What is a Class 1a to 1b conversion?

The National Construction Code defines Class 1a and Class 1b buildings in Part A6 of NCC Volume 2. A Class 1a building is a single dwelling. It’s a detached house or one of a group of attached dwellings like a townhouse or villa unit, each separated by a fire-resisting wall.

A Class 1b building is a boarding house, guest house, hostel, or similar. The NCC caps it at a total floor area of 300 square metres and ordinarily no more than 12 people. Go past either of those numbers and the building moves into Class 3, which is a hotel or backpacker classification with significantly heavier fire safety and accessibility requirements. The 300-square-metre and 12-person thresholds are the boundary between what is achievable on a house conversion budget and what becomes a commercial-grade project.

The building surveyor is not doing a paperwork exercise. They are certifying that the building meets the construction standards for its new classification. If a fire starts in a Class 1b building that was certified without adequate fire separation or smoke detection, the surveyor wears that liability. This is why reclassifications are scrutinised more closely than a standard building permit for a renovation. The surveyor’s licence is on the line.

The conversion also triggers a change of use under the relevant state planning framework. A building permit alone does not authorise a new use if the planning scheme requires a planning permit for it. The two permits are separate processes with separate assessment criteria, and getting the building permit does not mean the planning use is lawful.

Fire safety: where the cost concentrates

The single largest cost driver in a 1a to 1b conversion is fire safety. The NCC requirements for a Class 1b building are materially higher than for a Class 1a house. What was adequate for a family living under one roof is not adequate for residents in separate rooms who do not know each other and will not necessarily alert each other in a fire.

The fire safety upgrades that typically apply to a 1b conversion include:

• Smoke detection and alarm system. Hard-wired interconnected smoke alarms with battery backup in every bedroom and every corridor serving bedrooms. This is a step up from the interconnected alarms required in Class 1a dwellings, and in most states the 1b standard requires an AS 3786-2014 compliant photoelectric system rather than the residential smoke alarm standard.
• Fire separation. The NCC requires fire-resisting construction between the Class 1b building and any adjoining Class 1a dwelling on the same allotment. If the 1b is a converted section of an existing house and the owner lives in the other section, a fire-rated wall is mandatory.
• Emergency lighting. Class 1b buildings require emergency lighting in passageways, stairways, and other egress paths that would be in darkness if mains power failed. This is not required in a Class 1a house.
• Exit signage. Where the exit path from a bedroom is not immediately obvious, illuminated exit signs are required. The NCC Part G4 and Specification 18 set out the specific requirements.
• Fire extinguishers and fire blankets. Portable fire extinguishers in kitchens and near exits, plus fire blankets, are standard for Class 1b.
• Evacuation plan and occupant safety. Several states require a fire safety management plan or evacuation diagram for Class 1b buildings, particularly where residents are short-stay and unfamiliar with the layout.

These upgrades are not negotiable. If the building surveyor certifies the reclassification without them, the certification is defective and the insurer will walk away. We have seen projects where the fire safety retrofit alone added 15 to 20 per cent to the total project cost because the existing house layout made fire separation awkward and the electrical board needed upgrading to handle the detection system.

State-by-state approval pathways

The NCC is national. The building approval process is state legislation, and the planning trigger for change of use is council-dependent. Here is how the pathway works in each of the four states Draftee operates in.

Victoria

The Victorian pathway runs through the Building Act 1993 and the Building Regulations 2018. A building permit is required for the change of classification from 1a to 1b, issued by a registered building surveyor. There is no exemption for small-scale conversions.

The planning side depends on the council. A change from a dwelling to a boarding house or rooming house is a change of use that, in most Victorian residential zones, triggers a planning permit. Some councils have specific local policies for rooming houses. Others assess them under the general residential provisions. The answer depends on the zoning, the number of proposed residents, and whether the area has a rooming house density cap under the local planning scheme.

Fire safety in Victoria is governed by the building surveyor’s assessment against the NCC, with the Country Fire Authority imposing additional requirements if the property is in a bushfire-prone area. The Metropolitan Fire Brigade and CFA both have specific guidelines for Class 1b boarding houses that the building surveyor will reference at the building permit stage.

For owner-builders: if the conversion work exceeds $16,000 in value and the owner is doing the work themselves, an Owner Builder Certificate from the Victorian Building Authority is required. That threshold catches most 1b conversions.

Queensland

Queensland uses the Building Act 1975 and the Queensland Development Code. Under section 119 of the Building Act 1975, occupying a building in a classification for which it has not been approved is an offence. The practical pathway is:

1. A building approval for the change of classification from 1a to 1b, issued by a private building certifier.
2. A plumbing compliance permit from the local council for any plumbing work involved in adding or modifying bathrooms, kitchens, or sanitary facilities.
3. A development application if the council’s planning scheme requires one for the change of use from a dwelling to a boarding house or rooming house.

The Queensland Development Code MP 1.1 or 1.2 sets out the siting requirements depending on lot size. If the existing house already complies with its original siting approvals, the reclassification usually does not trigger new siting assessment. But if any new construction is proposed, extensions or separate entries or additional bathrooms, the siting provisions re-engage.

