The National Construction Code sorts every building into one of ten classes by the purpose it was built for. Most people never learn theirs. Then a new section of the Building Act arrived that switches on only if a building is Class 2, and suddenly the label matters. This sets out the ten classes, the rule that decides them, the 10 per cent exemption that catches people out, and where the regulator's plain-English summary and the Act itself do not quite line up.
01What a building class actually is
One rule decides it, and it is not the one most people assume.
The National Construction Code does not sort buildings by size, or height, or what they are made of. It sorts them by what they are for. In the Code's own words, it "groups buildings and structures by the purpose for which they are designed, constructed or adapted to be used".
That single idea does more work than it looks like. It means a building's class can change without a brick moving, if the use changes. It means two buildings that look identical from the street can carry different classes. And it means the question is never "what does this building look like", it is "what is this part of it for".
The classification then decides which technical requirements apply. Fire separation, exits, sanitary facilities, energy efficiency, access: the Code sets different requirements for different classes because the risks are different. A warehouse and a childcare centre do not need the same things, and the class is how the Code knows which is which.
There are ten classes, numbered 1 to 10, and several carry sub-classes marked by a letter. Class 1a is a house. Class 1b is a small guest house. Class 10a is a garage or shed. That lettering is where most of the detail lives.
02The ten classes
Grouped by what they are for, which is easier to remember than the numbers.
The numbering is not intuitive, and there is no point pretending otherwise. Class 7a is a carpark and Class 7b is a warehouse. Class 9b covers both a primary school and a night club. The fastest way to hold it is to stop reading it as a sequence and start reading it as four families.
Tap any class for the full definition and the rules that catch people out.
A few of those deserve expanding, because they are the ones people get wrong about their own property.
Class 1a is the one most readers live in. A detached house, or one of a group of attached dwellings separated by a fire-resisting wall, which is what a terrace or a townhouse is. Class 1b is a boarding house, guest house or hostel with not more than 12 occupants and a floor area not exceeding 300 square metres, or four or more single dwellings on one allotment used for short-term holiday accommodation.
Class 2 is apartments: a building containing two or more sole-occupancy units, each treated as a separate dwelling. This is the class that section 40A turns on, and it is covered in full below.
Class 10 is everything nobody lives in. 10a is a private garage, carport or shed. 10b is a structure rather than a building: a fence, a mast, an antenna, a retaining wall, a free-standing wall or a swimming pool. 10c is a private bushfire shelter. If you have ever wondered why a retaining wall or a pool fence attracts building requirements at all, this is why. They are classified structures.
03The edge cases that catch people out
Five rules that are not obvious from the class list, and change the answer.
None of this is trivia. Each of those rules moves a building into a different set of technical requirements, and requirements are what a defect is measured against. An owner who does not know their class is arguing about compliance without knowing which rules they are arguing under.
04One building can hold several classes at once
Each part is classified separately, and that is the rule most people have never heard.
A building is not given a single label at the front door. The Code requires that "each part of a building must be classified according to its purpose and comply with all the appropriate requirements for its classification". A mixed-use block is not a Class 2 building with some shops in it. It is a Class 2 part, a Class 6 part, a Class 5 part and a Class 7a part, each carrying its own requirements.
Two more allocation rules follow from the same idea and both are useful. A room containing a mechanical, thermal or electrical facility that serves the building takes the classification of the major part or principal use, rather than being classified on its own. And unless another classification is more suitable, an occupiable outdoor area takes the class of the part it is associated with. A rooftop terrace attached to apartments is not a separate thing; it goes with the apartments.
Where it genuinely is not clear, the Code hands the decision to the appropriate authority, which weighs what the building most closely resembles, the likely fire load, and the likely consequences of the risks. They will also look at relevant court decisions and determinations, and at any guidance the State or Territory has issued. In other words, classification is a judgement, and it is a judgement someone else makes about your building.
05The 10 per cent rule
The exemption that decides whether a small part of a building gets its own classification.
If every part had to be classified separately with no tolerance, a factory with a desk in the corner would be two buildings. So the Code carries an exemption. Where a part has been designed, constructed or adapted for a different purpose and is less than 10 per cent of the floor area of the storey it is on, that part takes the major classification instead of its own.
The Code supplies its own worked example, and it is worth seeing to scale.
