Granny flat exemption expands

Changes taking effect from 8 October 2026 will allow certified composite structural building products to be used for consent-exempt granny flats and provide greater flexibility around water and drainage services

Builders will have more options when constructing granny flats under the building consent exemption from 8 October 2026.

The changes will allow certified composite structural building products to be used, including some Structural Insulated Panel systems and solid timber building systems.

They will also make it easier to use or modify existing water supply, wastewater and stormwater services, subject to the suitability and capacity of those systems.

Composite products permitted

Composite structural building products used for an exempt granny flat must have a registered product certificate under the CodeMark scheme.

The product must be used within the scope and limitations of its certificate and installed in accordance with the manufacturer’s instructions to ensure it complies with the Building Code.

The Ministry of Business, Innovation and Employment (MBIE) says the change supports modern construction methods, including prefabricated and off-site manufactured homes, while providing more design and construction options.

Existing services may be used

Property owners may be able to use existing water supply, wastewater and stormwater services for a granny flat instead of installing separate systems.

Where a property is connected to a council network or another utility provider, the existing connection may be used or modified if the provider agrees it is suitable.

A building consent will not be required to make those changes, although any approvals required by the relevant utility provider must still be obtained.

Properties that rely on onsite infrastructure may also be able to use or upgrade existing systems. These could include rainwater supply, wastewater treatment and stormwater disposal systems.

Any onsite system serving the granny flat must have sufficient capacity and comply with the New Zealand Building Code.

Where council or utility services are unavailable or cannot support another connection, a new onsite system may be installed for the granny flat.

Foundation options clarified

New guidance sets out a pathway for assessing ground conditions and selecting foundations for consent-exempt granny flats of up to 70m².

Before a foundation is selected, preliminary site research and a shallow ground investigation are required. The guidance says Scala Penetrometer testing, supported by hand augering, should be used to demonstrate adequate soil strength to a minimum depth of 3m – even where previous information about the site is available.

Standard NZS: 3604 concrete slab, timber pile or concrete pile foundations may be used where the site is confirmed as ‘good ground’.

The guidance also provides specifications for a Modified Enhanced Waffle Slab that may be used on some sites with Class S or M expansive soils, TC2-equivalent liquefaction vulnerability or lower bearing capacity.

However, sites affected by more serious natural hazards, steep gradients or complex ground conditions fall outside the guidance. These sites may require a detailed geotechnical assessment, specific engineering design and a building consent.

Updated resources coming

Updated granny flat guidance, quick guides and supporting resources will be published on the Building Performance website when the changes take effect on 8 October.

The Granny Flats Foundations Guidance is available now.