Ten times the certifier was right, and the clause that says so

01
Every storey needs a smoke alarm, bedrooms or not, and more than one means interconnected.
The rule is one alarm per storey, not one per sleeping area, so the lower floor with no bedrooms still gets one, and once there are two they are interconnected, mains powered where mains power is supplied, and to AS 3786. This is the most common smoke alarm pushback, and the certifier wins it every time.
NSWClass 1NCC 2022
The questionOur 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 answer. The certifier is right. In a Class 1a house every storey needs a smoke alarm, including a storey with no bedrooms, and once there is more than one alarm they must all be interconnected, mains powered where mains power is supplied, and comply with AS 3786.
ABCB Housing Provisions Standard (NCC 2022), Schedule 5 New South Wales, NSW 9.5.1 Smoke and heat alarm requirements, p. 477
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02
A laundry is a wet area whether or not it has a shower.
A laundry is its own wet area, shower or no shower. The whole floor water resistant, the wall and floor junctions too, with extra treatment where the tub sits against a wall. The tiler's rule of thumb is about showers. The Housing Provisions are about laundries.
ACTClass 1NCC 2022 (NCC 2025 electable to 30 April 2027)
The questionOur Canberra tiler says the new laundry does not need waterproofing because there is no shower in it. The certifier wants the floor treated anyway. What does the code require for a laundry?
The answer. The certifier is right. The Housing Provisions treat a laundry as its own wet area: the whole floor must be water resistant and so must the wall/floor junctions, with extra wall and junction treatment where the tub sits against a wall, whether or not there is a shower.
ABCB Housing Provisions Standard (NCC 2022), 10.2.5(1) Other areas: walls adjoining vessels (sink, basin or laundry tub), p. 247
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03
A door onto more than three risers needs a threshold landing, and the Standard gives the size.
More than three risers, or more than 570 mm, and a threshold landing is required at the door: 750 mm long, the full width of the doorway, no steeper than 1:50. The surveyor asked for it because the Standard draws it.
VICClass 1NCC 2025
The questionOur Bendigo design has the laundry door opening straight onto a four-riser external stair down to the yard. The building surveyor wants a landing at the door. When is a threshold landing required, and how big does it need to be?
The answer. The building surveyor is right. A doorway that opens onto a stair of more than 3 risers or more than 570 mm needs a threshold landing at least 750 mm long, the full width of the doorway, with a gradient no steeper than 1:50.
ABCB Housing Provisions Standard (NCC 2025), 11.2.5(1) and (2) Landings; Figures 11.2.5a and 11.2.5b, p. 311
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04
External stair treads need P4 or R11, and a rated nosing strip is a genuine alternative.
An external stair is a wet surface, so treads need P4 or R11 to AS 4586. The surveyor was right to raise it, and the answer also gives the way through: a tested nosing strip rated P4, provided the strip itself carries the classification.
VICClass 1NCC 2025
The questionThe building surveyor flagged the timber stair off our Geelong deck for slip resistance on the treads and nosings. What classification do external stair treads need, and does a nosing strip get us out of treating the whole tread?
The answer. The surveyor is right to raise it. A stair exposed to the weather is a wet surface, so treads need P4 or R11 when tested to AS 4586. A tested nosing strip rated P4 is a genuine alternative to treating the whole tread, but the strip itself must carry that classification.
ABCB Housing Provisions Standard (NCC 2025), 11.2.4(2) and (4) Slip resistance; Table 11.2.4, p. 310
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05
Roof lights need 900 mm from a separating wall or boundary, and together no more than 20 per cent of the roof.
900 mm from the vertical projection of a separating wall, 900 mm from a side boundary, and no more than 20 per cent of the roof in total. The certifier raised fire separation because three skylights running to a party wall is exactly what the clause is for.
SAClass 1NCC 2022
The questionOur Adelaide terrace design has three skylights in a roof that runs to within a metre of the party wall, and the certifier has raised fire separation. How close to a boundary or separating wall can a roof light sit, and how much of the roof can they take up?
The answer. The certifier is right to raise it. A combustible roof light must sit at least 900 mm from the vertical projection of a separating wall and 900 mm from a side boundary, and all the roof lights together can take up no more than 20% of the roof or part of the roof.
ABCB Housing Provisions Standard (NCC 2022), 9.3.3 Roof lights, p. 232
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06
The step-free entry exemption has two limbs, and you need both.
