Our certifier says the study doesn't qualify as a habitable room because the window is only 9% of the floor area. How is natural light measured, and can we borrow light from the living room next door?

The certifier is applying the right test. A habitable room needs windows with an aggregate light-transmitting area of at least 10% of the room's floor area, and that area is measured exclusive of framing members and glazing bars. Depending on how the 9% was calculated, correctly excluding the framing may make the shortfall larger or smaller. The window must also face the sky, a court, or an open verandah or carport, and a window facing a neighbouring boundary must sit at least 900 mm from it. [1]

Borrowed light is expressly allowed, with two conditions that both must be met: [1]

• the glazed panel or opening between the study and the living room must itself be at least 10% of the study's floor area; and
• the living room's own windows must be at least 10% of the combined floor area of both rooms. The living room has to carry the daylight for both spaces, not just itself.

Two details catch people out. If a doorway is the opening, it must satisfy the requirement with the door closed. And where the study already gets some direct light, as yours does at 9%, the borrowed-light areas may be reduced to suit, so the existing window counts toward the solution rather than being wasted. [2]

That gives three compliance branches: enlarge the study window to reach 10%; add a roof light, which only needs 3% of the floor area, with a worked method for combining window and roof light contributions; or borrow from the living room under the two conditions above. [1] [2]

Queensland is on NCC 2022 until 1 May 2027 and the numbers are unchanged in NCC 2025, which also adds a performance route based on a 2% average daylight factor. [3]