Foundation design

What we need from you

To ensure the foundations for the structure are designed correctly, the following is typically required:

1. Geotechnical reportGround conditions, bearing capacity and the foundation recommendations for your site.
2. Architectural plans outlining foundation extentsFloor plans and elevations showing the footprint the foundation has to support.
3. Site survey — if the site is slopingA contour plan so we can set levels, steps and cut or fill.
4. Truss / roof loading plan — single storeyPoint loads and bearing positions from the roof structure.
5. Truss / roof and floor loading plans — double or triple storeyAll point loads from upper floors and the roof, so beams and pads can be sized.

Why a geotechnical report should be your first step

Everything starts with the ground

You wouldn’t build a house without a plan. You shouldn’t build one without knowing what’s underneath it either.

A geotechnical report tells you exactly what your site is made of — what the soil can hold, what it can’t, and what hidden problems might be lurking below the surface. For anyone building or renovating in New Zealand, it’s not optional. It’s essential.

What does a geotechnical report actually do?

A qualified geotechnical engineer visits your site, takes samples, runs tests, and produces a report that covers:

• Soil type and profile — what layers of material sit beneath your site
• Bearing capacity — how much weight the ground can safely support
• Groundwater levels — where the water table sits and how it might affect your foundations
• Problem soils — any conditions that could cause your building to move, crack, or fail
• Foundation recommendations — what type of foundation your site needs

The Building Code requires it

New Zealand’s Building Code Clause B1 (Structure) requires your foundation design to be fit for the ground conditions. Building Consent Authorities won’t approve a consent application without the ground information to back it up.

Most residential buildings in New Zealand are designed to NZS 3604, which allows standardised foundation designs — but only if the site qualifies as “good ground.” If it doesn’t, you need a geotechnical report to determine what your site actually requires.

Getting this wrong can mean delays, rework, or consent refusal. Getting it right means your project moves forward smoothly.

Two major ground risks in New Zealand

Every site is different, but there are two particularly common and costly ground conditions in New Zealand that property owners need to know about.

Liquefaction

When earthquake shaking hits loose, saturated soil, that soil can behave like a liquid. Buildings on the surface can sink, tilt, or shift — sometimes beyond repair.

The 2010–2011 Canterbury Earthquake Sequence made this painfully clear. Thousands of homes across Christchurch and surrounding areas were damaged or destroyed by liquefaction. Entire subdivisions were written off. Brand-new homes were just as affected as older ones.

Liquefaction isn’t a Canterbury-only problem. Risk areas exist in Auckland, Tauranga, Invercargill, the Hutt Valley, and other parts of the country. If your property sits on or near a floodplain, river flat, or reclaimed land, the risk could be real.

A geotechnical report identifies whether your site is at risk and what foundation solutions — such as ground improvement or deep foundations — are needed to manage it.

Expansive soils

Some soils expand when wet and shrink when dry. This constant cycle of movement pushes against foundations, causing cracks in walls, uneven floors, and long-term structural damage.

Across the Auckland region and parts of Northland, expansive soils are common. The problem has been documented since the 1970s, when severe drought cracked foundations on dozens of homes in Pakuranga.

NZS 3604 identifies expansive soils by their Liquid Limit (above 50%) and Linear Shrinkage (above 15%). If your site tests into this category, a standard slab foundation won’t do the job. You need a foundation designed to cope with the ground movement your site will experience.

New Zealand’s current testing methods for classifying expansive soils have been questioned by the geotechnical community. The Shrink-Swell Index test, inherited from the Australian Standard AS2870, has been shown to be unreliable for New Zealand soils. The New Zealand Geotechnical Society has flagged this and is pushing for better methods. Until that happens, getting expert site-specific advice is more critical than ever.

Climate change is expected to make this worse. More frequent and more severe droughts mean more ground movement — and more risk to foundations that weren’t designed for it.

The cost of skipping it

Some people try to save money by skipping the geotechnical report. It almost always costs them more in the long run.

Without a proper ground investigation:

• Foundations can be under-designed for real conditions
• You risk failing to meet Building Code requirements
• Building consent can be delayed or rejected
• Unexpected ground conditions can blow out your construction budget
• Remediation after the fact is far more expensive than getting it right upfront
• Insurance complications can arise if ground conditions weren’t properly assessed

How Foenix Foundations can help

At Foenix Foundations, we design foundations based on real ground data. Every site we work on starts with a proper understanding of the soil conditions, and every foundation we design is tailored to suit.

We don’t believe in one-size-fits-all solutions. Your site is unique, and your foundation should be too.

If you’re planning to build, buy, or renovate, start with the ground. We can help you make sense of what’s down there and design a foundation that will stand the test of time.