Hazard maps in Japan: what property buyers can and cannot learn

Three official tools do different jobs

The national Overlay Hazard Map, labelled Kasaneru Hazard Map (重ねるハザードマップ), places several datasets on one screen. It is useful for seeing river-flood inundation, surface-water flooding, storm surge, tsunami, sediment-disaster zones, landform and evacuation-site layers across municipal boundaries. It is an efficient comparison screen, not a national certificate of safety.

The Municipal Hazard Map Directory, labelled Wagamachi Hazard Map (わがまちハザードマップ), has a different role. It leads to maps published by municipalities. Those local maps can carry the municipality's own legend, evacuation information, dates and notes about the assumptions behind the display. The Ministry's 2020 disclosure rules point to the latest available municipal water hazard map where one exists, not merely to any national web layer showing similar colours. If the municipality confirms that no applicable map exists, that absence is explained instead.

The Real Estate Information Library combines disaster information with landform, large-scale embankment, liquefaction tendency, disaster history, planning and price layers. That breadth makes it useful for cross-checking which issues at a location merit further investigation. The Library's own Q&A states that its disaster layers are not the municipal water hazard map prepared under the Flood Control Act. Where an applicable municipal map exists, that municipal map is used for the statutory explanation; when none exists, the Ministry's Q&A calls for its absence to be explained.

Water maps cover three different mechanisms

The Flood Control Act does not use one undivided category of flooding. Article 14 addresses kozuishinsui sotei kuiki (洪水浸水想定区域), an area projected to be inundated when a river floods under the stated maximum-scale rainfall assumptions. The designation identifies the area and expected depth, with further matters set by regulation.

Article 14-2 addresses usuishussui (雨水出水), surface-water or pluvial flooding. Its mechanism is rainwater that cannot be drained by the relevant drainage facilities or cannot be discharged from them to a river or other public water body. This is often described as inland flooding, but that English phrase can hide the drainage-system definition in the article.

Article 14-3 addresses storm surge. It concerns areas projected to be inundated if a maximum-scale storm surge meeting the national standard causes coastal flooding. Storm surge is not a synonym for high tide, and it is not a tsunami. The law, map legend and source date need to remain attached to the colour being read.

Sediment and tsunami designations are separate

A sediment-disaster zone is not part of the three water-map categories above. Article 7 of the Act on Sediment Disaster Countermeasures allows a prefectural governor to designate a dosha saigai keikai kuiki (土砂災害警戒区域), a designated sediment-disaster hazard zone—the statutory sediment-disaster warning zone—where the warning and evacuation system particularly needs to be developed. The designation is made for each type of natural phenomenon that could cause the disaster.

Article 9 adds a dosha saigai tokubetsu keikai kuiki (土砂災害特別警戒区域), a designated special sediment-disaster hazard zone. The article describes a subset in which building damage could cause serious harm and where restrictions on development and structural requirements for buildings may apply under the statute. Whether a restriction applies to a particular site requires a separate determination. The two designations therefore should not be collapsed into the labels “yellow” and “red” without checking the legend and legal designation on the source map.

Tsunami is also separate. Article 53 of the Act on Regional Development for Tsunami Disaster Prevention allows a prefectural governor to designate a tsunami-disaster warning zone based on the tsunami inundation projection. This is a standing legal zone, not an active warning. The designation specifies both the zone and a reference water level used for evacuation and for specified controls.

What the statutory explanation identifies

Article 16-4-3 of the Real Estate Brokerage Act Enforcement Regulations puts three related but separate matters into the pre-contract framework. Item (ii) concerns whether the land or building is inside a sediment-disaster warning zone designated under Article 7(1). Item (iii) concerns whether it is inside a tsunami-disaster warning zone designated under Article 53(1). Item (iii-2) concerns the location of the land or building on a drawing supplied by the municipality under the Flood Control Act regulations.

The Ministry's guidance on the water-map item is deliberately narrower than a complete disaster report. Where an applicable municipal map exists, the licensed real estate brokerage presents the latest available map for river flooding, surface-water flooding or storm surge and shows the property's approximate location. If the municipality confirms that no applicable map exists, that absence is explained. The Q&A says that an exact parcel boundary or cadastral position is not required by this item. It also says that the location must still be shown when it lies outside the projected inundation area.