There is an exemption for a sloping block, and the certifier was right that it exists. It has two limbs, both required: no step-free route via a garage, carport or parking space, and a site condition such as a slope steeper than 1:14. It switches off the access path clause only.
ACTClass 1aNCC 2025 (electable in the ACT to 30 April 2027)
The questionOur sloping Canberra block makes a step-free entry very hard. The certifier mentioned the livable housing provisions have an out for exactly this. When is step-free entry required and what relief exists?
The answer. The certifier is right that there is an exemption, but it has two limbs and you need both: no step-free route via a garage, carport or parking space, and a site condition such as an average slope steeper than 1:14. It only switches off the access path clause; everything else in the Livable Housing Design Standard still applies.
NCC 2025 Volume Two, Building Code of Australia, H8P1, p. 157
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07
A rainwater tank is only mandatory in Queensland where the council has opted in, and a greywater plant can meet it instead.
A tank is only mandatory for a new Queensland house where the council holds a Ministerial approval making QDC MP 4.2 apply to its area, and even then a greywater plant or other supplementary supply can meet it. The client heard a rule. The certifier knew it was a map.
QLDClass 1NCC 2022 (with QDC MP 4.2)
The questionOur client insists every new Queensland home needs a rainwater tank, but the certifier says it depends on the council. What does the code actually require?
The answer. The certifier is right. A tank is only mandatory for a new Queensland house where the council holds a Ministerial approval making QDC MP 4.2 apply to its area, and even then a greywater plant or other supplementary supply can meet the requirement instead.
Queensland Development Code MP 4.2, Rainwater tanks and other supplementary water supply systems (Version 2.0, 2013), Section 4, Approval to make Part mandatory for an area, p. 3
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08
A flight with less than 1 m of rise is exempt from the handrail rule; the main stairs are not.
A flight with a change in elevation under a metre is exempt from the handrail requirement, while the main stairs need one on at least one side, continuous, at 865 mm or more above the nosings. The certifier asked for exactly what the Standard asks for and nothing more.
WAClass 1aNCC 2025
The questionOur split-level has a short flight of five steps (about 850 mm total rise). The certifier wants a handrail on the main stairs but not there. Is that right?
The answer. Yes: a flight with a change in elevation under 1 m is exempt from the handrail requirement, while the main stairs need one on at least one side, continuous, at 865 mm or more above the nosings.
NCC 2025 Volume Two, ABCB Housing Provisions, 11.3.5 Handrails, p. 319
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09
On reactive clay, articulation joints are mandatory, and the wall sets the spacing, not the soil.
On reactive clay the joints are mandatory, because the only exemption is for soil classes A and S. The spacing is set by the wall: six metre centres without large openings, five where openings exceed 900 by 900, plus joints at changes in the wall. The engineer flagged it because the soil report made it unavoidable.
SAClass 1NCC 2022 (NCC 2025 from 1 May 2027)
The questionOur Adelaide brick veneer house on a reactive clay site has long unbroken walls, and the engineer flagged articulation joints. How far apart do articulation joints need to be in masonry veneer, and what sets the spacing?
The answer. The engineer is right. On a reactive clay site articulation joints are mandatory, because the only exemption is for soil classes A and S. Spacing is not set by the soil class: it is set by the wall, at not more than 6 m centres for walls without large openings and not more than 5 m centres where openings exceed 900 mm x 900 mm, plus joints at specific changes in the wall.
ABCB Housing Provisions Standard (NCC 2022), 5.6.8 Vertical articulation joints, p. 98
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10
Cutting insulation for downlights lifts the R-Value the rest of the ceiling must reach.
Where ceiling insulation is cut back for downlights, the remaining insulation must be lifted to the R-Value in Table 13.2.3w, by the percentage of ceiling left uninsulated. The assessor was right about the rule. Whether the table binds a Perth renovation depends on the compliance path, which is the next question to ask.
WAClass 1NCC 2025 (NCC 2022 electable)
The questionOur Perth renovation adds 20 downlights to an insulated ceiling. The energy assessor says we now need thicker insulation to make up for the gaps. Is that a real requirement, and how is the loss worked out?
The answer. It is a real rule: where ceiling insulation is cut back for fittings like downlights, the remaining insulation must be lifted to the R-Value in Table 13.2.3w, set by the percentage of ceiling left uninsulated. Whether that table binds a Perth renovation depends on the compliance path, because WA lets alterations to existing homes use a different route.
ABCB Housing Provisions Standard (NCC 2025), 13.2.3(5) Roofs and ceilings, p. 347
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The pattern

