Open-source granny flat plans project launched

A University of Auckland research project aiming to develop open-source plans for small standalone dwellings has begun, with researchers seeking to address affordability, site constraints and liability questions under New Zealand’s new granny flat rules

The 18-month project, led by Professor of Architecture Anthony Hōete, follows recent changes allowing some standalone dwellings of up to 70m² to be built without building consent, subject to meeting exemption criteria.

Hōete said the research formally began on 1 April, after earlier pilot work involving around 120 students.

The project aims to produce a set of downloadable designs that could be used across different parts of the country, while accounting for variations in climate, wind and seismic conditions.

Design work yet to be finalised

Hōete said the designs have not yet been completed, with the current phase focused on understanding site conditions and testing feasibility.

“In order to design it, we need to understand what New Zealand’s available land looks like,” he said.

The research will include a large-scale sampling exercise using spatial data to assess how many properties could accommodate a detached minor dwelling.

Hōete said early analysis suggests site size may be a limiting factor. A 70m² dwelling, combined with setbacks, could require a minimum section size of around 150m².

“That’s no problem if you have the classic quarter acre section,” he said. “But what about in more medium density areas?”

The team is also considering whether smaller designs – such as 40m² or 50m² – may be more viable across a wider range of sites.

Affordability central to project scope

Hōete said affordability would be a key focus, but the research had not yet defined what price point would be considered affordable.

“We want to find out what affordable means,” he said. “Is $3,500 per m² affordable? Many people may not be prepared to pay $250,000 or $300,000 for a granny flat – we need to find out if they can.”

He said the project would involve engagement with homeowners, builders, suppliers and merchants to better understand cost expectations and material availability.

Hōete said the project would also consider how construction costs align with funding options, noting that financing from banks could affect uptake.

“If you can’t get a bank loan to build a granny flat, where’s that going to go?” he said.

Liability and design responsibility under review

Hōete said liability was an area still being worked through, particularly in relation to open-source designs.

“The liability we may have to investigate is only in relationship to the design,” he said. “We wouldn’t be liable for any construction issues that arise.”

He said the intention was that designs would be compliant if followed as specified, but responsibility for construction would remain with licensed building professionals.

“Someone will have to take the responsibility for ensuring what’s built is in accordance with what’s in the drawings,” Hōete said.

He said changes to a design could shift liability.

“If you download a plan and make an amendment, well, that shifts where liability will sit.”

Hōete also confirmed discussions were underway with insurers and industry bodies to understand how an appropriate insurance framework might operate.

“I haven’t got final details, but I can tell you conversations have been going on between the university’s risk insurance team, the NZ Registered Architects Board and private insurers.”

Councils emphasise owner responsibility

Before proceeding with building work, homeowners must apply for a project information memorandum (PIM) from their council and engage a licenced building practitioner to carry out the building work. Once construction is complete, homeowners must provide council with final design plans for the building, along with additional required documents such as Records of Work (RoW).

Councils contacted for comment said their role in overseeing exempt granny flats is limited.

Wellington City Council said it does not approve designs under the exemption pathway, and that “the owner is responsible for ensuring all requirements of the exemption are met no matter whether they choose an open-source approach or not”.

The council said removing independent inspection “can increase the risk of non-compliant building work and reduce consumer protections”.

Christchurch City Council said it has “little, if any, role in overseeing the design” of non-consented dwellings; however, they did add that enforcement action may still be taken if work is non-compliant.

Auckland Council said its role is limited to assessing proposals through a project information memorandum, and that “there is no building code assessment”.

“It is for that owner and the licensed building professional… to ensure that both the design and construction… is building code compliant,” said general manager of building consents Ian McCormick.

Planning and site constraints remain

All three councils said open-source designs would still need to comply with planning rules and site-specific constraints.

Wellington City Council said national permitted pathways do not remove the need to consider hazards, infrastructure and other planning requirements.

Christchurch City Council said a project information memorandum would identify site-specific considerations that designers must address.

Auckland Council said resource consent may still be required in some cases, including where natural hazards are present.

Next steps and timeline

Hōete said the project is moving into a stakeholder engagement phase, involving designers, builders, homeowners and suppliers. A website is expected to be launched to support public engagement and provide information about the project.

The research is scheduled to run for 18 months, with further findings expected as data collection and analysis progresses.

Hōete said the team expects to have clearer direction within the next six months, particularly on design sizes and site feasibility.