Showing nearby evacuation sites is described as desirable rather than as part of the required location item. The Ministry further recommends showing several nearby sites so that one marked location is not mistaken for the optimal destination. Questions about the detailed contents of a municipal map are directed to the municipality that prepared it.

This article is not that statutory explanation. Where a licensed real estate brokerage handles a transaction and Article 35 applies, the explanation must be provided by that brokerage through a qualified real estate transaction agent, or takken-shi (宅地建物取引士). A general web page cannot establish which current designation or municipal document applies to an actual property.

Outside a coloured area does not mean zero risk

The clearest warning in the Ministry's Q&A is also the easiest to overlook when looking at a map: being outside a projected inundation area does not mean that water risk is absent. The guidance gives examples of map wording that warns that flooding can occur outside the displayed area because rainfall patterns, land use and other conditions can change.

A blank area can also reflect the scope of the dataset, the hazard selected, the scale at which it is displayed, or a gap between the source's update and the web service's update. The Real Estate Information Library warns that content dates differ, that some content may not cover every area, and that overlaid features can be misaligned with one another or with the background map. Those are reasons to preserve the source, legend and confirmation date instead of preserving only a cropped colour.

The reverse is equally important. A coloured area represents the output of a model or a formal designation; it does not say that a particular building will suffer a stated loss. Building height, entrances, electrical and mechanical equipment, drainage, retaining walls, access and evacuation conditions are not captured by the map layer.

The Real Estate Information Library is a cross-check

The Library is valuable precisely because it goes beyond the statutory water-map item. Its landform layers include shaded relief, land-condition classification and large-scale embankment maps. Its disaster layers include projected river-flood inundation, sediment zones, tsunami inundation, storm-surge areas, designated emergency evacuation sites, disaster-risk areas, steep-slope failure areas, landslide-prevention areas, liquefaction tendency and selected disaster-history data.

Each layer answers a narrower question than its label can suggest. The liquefaction layer, for example, is described as a five-level relative tendency based on landform classification. It is not a site investigation. Disaster-history data are selected records assembled through the land-classification survey, not a promise that every event affecting a parcel is present. The large-scale embankment layer identifies mapped embankments; it does not evaluate their current condition.

The Library's terms also state that its content is intended as reference information and is not guaranteed for use in procedures such as an Explanation of Important Matters or a building-confirmation application. Its map is therefore a strong place to find the next source to open, and a weak place to stop the inquiry.

An evacuation site is not automatically the best route

A shitei kinkyū hinan basho (指定緊急避難場所) is a designated emergency evacuation site. It should not be confused with a designated shelter used for a longer stay. The suitability of an emergency site can also depend on the kind of disaster for which it is designated.

A straight line from a property to the nearest symbol says nothing about whether the route crosses a river, low-lying road, slope, underpass, seawall opening or another displayed hazard. This is why the Ministry's water-map Q&A cautions against presenting one nearby site as necessarily optimal. A map can show several candidates and routes; the public authority's current evacuation information supplies the context.

A map does not describe the building

A hazard map describes an external hazard or a legal zone. It does not establish a building's resistance to shaking, water, ground movement or fire. Japan's 1981 seismic-code benchmark, confirmation certificate, completion certificate and seismic-evaluation system are covered separately in the guide to earthquake building standards. None of those documents is replaced by a hazard-map screenshot.

The same projected flood depth can affect different buildings in different ways. A detached house and a condominium can place entrances, parking, electrical switchgear, pumps, lifts and water equipment at different levels. The guide to buying a condominium in Japan explains the management records and reserve-fund arrangements for the building as a whole rather than for the map.

Earthquake insurance also uses its own statutory property categories and loss thresholds. The article on earthquake insurance in Japan explains that system without treating a hazard-map layer as a substitute for an insurance assessment. An insurer's underwriting, terms, exclusions and price cannot be inferred from the national map alone.

A small evidence record prevents later confusion

A map can change between the first viewing of a listing and the contract date. A concise record makes later comparisons possible without turning the record into a safety judgment. The useful fields are factual:

• ■The official source and the public authority that produced the underlying information.
• ■The map or layer name, its displayed date or edition, the legend and the viewing date.
• ■The address or map point used, with a note that a search point is not an exact parcel boundary.
• ■Which layers were available, which were unavailable and which showed a designation or projected area.
• ■The source URL and an unedited screenshot that keeps the legend and scale visible.
• ■Questions left for the municipality, the licensed real estate brokerage or the relevant qualified professional.

