Japan’s 1981 earthquake building standard: what 旧耐震 vs 新耐震 really means for buyers

The line: June 1, 1981

On June 1, 1981, an amendment to the enforcement order of Japan’s Building Standard Act (建築基準法施行令) brought the revised seismic design standard — the 新耐震基準 (shin-taishin) — into force. Everything designed to the rules before that day is called 旧耐震 (kyū-taishin), the old code. The revision followed the 1978 Miyagi-ken-oki earthquake (M7.4), whose damage in the Sendai area exposed the old standard’s weakness under strong shaking. The new standard requires buildings to survive rare, severe shaking without collapse — the old standard was calibrated to moderate events.

This single date is the most consequential line in Japanese residential due diligence: lending terms, seismic-retrofit subsidies, insurance discounts, and resale value all key on which side of it a building falls.

The date that legally decides: 建築確認, not completion

Here is the part most buyers — and many listings — get wrong. The law does not ask when a building was finished. It asks when its building certification (建築確認) was approved. Official retrofit-subsidy programs define an old-code building as one whose certification was approved on or before May 31, 1981. A house whose plans were approved in May 1981 and completed in 1982 is legally old-code. A registry that says “built 1982” proves nothing by itself.

Practically: for completion years up to about 1983 — longer for larger buildings, which take longer to build — the completion year alone cannot classify the building. The certification date is a stated fact in the building’s paperwork, and in a court auction it is usually written in the court appraisal. That is the number to find.

What the official damage data shows

The boundary is not theoretical. After the 1995 Great Hanshin-Awaji (Kobe) earthquake, the national damage analysis found severe structural damage overwhelmingly concentrated in buildings certified before June 1981 — the finding that produced the Seismic Retrofit Promotion Act (耐震改修促進法) the same year, the law under which 耐震診断 (seismic diagnosis) and retrofit subsidy programs still run.

The 2016 Kumamoto earthquakes produced the cleanest era-by-era numbers on record. In the Architectural Institute of Japan’s (日本建築学会) exhaustive building-by-building survey of central Mashiki — the town that recorded seismic intensity 7 twice — analysed in the MLIT damage-cause committee report, wooden houses collapsed at 28.2% when certified before June 1981, at 8.7% when certified between June 1981 and May 2000, and at 2.2% when built to the June 2000 standard or later. One boundary cut the collapse rate by two-thirds; the second cut the remainder by three-quarters again.

The second line: the 2000 standard for wooden houses

新耐震 is not the end of the story for wood-frame houses. The June 2000 revision — the 2000年基準 — added what the 1995 and 2016 earthquakes showed was missing: foundations specified against actual ground bearing capacity, prescribed metal hardware at frame joints (継手・仕口), and checks on the balance and placement of braced walls, not just their total quantity. A wooden house certified in, say, 1992 is new-code — and still pre-dates all three of those requirements. For wooden buildings, the honest mental model is three eras, not two.

Checking a court-auction lot

In a Japanese court auction (競売, keibai), the file that answers the question is the 三点セット — specifically the court appraisal (評価書), which usually states the building’s 建築確認 date and number. Yudane’s free lot pages flag construction era from the court record: pre-1981 lots carry the old-code fact, and completions from 1981 to 1983 carry a boundary-window note telling you exactly this — that the approval date, not the year, decides, and where to find it. The paid deep-read translates the appraisal itself, fact by fact, so you can read the stated certification date rather than infer it.

None of this is a verdict on any lot. Old-code buildings are bought every day in Japan — at prices that assume a 耐震診断 and possibly a retrofit. The failure mode is not buying old-code; it is bidding as if the question did not exist. Browse the board →

This is information about public standards and public court records, not legal or structural advice. A seismic diagnosis is an engineer’s instrument; get one.