H1 Compliance for Alterations and Additions in NZ: How to Stop Over-Insulating

What are the H1 rules for alterations and additions in New Zealand?

Under the New Zealand Building Act, extending or altering a home does not mean you must upgrade the entire house.

The rule is simple:

the new building work must comply with current H1 standards, and the existing parts of the house you touch must not be made thermally worse.

Any untouched existing fabric is excluded from the compliance assessment.

While the rule sounds simple, applying it in the real world is where architects and developers often get trapped into over-specifying expensive insulation.

With the H1/AS1 Sixth Edition removing the old "Schedule Method," as of November 2026, builders can no longer just guess the insulation requirements. You now have to calculate or model your offsets. At Reshaped Consulting, here is how we handle the most common alteration scenarios to keep your design intact and your build costs down.

Do I need an H1 if I swap or enlarge my windows during a renovation?

One of the most common alterations is upgrading or expanding glazing. How this is treated under H1 depends entirely on what you are doing—and how you prove it.

• Like-for-Like Upgrades: If you are keeping a window the exact same size and simply upgrading it from single to double glazing, you do not need to do an H1 upgrade elsewhere. It complies as-is because the building is not being made worse.

• Enlarging Windows: If you smash out a wall to install massive architectural sliding doors, you are losing insulated wall space. If you use the standard Calculation Method, the council will force you to "offset" this heat loss by dumping extra insulation elsewhere (usually the roof).

The Modeling Solution: The Calculation Method is highly restrictive because it only looks at heating energy (heat loss). When we use Thermal Modeling (Verification Method H1/VM1) for our clients, we assess both heating and cooling energy. Modeling accounts for the passive solar heat coming in through those new large windows. More often than not, this free solar heat offsets the loss, meaning we don't have to awkwardly cram extra insulation into your roof to make the design comply.

How does H1 compliance apply when converting an attached garage into a living space?

Converting an attached garage into a bedroom or living space is a massive headache under standard H1 calculators.

Most attached garages sit on an existing, uninsulated concrete slab. Basic calculation tools will tell you that because the old slab performs poorly, your new walls and roof have to work overtime and be heavily over-insulated to compensate and drag the average up.

How we solve it: As engineering consultants, we take a practical approach. We successfully argue to the council that because the uninsulated slab is existing fabric, it is excluded from the H1 assessment level. This means your new walls and roof only have to meet their own standard requirements—they do not have to work overtime to compensate for a 20-year-old slab. We explicitly state this in our H1 reports, and we have never had a council push back on it.

Will an addition or alteration force me to upgrade the insulation in the rest of my existing house?

Clients often worry that touching one part of an old 1970s house will trigger a mandatory thermal upgrade for the entire home.

The Building Act is very clear: you do not have to upgrade the untouched parts of the house. We draw a strict boundary in our compliance reports between the new work, the altered work, and the untouched existing envelope. By providing crystal-clear documentation to the council, we ensure you are never forced into unnecessary whole-house upgrades.

Can I still use the Schedule Method to guess insulation offsets for minor alterations under the Sixth Edition?”

Before the H1/AS1 Sixth Edition, builders doing minor alterations could often just guess what insulation was needed using the basic Schedule Method.

That era is over. The Schedule Method has been removed for these scenarios, meaning every alteration and offset must now be formally proven.

• The Calculation Method might save you a little bit of time upfront, but because it is so rigid, it almost always forces you to buy thicker, more expensive insulation or costlier double or tripple glazed windows.

• Thermal Modeling takes a closer look at how the building actually performs. In the long run, investing in proper thermal modeling will always save you more money on building materials than relying on the rigid calculation method.

Don't Let Standard Calculators Dictate Your Renovation

Standard online calculators will give you standard answers—which usually means buying more insulation than you actually need.

If you are planning a high-end alteration, addition, or conversion anywhere in New Zealand, let our nationwide team at Reshaped Consulting handle the H1 compliance. We use advanced thermal modeling to protect your design, satisfy the council, and keep your material costs under control.

Contact us today to discuss your upcoming alteration project.