Granny Flats Exemption.

The Granny Flat Exemption: An Engineer’s Guide to the Checklist.

With the government’s push to streamline housing, the "Granny Flat Exemption" has become a hot topic for Kiwi homeowners looking to add value or accommodate family. While the idea of skipping the full building consent process is attractive, "exempt" doesn't mean "anything goes."

At Nemean, we often see clients trip up on the specific engineering and design limitations hidden in the fine print. We’ve summarised the official Building Design Conditions Checklist to break down what you need to know to ensure your project stays compliant—and safe.

Exemption Conditions.

To qualify for the exemption, your design must be strictly residential and low risk. From a structural perspective, the checklist imposes rigid constraints to ensure the building doesn't require complex engineering oversight.

If your design steps outside these bounds, you are back in Building Consent territory:

• Single Storey Only: No mezzanines, no second floors.

• Detached: It must be standalone. You cannot attach this to your existing garage or house.

• Size Cap: The floor area must be 70 square metres or less. it depends on your council. Each one has its own rules and regulations and often differ in terms of size limits – typically, you’re allowed 60-65m², but Whangarei District Council, for example, allows 90m² (including decks)

• Zoning + land use – again, these are council-specific. But you might not get approval if your home is, say, in a commercial zone, heritage-listed or in a natural hazard area.

• Height Restrictions: The max building height is 4 metres above the floor level. Crucially for sloped sites, the floor level cannot be more than 1 metre above the supporting ground. If you are on a steep section requiring tall piles, this exemption likely won't work for you.

Materials Matter: Weight Limits

The exemption relies on the building being "lightweight" to reduce the load on foundations and the ground. The checklist specifies:

• Framing: Must be light steel or timber.

• Roofing: Must be lightweight cladding (max 20kg/m²). Heavy concrete tiles are out.

• Cladding: Maximum weight of 220kg/m². This allows for some brick veneer, but heavy stone or concrete panels might push the limit.

Civil Considerations: The Three Waters

Just because you don't need a consent doesn't mean you can ignore drainage. The checklist requires you to identify a specific compliance pathway (Acceptable Solutions or Verification Methods) for your water infrastructure.

1. Water Supply
You must connect to the council supply (NUO) if available. If you are rural or off grid, you can use an on-site system, but you must verify compliance via G12/AS1 or G12/AS3.

2. Stormwater (Surface Water)
This is a critical area for civil engineering. You cannot simply pipe roof water onto the ground. You must connect to the public system or use an on-site system (like soak pits) designed to E1/AS1, E1/AS2, or E1/VM1. Improper stormwater handling is a leading cause of localized flooding and ground instability.

3. Wastewater (Foul Water)
You must connect to the sewer if possible. If not, on-site wastewater management must meet G13 standards.

Plumbing Constraints:

• Maximum 30 fixture units total (roughly enough for a bathroom, kitchen, and laundry).

• No pumped systems inside the building.

• Main drains must be at least DN100 at a 1:60 grade.

The "Self-Contained" Catch

Finally, the unit must be fully self-contained. It requires its own independent electricity and gas supply. It is intended for a single household.

Do You Need an Engineer?

Even with an exemption, the Building Code still applies. The checklist explicitly states the build must comply with the requirements for 'Housing - detached dwelling'.

If your site has "bad ground" (peat, soft clay, liquefaction risk, etc) or if you are struggling to meet the surface water drainage requirements, you may still need professional advice. The exemption covers the process, not the laws of physics.

Planning a granny flat? If you are unsure if your soil conditions or drainage plan meet the "Acceptable Solutions" required by this checklist, get in touch with the team at Nemean. We can help verify your ground conditions and drainage design to ensure your exempt build stands the test of time.

Disclaimer: This article is based on the "Granny flats exemption: Building design conditions checklist” (BP 12331_12/25). Regulations can change; always verify with a professional before starting construction.