NZ House Foundations & 'Good Ground' (NZS 3604)

The requirement

In New Zealand, a standard timber-framed house is built to NZS 3604:2011. It’s the recipe most homes use, and the council accepts it under the Building Code (Clause B1 Structure, via B1/AS1). But there’s a catch that comes before any footing dimension: your block has to be “good ground.”

What “good ground” means. Good ground is soil or rock that can permanently hold an ultimate bearing capacity of 300 kPa — roughly 100 kPa once you take off the standard safety factor. That’s the number in NZS 3604:2011 clause 1.3. Just as important is what good ground is not: any ground that could move 25 mm or more for any reason is out. That rules out expansive clay, uncontrolled fill, soft or loose soil — and frost heave. If your site isn’t good ground, NZS 3604 stops applying and the footing goes to an engineer.

How deep the footing goes. The footing has to sit at least 200 mm below the cleared ground level, OR down onto good ground — whichever is deeper. So 200 mm is the minimum. If good ground is further down, you keep digging until you hit it. NZS 3604 doesn’t set a separate depth for houses versus other buildings inside its scope.

NZS 3604:2011 cl 1.3 & s7.5 (via NZBC B1/AS1)

1 footing
2 reinforcement
3 ground

No frost-line rule. Here’s the big difference from the US and Canada: New Zealand has no frost-depth requirement at all. NZS 3604 doesn’t set a frost line to dig below. Frost is handled the other way — through the good ground test. Any ground that could heave 25 mm or more, frost included, is simply not good ground and has to go to specific engineering design. So don’t go looking for a frost depth on a New Zealand job. There isn’t one to find.

What we won’t print — the widths. The actual width and reinforcing of your perimeter foundation come off the dimensioned figures in NZS 3604:2011 section 7.5 (Figures 7.13, 7.14, 7.16 and the like). They change with the wall type — timber frame, masonry veneer, concrete masonry — and the number of storeys. There is no single right width, so we won’t print one. Read it off the standard or your consented plans.

Work it out on site

The depth check is quick. Set your string line at the cleared ground level — the ground after you’ve stripped the topsoil off. Measure straight down to the underside of your footing. That gap has to be at least 200 mm.

Cleared ground to underside of footing: needs ≥ 200 mm
Your trench measures: 250 mm — that clears the minimum.

But there’s the second half: is the base sitting on good ground? If the soil tester says good ground starts at 400 mm down, then 250 mm isn’t enough — you dig to 400 mm, because it’s the deeper of the two rules that wins.

Once the depth is sorted, the concrete volume is just maths. Say you’re pouring a perimeter foundation — these are example numbers, your real width comes off the plan:

Total length of foundation: 40 m
Foundation width: 0.35 m (example only — read yours off NZS 3604 / the plan)
Depth poured: 0.3 m

Volume = length × width × depth:

40 × 0.35 × 0.3 = 4.2 m³

So you’d order about 4.2 cubic metres, then add a bit for spillage and an uneven trench — most builders tack on 5 to 10 per cent. Swap in your own numbers and the calculator does the sum.

Do it in 5 seconds

Part of the New Zealand building code guide — one national rule book, with the Acceptable Solutions and calculators that solve it.
Setting out the stairs on the same build? See NZ stair & barrier rules.

Sources

NZS 3604:2011 Timber-framed buildings — clause 1.3 (“good ground”) and section 7.5 (Foundations) — the deemed-to-comply route cited under NZ Building Code Clause B1 Structure via B1/AS1 (with MBIE modifications, notably Amendment 11). The good-ground threshold and the 200 mm embedment are restated by MBIE and building consent authorities: building.govt.nz — using NZS 3604 and MBIE CodeHub — good ground vs cleared ground.
Adoption / edition caveat: NZS 3604:2011 is the current edition and the standard path for most New Zealand homes, but it is an Acceptable Solution — not the only legal route. A designer may instead use a Verification Method or an Alternative Solution, which can carry different numbers. The standard itself sits behind Standards New Zealand registration, so the figures here come from official MBIE material restating NZS 3604 s7.5 as modified by B1/AS1 Amendment 11 — confirm against the current standard and amendment before you sign off. The exact footing widths are set per-detail in the NZS 3604 s7.5 figures and must be read from the standard or your consented plans, not a website. A draft revision (DZ NZS 3604) went to consultation in 2024–25, but the 2011 edition remains current as of July 2026. Any site outside NZS 3604’s scope needs specific engineering design (SED) by a chartered professional engineer.

Frequently asked questions

What is 'good ground' in NZS 3604?

It's the ground you're allowed to build a standard timber-framed house on without an engineer. NZS 3604:2011 (clause 1.3) says good ground can permanently hold an ultimate bearing capacity of 300 kPa — that's about 100 kPa once you take the safety factor off. Ground that could move 25 mm or more for any reason — expansive clay, fill, soft soil, frost heave — is NOT good ground. If your block isn't good ground, the standard doesn't cover it and you need a chartered engineer.

How deep does my footing have to go?

At least 200 mm below the cleared ground, OR down onto good ground — whichever is deeper. So 200 mm is the floor, but if good ground sits lower than that, you keep digging until you reach it. NZS 3604 doesn't set a different depth for houses versus other buildings within its scope.

Does New Zealand have a frost-line rule like the US or Canada?

No. NZS 3604 sets no frost-depth number at all. Instead of a frost line, the standard handles frost through the 'good ground' test: any ground that could heave 25 mm or more — frost included — is ruled out as good ground and has to go to specific engineering design. So there's no frost depth to dig to; there's a movement limit to check.

What width do I make the perimeter foundation?

That comes off the dimensioned figures in NZS 3604:2011 section 7.5 (like Figures 7.13, 7.14 and 7.16), and it changes with the wall type — timber frame, masonry veneer, concrete masonry — and the number of storeys. There's no single width for every house, so read it off the standard or your consented plans. Don't guess it from a website.

What if my site isn't good ground?

Then NZS 3604 stops applying and the footing has to be designed by a chartered professional engineer — specific engineering design (SED), verified through the geotechnical route (B1/VM1 or B1/VM2). This covers weak soil under 300 kPa, expansive soil, uncontrolled fill, liquefaction-prone or frost-heave-prone ground, or anything that could move 25 mm or more.