Four percentage points is the whole difference. At 8 per cent the office is simply part of a Class 8 factory. At 12 per cent it is a Class 5 office that happens to be inside one, with Class 5 requirements attached to it.
There is a limit on the exemption and it matters more than the exemption does. It does not apply where the minor use is a laboratory, a Class 9b early childhood centre, or a Class 2, 3 or 4 part of a building. So a small childcare room inside a larger building does not get absorbed by the major classification no matter how small it is, and neither does a single apartment inside a commercial building. The Code has decided those uses carry risk that a percentage cannot wash out.
The Code notes that where a storey has a very large floor area, the 10 per cent concession is also large. Ten per cent of a big warehouse floor is a substantial room. The exemption is not a small-print allowance for a cupboard; on a large building it can absorb a genuinely sizeable space into a classification that carries lighter requirements than the space would attract on its own.
06Why the label suddenly matters: section 40A
A duty that arrived on 1 July 2026 and is switched on entirely by classification.
Until recently a homeowner could go a whole build without learning their building class. Then section 40A was inserted into the Victorian Building Act 1993, and a new obligation arrived that applies to some buildings and not others, on the basis of the class.
"The developer of a residential apartment building must ensure notice of intention to apply for an occupancy permit is given to the Authority in accordance with subsection (2) before the end of the applicable period in respect of the residential apartment building."
"Penalty: 500 penalty units, in the case of a natural person; 2500 penalty units, in the case of a body corporate."
A Victorian penalty unit was fixed at $209.10 for the financial year commencing 1 July 2026. On that figure, and the multiplication is ours rather than the Act's, the maximum is about $104,550 for an individual and $522,750 for a company. The regulator's own guidance page describes the duty without mentioning the penalty at all.
Whether it applies to a given building is a classification question, and the Act defines it tightly.
The definition in section 3(1) is a "class 2 building within the meaning of the Building Code of Australia with a rise in storeys of more than 3", or part of one, "that contains 2 or more homes", excluding anything the regulations carve out. The regulator's guidance renders the height test as "a rise in storeys of 4 or more", which is the same thing said differently. It does not mention the two-or-more-homes limb.
The part that is easy to get wrong
The guidance page says to notify "at least 6 months prior to making an application for an Occupancy Permit". Read plainly, that sounds like a deadline with an open runway behind it: any time up to six months out is fine, and earlier is safer.
The Act does not say that. It defines an applicable period with a start and an end.
For a normal build the period commences 12 months before the occupancy permit application date and ends six months before it. For work to be completed within six months, it commences six months before and ends 30 days before. On the section's own wording the window shuts, and it also opens. Notifying eighteen months out is not obviously compliance with a period that commences at twelve.
We are not saying the regulator's page is wrong. It is plain-English guidance and it is entitled to summarise a definition that runs to two limbs. But guidance is not the instrument, and anyone relying on this should read section 40A(5) rather than the summary of it. That is a general principle worth more than this one section: the summary tells you a rule exists, the instrument tells you what it says.
One more limb sits underneath. If notice has already been given and the developer becomes aware of a change in circumstances affecting the application date, a further notice must be given within five business days. And according to the regulator, the requirement reaches building permits issued both before and after 1 July 2026, so projects already under way are caught.
If you are buying off the plan or taking handover in an apartment building, this is worth knowing for a reason that has nothing to do with compliance paperwork. A regulator that is told six months ahead which apartment buildings are approaching an occupancy permit is a regulator in a position to look at them.
07What to do with this
Four practical uses, and one that saves an argument.
The reason any of this matters to an owner is narrow and specific. A defect is not an opinion about workmanship. It is a departure from a requirement, and the requirements that apply to your building are chosen by its class. Argue about a defect without knowing the class and you are arguing about the wrong rulebook.
That is the part we do. Site Inspections documents defects against the requirement they actually engage, for the class the building actually is, in the format an insurer, a tribunal or a rectifying builder can act on.
08Sources
Every quotation on this page comes from one of these, with the section or clause named beside it.
Every figure on this page was drawn by Site Inspections from the rules described. No diagram, chart, table or image from the Australian Building Codes Board or the Building and Plumbing Commission has been reproduced, adapted, redrawn or traced. Both bodies reserve their material, and the BPC expressly names diagrams. The classifications themselves are rules and carry no copyright; the way a regulator chooses to present them does.
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