What these have in common.

Look at what the ten have in common. In every one the certifier was applying a rule that is broader than the room it was raised in: every storey, not just the bedroom storey; every wet area, not just the shower; every opening, not just the door. The pushback came from a designer applying the rule to the thing in front of them.

The fix is not to defer to the certifier. It is to read the clause as widely as the certifier does, before the drawings go in. The companion list is the four times the certifier read it too widely.

Where these came from

Every answer above was produced by DatumDesk.

DatumDesk answers Australian National Construction Code (NCC) compliance questions with clause citations, drawing on NCC 2022 and NCC 2025, state and territory variations, and ABCB handbooks. The commentary on this page is ours. The question, the verdict and the citation on each item are the published answer, verbatim, and the full answer with every source is one click away.

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Questions practitioners ask

Before you rely on any of this.

Does this mean I should always accept the certifier's call?

No. It means you should read the clause before you argue. Most calls are right. Some ask for more than the provision does, and the companion list sets those out with the same citations.

Which NCC edition applies in my state right now?

It depends on the state and, in some, on the lodgement date. Victoria and Western Australia are on NCC 2025, with Western Australia allowing an election for NCC 2022 until 30 April 2027. The ACT allows either edition until the same date. New South Wales, Queensland, South Australia (for building work) and Tasmania move on 1 May 2027. The Northern Territory has disapplied NCC 2025 until the next edition. DatumDesk keeps a per-state status page at datumdesk.ai/ncc-2025 and every answer leads with the edition operative in your jurisdiction. Adoption differs by state, so confirm with your state building authority.

How are DatumDesk answers checked?

Answers are generated only from an indexed corpus of the NCC volumes, the Housing Provisions, state and territory variations, ABCB handbooks and referenced regulation. Every numeric value is cited to a document, clause and page. Practice notes are labelled as interpretive. Licensed Australian Standards are named and deferred to, never reproduced. Where the corpus does not hold the answer, DatumDesk says so instead of guessing.

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Ten times the certifier was right, and the clause that says so

Every storey needs a smoke alarm, bedrooms or not, and more than one means interconnected.

A laundry is a wet area whether or not it has a shower.

A door onto more than three risers needs a threshold landing, and the Standard gives the size.

External stair treads need P4 or R11, and a rated nosing strip is a genuine alternative.

Roof lights need 900 mm from a separating wall or boundary, and together no more than 20 per cent of the roof.

The step-free entry exemption has two limbs, and you need both.

A rainwater tank is only mandatory in Queensland where the council has opted in, and a greywater plant can meet it instead.

A flight with less than 1 m of rise is exempt from the handrail rule; the main stairs are not.

On reactive clay, articulation joints are mandatory, and the wall sets the spacing, not the soil.

Cutting insulation for downlights lifts the R-Value the rest of the ceiling must reach.

What these have in common.

Every answer above was produced by DatumDesk.

Before you rely on any of this.

The next one is on its way.