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

The requirement

This guide covers perimeter foundations in NZS 3604 section 7.5. The 200 mm embedment is not a rule for every foundation. Check the figure and consented design. The footing must also bear on good ground, so the site can require a deeper footing. Good ground must hold an ultimate bearing capacity of 300 kPa. Frost and ground movement still need a site check.

Here’s the background. A standard timber-framed house in New Zealand is built to NZS 3604:2011. It’s the recipe most homes use. The council accepts it under the Building Code (Clause B1 Structure, via B1/AS1). But one check comes before any footing size: 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. That’s roughly 100 kPa once you take off the standard safety factor. The number sits in NZS 3604:2011 clause 1.3. What good ground is not matters just as much. Any ground that could move 25 mm or more, for any reason, is out. That rules out expansive clay, uncontrolled fill, and soft or loose soil. It rules out frost heave too. If your site isn’t good ground, NZS 3604 stops applying. The footing then goes to an engineer.

How deep the footing goes. For this perimeter detail, check at least 200 mm below the cleared ground level, or down onto good ground. The deeper of the two wins. So 200 mm is the minimum. If good ground is further down, you keep digging until you hit it. Other foundation types have different details. Check the applicable figure and the approved design.

1. 1 footing
2. 2 reinforcement
3. 3 ground

Check frost and ground movement. B1/AS1 excludes ground where foreseeable movement could reach 25 mm, including frost heave. That does not prove frost is harmless on other sites. Ask the designer to assess the ground and set the foundation detail. This guide does not establish a safe frost depth for your location.

The widths we won’t print. The width and reinforcing of your perimeter foundation come off the dimensioned figures in NZS 3604:2011 section 7.5. Think Figures 7.13, 7.14 and 7.16. They change with the wall type: timber frame, masonry veneer, or concrete masonry. They change with the number of storeys too. 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

For the volume calculation, use the concrete element’s thickness from the approved drawings. This is different from its embedment below ground level. Calculate separate concrete walls or beams separately, without double-counting intersections.

The depth check is quick. Set your string line at the cleared ground level. That’s 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, so it clears the minimum.

But there’s a second half: is the base sitting on good ground? Say the soil tester finds good ground at 400 mm down. Then 250 mm isn’t enough. You dig to 400 mm, because the deeper rule 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 or the plan).
• Depth poured: 0.3 m.

Volume is length × width × depth. So 40 × 0.35 × 0.3 = 4.2 m³. 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

Related

• 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 the modifications in the applicable B1/AS1 edition). The good-ground threshold and the 200 mm embedment are restated by MBIE and building consent authorities: MBIE B1/AS1 current editions 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 the applicable B1/AS1 edition — 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 applicable B1/AS1 edition must be checked. The second edition took effect on 28 July 2025. The first-edition transition for new consent applications ended on 31 July 2026. Any site outside NZS 3604’s scope needs specific engineering design (SED) by a chartered professional engineer.