 9.5.2 confirm the alarms shown are required to be interconnected by 9.5.1(d). [4]

Interconnection stops at the dwelling. The explanatory information says the interconnection requirement only applies within a single dwelling, so alarms in a Class 1a dwelling need not be interconnected with alarms in another dwelling. [3] For a freestanding house that is academic; for a semi or terrace it answers a common question.

Where on the ceiling. Alarms must go on or near the ceiling: [5]

• on the ceiling: at least 300 mm from the corner junction of wall and ceiling, and, in a room with a sloping ceiling, between 500 mm and 1500 mm from the high point or apex;
• on the wall, only where the ceiling is not possible: between 300 mm and 500 mm off the ceiling at the wall junction.

A downstairs alarm tucked hard into a corner, or right at the top of a raked void, fails even though it is on the right storey.

The NSW extra you probably will not need. NSW 9.5.1 also requires interconnected heat alarms in a Class 10a private garage beneath a garage top dwelling that does not belong to that dwelling. [1] It does not apply to an ordinary house with its own garage.

Practical next step. Mark on the plans one alarm in each bedroom hallway upstairs and at least one alarm downstairs, note them as mains powered and interconnected, and dimension each one against the 300 mm corner clearance (and the 500 mm to 1500 mm band under any sloping ceiling).

NSW edition note. NSW is on NCC 2022 (Amendment 2, in effect since 29 July 2025) until 30 April 2027, and NCC 2025 applies from 1 May 2027 with no election window. NCC 2025 carries the same location, installation and interconnection rules, and the same NSW 9.5.1, so the answer does not change when the edition moves. [6]