The property register serves a different purpose. It records land and building entries and rights; it is not a disaster map. The guide to Japan's property register explains that boundary. A vacant house raises further questions involving road access, agricultural land and municipal management, covered in the guide to buying an akiya.

What this page does not decide

This article does not analyse any specific address or classify any property as inside or outside a zone. It does not say whether any map is complete or current for a particular municipality. It does not identify an evacuation destination, interpret a disclosure document, estimate damage, review a building, assess a retaining wall or soil, calculate a price adjustment, or state whether insurance or financing is available.

No prefectural or municipal ordinance, local map, disaster plan, evacuation instruction or historical record has been surveyed for this article. Real-time warnings from the Japan Meteorological Agency are a different system and are not described here. The page also does not address seller disclosure duties outside the national provisions named above or apply any law to a transaction.

Sources

Statutory text was retrieved from e-Gov API version 2 with asof=2026-09-25. Ministry and portal materials were read on the same date. The English descriptions are reference translations; the Japanese originals govern.

• Real Estate Brokerage Act Enforcement Regulations (宅地建物取引業法施行規則) — Art. 16-4-3

  Read through e-Gov API version 2 with asof=2026-09-25, revision 332M50004000012_20260401_507M60000802002. Items (ii), (iii) and (iii-2) separate sediment-disaster warning-zone status, tsunami-disaster warning-zone status and the property's location on the municipal drawing supplied under the Flood Control Act regulations.

• Flood Control Act (水防法) — Arts. 14, 14-2, 14-3 and 15

  Read through e-Gov API version 2 with asof=2026-09-25, revision 324AC0000000193_20260529_507AC0000000086. The Act separates river-flood, surface-water and storm-surge projected inundation areas and requires municipalities containing those areas to take measures to communicate the matters in their disaster plans.

• Act on Sediment Disaster Countermeasures (土砂災害防止法) — Arts. 7–9

  Read through e-Gov API version 2 with asof=2026-09-25, revision 412AC0000000057_20250601_504AC0000000068. Article 7 defines the warning-zone designation and Article 9 defines the special warning zone, where restrictions on development and structural requirements for buildings may apply under the statute. Whether a restriction applies to a particular site requires a separate determination.

• Act on Regional Development for Tsunami Disaster Prevention (津波防災地域づくり法) — Arts. 53 and 55

  Read through e-Gov API version 2 with asof=2026-09-25, revision 423AC0000000123_20260717_508AC0000000062. Article 53 provides for designation of tsunami-disaster warning zones and Article 55 requires public communication measures for evacuation information inside them.

• MLIT — 2020 addition of water hazard maps to the Explanation of Important Matters

  Read on 25 September 2026. The Ministry identifies river flooding, surface-water flooding and storm surge; use of the latest available municipal map where one exists; explanation when the municipality confirms that no applicable map exists; the property's approximate location; the recommended display of nearby evacuation sites; and the warning not to equate a location outside an inundation area with no water risk.

• MLIT — Water hazard map disclosure Q&A

  Read on 25 September 2026. Q3-1 to Q3-8 distinguish the required location explanation from exact parcel identification, state that location is still shown outside a projected inundation area, and caution against presenting one nearby evacuation site as necessarily optimal. Q4 says that the absence of an applicable municipal map is explained when the municipality confirms that none exists.

• GSI and MLIT — Hazard Map Portal Site

  Read on 25 September 2026. The portal separates the Overlay Hazard Map, which places several national datasets on one map, from the Municipal Hazard Map Directory, which links to maps produced by municipalities.

• MLIT — Real Estate Information Library manual and Q&A

  Read on 25 September 2026. The library combines price, landform, disaster, planning and facility layers. Its Q&A states that its disaster layers are not the municipal water hazard maps prepared under the Flood Control Act and must not replace them in the statutory explanation.

• MLIT — Real Estate Information Library Q&A, Q8

  Read on 25 September 2026. The answer states that the Library's disaster layers are not municipal water hazard maps prepared under the Flood Control Act. Where an applicable municipal map exists, licensed brokerages use that municipal map for the statutory explanation; the Ministry's disclosure Q&A addresses the separate case in which a municipality confirms that no applicable map exists.

Thank you for taking the time to read the source boundaries as well as the map labels. Care at that stage prevents a coloured layer—or a blank one—from being asked to prove more than it can.