Queensland’s smoke alarm legislation under the Fire Services Act 1990 requires interconnected photoelectric smoke alarms complying with AS 3786-2014 in every bedroom, in hallways serving bedrooms, and on every storey for all domestic dwellings. For a Class 1b conversion, the standard is effectively the same as the mandatory rental property standard, but the building certifier will also check against the NCC Part G3 requirements, which may be higher depending on the layout.

Queensland introduced compulsory seller disclosure of unapproved structures from 1 August 2025 under the Property Law Act 2023. If a 1b conversion was done without approval, it must be declared at sale. That has pushed a wave of retrospective certification work through the system.

New South Wales

NSW operates under the Environmental Planning and Assessment Act 1979 and the Environmental Planning and Assessment Regulation 2021. A change of building classification from 1a to 1b requires a construction certificate or a complying development certificate, depending on the pathway.

The planning trigger is the more variable part in NSW. A boarding house or rooming house is a defined land use in most NSW local environmental plans, and the permissibility depends on the zoning. In R1 and R2 residential zones, boarding houses are often permissible with consent. In R3 medium-density zones, they are generally permissible. The State Environmental Planning Policy (Housing) 2021 does not specifically cover Class 1b boarding houses in the same way it covers secondary dwellings, so the assessment defaults to the local LEP.

Fire safety in NSW is enforced by the relevant council and Fire and Rescue NSW. A fire safety schedule may be imposed as a condition of development consent, listing the specific fire safety measures required and the performance standard for each. The building surveyor or certifier will reference this schedule when issuing the construction certificate.

The BASIX energy efficiency requirements apply to new Class 1a dwellings but not typically to a change-of-classification from 1a to 1b where no new floor area is being constructed. If the conversion involves an extension or new construction, BASIX re-engages for that component.

Tasmania

Tasmania’s building system runs on the Building Act 2016, the Building Regulations 2014, and the Director’s Determination on Categories of Building and Demolition Work. Consumer, Building and Occupational Services (CBOS) administers the system.

A building permit is required for a change of classification. The building surveyor is a statutory officer in Tasmania and issues the Certificate of Likely Compliance. The planning trigger depends on the council’s planning scheme, which in Tasmania varies more between municipalities than in mainland states. A change from a single dwelling to a boarding house will require a planning permit in most residential zones.

Tasmania’s fire service has specific guidelines for boarding houses that sit alongside the NCC requirements. The building surveyor will coordinate with the Tasmania Fire Service during the building permit assessment if the layout or number of residents triggers their involvement.

Bushfire-prone areas and landslip hazard areas add specific construction requirements under the Building Regulations 2014. If the property is in a landslip hazard band A or B, a geotechnical assessment may be required before the building permit is issued, even for an internal conversion with no new structural work.

What Draftee does at this stage

Most of the 1b conversion enquiries we receive land after the owner has already investigated the market for boarding house accommodation in their area, budgeted the renovation, and spoken to a builder about bed-and-bathroom counts. What they have not done is run the planning property report or spoken to a building surveyor about the fire safety requirements.

We design the conversion to meet the regulatory standard the building surveyor will assess against. That means the floor plan drawn at the start accounts for fire separation, egress paths, accessible bathrooms, and the emergency lighting layout before the builder prices the job. A design that anticipates the surveyor’s requirements is a design the surveyor signs off on first pass. A design drawn in ignorance of those requirements gets sent back with a list of changes, and the builder re-quotes each round.

For a 1b conversion, the design package typically includes the floor plan with room classifications noted, the fire safety layout showing detection, separation, egress, and emergency lighting, the wet area waterproofing and sanitary facility schedule, and the energy efficiency documentation if the conversion involves new construction. If you are converting an existing house to a boarding house, the question is whether the existing layout can accommodate the fire safety upgrades without gutting the floor plan. Some houses can. Some cannot. The time to find out is before you buy materials.

Talk to Draftee about a 1b conversion

What does a Class 1a to 1b conversion usually cost?

We are not giving a build cost. Every house is different, the fire safety retrofit cost is specific to the layout, and the builder’s quote is the only number that counts for the build. What we can give is the documentation cost.

For a Class 1a to 1b conversion that stays within the 300-square-metre, 12-person NCC limit, the design and approval documentation typically covers:

• Existing conditions survey and floor plan.
• Architectural drawings showing the proposed layout with room classifications, fire separation, egress paths, and accessible facilities.
• Fire safety design showing detection, emergency lighting, exit signage, extinguisher locations, and evacuation diagram.
• Structural engineering assessment if any load-bearing walls are being modified.
• Energy efficiency report if new construction is involved.
• Wet area waterproofing schedule to AS 3740.
• Specifications and schedules for the building approval lodgement.

The documentation cost for a standard 1b house conversion usually lands between $4,000 and $8,500 depending on the house, the state, and whether structural work or overlays drive additional reports. The fire safety documentation is the part that varies most. A simple layout with a straight corridor and bedrooms off it is cheaper to document than a split-level house with multiple egress paths.