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TC1, TC2 or TC3? What Your Technical Category Means for Your Build

October 01, 2026

TC1, TC2 or TC3? What Your Technical Category Means for Your Build

If you're building, buying or renovating on Canterbury land, you've probably seen the letters "TC1", "TC2" or "TC3" on a LIM report or in a geotechnical assessment. These Technical Categories were introduced by the Government in 2011 to classify residential land after the Canterbury earthquakes, based on how likely a site is to suffer liquefaction damage in a future quake. Here's what each one actually means for your foundation design.

TC1: Low Liquefaction Risk

TC1 land is unlikely to experience significant liquefaction damage. Standard foundation solutions, such as a normal concrete slab or timber floor built to standard details, are generally acceptable. This is the most straightforward and cost-effective category to build on.

TC2: Minor to Moderate Risk

On TC2 land, minor to moderate liquefaction damage is possible, so foundations need to be enhanced beyond the TC1 standard. Typical options include standard timber-piled foundations with suspended timber floors for lighter, lightweight-clad homes, or an enhanced concrete foundation such as a waffle slab (Ribraft-type) system. These foundations cost more than a basic TC1 slab but still generally follow standard, pre-engineered details.

TC3: Higher Risk, Specific Design Required

TC3 land carries the highest risk of moderate to significant liquefaction damage, and a standard foundation solution is not enough. Every TC3 property needs a site-specific geotechnical investigation and a specifically engineered foundation design, tailored to that site's ground conditions. Depending on what the geotechnical report finds, a chartered professional engineer may recommend one of several approaches:

Deep piles extending roughly 12-20 metres down to reach stable gravel layers
Ground improvement, such as compaction or cement stabilisation, to strengthen the site so a lighter, TC2-style foundation can then be used
Shallow foundations with lightweight structures, such as raft foundations that sit on top of the ground rather than into it, often paired with a lightweight timber floor and bracing

Because every TC3 site is different, there's no one-size-fits-all answer — the right foundation depends entirely on what the geotechnical investigation turns up on your specific section.

A Word of Caution on Relying on TC Alone

It's worth knowing that TC categories are now over a decade old and were only ever meant to give a quick, general guide to land risk across Canterbury's green zone. Building science and geotechnical investigation methods have moved on since 2011. While the TC label still appears on LIM reports and other property documents, it's best treated as a starting point rather than the final word — a site-specific geotechnical report and an engineer's assessment of your particular section will always give you a more accurate picture than the zone label alone.

What This Means for You

If you're planning a new build, an addition, or buying a property in Canterbury, it pays to find out your site's Technical Category early — and to get a proper geotechnical and structural assessment rather than assuming the TC label tells the whole story. This can shape your budget significantly, particularly if your site turns out to be TC3.

Not sure what Technical Category your land falls into, or what foundation design is right for your site? Get in touch with Fil Engineers — we're happy to talk through your options.

This article is general information only and is not a substitute for site-specific engineering advice. Every property is different — always get a qualified geotechnical and structural assessment for your own site before making foundation decisions.

Sources

Fil Engineers

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