The %NBS short version.
%NBS is the percentage of New Building Standard your building meets in a seismic event. The triggers most owners care about:
- Below 34%NBS — earthquake-prone under the Building Act. Council-issued notice, 7–35 year compliance deadline depending on importance level.
- 34–67%NBS — earthquake-risk. Not formally non-compliant, but flagged in due-diligence reports and may affect insurance, valuation, and tenant covenants.
- Above 67%NBS — considered acceptable for most uses. The market-clearing threshold for institutional tenants and lenders.
Where URM fails first.
Unreinforced masonry has predictable weak points:
- Parapets and chimneys — high up, unbraced, and the first thing to fall. Often the cheapest fix, often the highest life-safety dividend.
- Wall-to-diaphragm connections — old timber floors and roofs typically just sit on the masonry. Anchoring them in stops walls from peeling outward.
- Out-of-plane wall failure — long, slender spans of brick without intermediate bracing.
- Soft storey ground floors — Victorian-era retail with shopfront glazing under solid upper floors.
The cost gradient.
Indicative ranges for a typical 2-storey URM commercial building of 600–1,000 m²:
- Get to 34%NBS (life-safety, off the earthquake-prone list) — $250k–$500k. Parapet bracing, diaphragm ties, key wall restraints.
- Get to 67%NBS — $700k–$1.4m. Adds in-plane strengthening to walls, often via steel moment frames or shotcrete jackets.
- Get to 100%NBS — $1.5m+. Approaches a full retrofit and is rarely the cheapest long-term play.
"The 34→67 jump is often worth doing in one go. The 67→100 jump rarely is — economic life of the building usually doesn't justify it."
IL2 vs IL4 — and why it matters.
Importance Level changes the seismic demand. An IL2 building (most residential and small commercial) faces lower loads than an IL4 building (post-disaster facilities, public assembly over 1,000 occupants). The same physical structure can come in at, say, 55%NBS as IL2 and 32%NBS as IL4. Change the tenant, change the rating.
Practical takeaway
If you own URM and don't have a current detailed seismic assessment (DSA), get one. The Initial Seismic Assessment (ISA) is a quick desktop — it tells you whether you need to keep going. The DSA gives you the number that drives every decision after it: insurance, leasing, strengthening, sell.