Short answer: if a house stands on or below a slope of more than about 2 metres, two sets of rules can apply. A local cliff ordinance (がけ条例) can require a certified retaining wall or special foundations before you are allowed to rebuild or extend, and an old retaining wall (擁壁) that does not meet today's standards is the owner's responsibility to keep safe — including liability if it collapses onto a neighbour or a road. Old stone walls are common on cheap hillside akiya and the cost of replacing one can exceed the price of the house. You can spot most of this from maps, photos and two questions to the agent before you pay anything.
The Building Standards Act requires buildings at risk from slope collapse to have a retaining wall or other safety measures (Art. 19(4)), and lets prefectures and designated cities add detailed rules by ordinance (Art. 40). Those local rules are what people call the cliff ordinance. Each prefecture or city writes its own, but they follow a common pattern:
| Element | Typical rule (check the local ordinance) |
|---|---|
| Definition of a cliff | Ground sloping at more than 30 degrees with a height of more than 2 m (some areas use 3 m). A slope with a certified retaining wall may still count, depending on the ordinance |
| Area affected | Land within a horizontal distance of about twice the cliff height from the top or the bottom of the cliff (some areas use 1.5 times) |
| What is required to build there | A retaining wall that meets current standards, or a building design that is safe without one — for a house above a cliff, foundations taken deep enough; for a house below a cliff, a reinforced concrete wall or structure facing the slope |
| When it bites | On new construction, rebuilding and many extensions. A lawful existing house can usually be lived in and repaired as it is |
In Tokyo, for example, Article 6 of the Tokyo Metropolitan Building Safety Ordinance (東京都建築安全条例) applies to cliffs over 2 m and the area within twice the cliff height; city pages such as Ota City's guidance on building near cliffs (Japanese) explain the local application. Other prefectures have equivalents with different numbers — search the prefecture's name plus がけ条例 or 建築基準法施行条例 がけ.
The cost is not theoretical. In a reported Tokyo District Court case (judgment of 18 November 2016), the buyers of a ¥64 million house learned after purchase that the lot fell under the cliff provisions of the Tokyo ordinance and that a protective wall and reinforcement would cost about ¥20.8 million; the court found the brokers liable for failing to explain the restriction. A cheap rural akiya will usually involve smaller numbers, but the ratio of wall cost to house price can be even worse.
A retaining wall more than 2 m high is itself a structure that needs building confirmation (確認申請) under the Building Standards Act (Art. 88), and walls built in regulated development areas needed permission under the former Residential Land Development Regulation Act. Many old walls predate these rules or were never inspected. Types that municipalities commonly say cannot be assumed safe:
| Wall type | Japanese | Why it is a concern |
|---|---|---|
| Dry-stacked or rounded-stone walls | 空石積み・玉石積み | Stones held by friction only; no drainage design. Common on rural lots and terraced hillsides |
| Oya stone walls | 大谷石積み | Soft volcanic tuff blocks that weather and crack; widely used in the Kanto region |
| Concrete block walls used as retaining walls | ブロック積み（コンクリートブロック） | Hollow blocks are not designed to hold back soil |
| Walls extended upward | 増し積み擁壁 | A newer layer built on top of an old wall, adding load the base was not designed for |
| Walls without weep holes, or with bulging or cracks | 水抜き穴なし・はらみ・クラック | Water pressure behind the wall is a leading cause of failure |
The strongest evidence that a wall is acceptable is a confirmation certificate and completion inspection certificate (確認済証・検査済証) for the wall, or a development permit record. Ask the agent whether either exists; for walls over 2 m, the municipality's building department may be able to tell you from its records.
Looking at a house on a slope? An Akiya Check report pulls the landslide zones, large-scale fill areas, elevation and zoning for the exact lot from official data and lists the questions to send the agent. Comparing a hillside house with a flat one? Compare 3 shows them side by side.
Single report $49 · Compare 3 $99 Free 12-point checklist| Japanese | English |
|---|---|
| この敷地にがけ条例は適用されますか。建て替え時に擁壁の新設は必要ですか。 | Does the cliff ordinance apply to this lot? Would a new retaining wall be required to rebuild? |
| 擁壁の高さと構造を教えてください。確認済証・検査済証はありますか。 | How high is the retaining wall and how is it built? Is there a confirmation or completion certificate for it? |
| 擁壁は誰の所有ですか（この敷地か、隣地か）。 | Who owns the wall — this lot or the neighbour? |
| 擁壁にひび割れ・はらみ・水抜き穴の詰まりはありますか。補修履歴は？ | Are there cracks, bulging or blocked weep holes? Any repair history? |
| 盛土規制法の規制区域や急傾斜地崩壊危険区域に入っていますか。 | Is the lot in a regulated area under the embankment law, or a steep-slope collapse hazard area? |
Ownership of the wall matters: a wall on the boundary may belong to you, to the neighbour above or below, or be shared, and the owner carries the repair cost and liability. A boundary survey is often what settles it.
Generally no. The rules mainly apply when you build, rebuild or extend. A lawfully built existing house can usually be occupied and repaired. The effect is on future building options and on value.
It is the vertical difference between the ground at the top and the bottom of the wall or slope. Local ordinances define the measurement points; ask the building department if the lot is borderline around 2 m.
Under Article 717 of the Civil Code, liability for a defective structure on land falls first on the person in possession and, if they took necessary care, on the owner — without the owner needing to be at fault. Buying a lot with an unsafe wall means buying that exposure.
No. Each prefecture or designated city sets its own rule under the Building Standards Act, so heights (2 m or 3 m), distances (1.5 or 2 times) and exceptions differ. Always read the ordinance for the municipality where the house is.
Partly. Elevation and slope maps, landslide zones, the fill land map and street view will show whether a wall or cliff is present. The wall's condition and certificates need photos from the agent and a question to the municipality; an engineer's opinion requires a site visit by someone.
See how to check a house in Japan before buying, liquefaction risk and pre-1981 seismic standards, or browse all guides.
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Get a $49 risk report Free 12-point checklistThis guide is general information based on public sources cited above. It is not legal, tax, or real-estate brokerage advice. Confirm every item for a specific property with the agent, the municipality, and licensed professionals.