How to read flood hazard maps ― what the colours mean, the \"maximum assumed scale\", and how they are handled in the explanation of important matters

Since August 2020, showing where a property sits on the flood hazard map has been mandatory in the explanation of important matters for real estate sales and rentals. The colours of a flood inundation assumed area show the depth of flooding; the \"maximum assumed scale\" means rain of a once-in-a-thousand-years class; and an \"assumed area for house collapse due to flooding\" means a risk of being swept away. This article sets out the difference between inland flooding and storm surge, guidance on evacuation, how it relates to flood insurance premiums, and how to look at inherited land or land before buying.

About this translation ― This page is an English translation of an article originally written in Japanese by KuraLab's operator, a licensed real-estate transaction specialist (宅地建物取引士) and administrative scrivener (行政書士) in Japan. The Japanese original is the authoritative text; where the two differ, the Japanese version prevails. The article explains Japanese law and procedure in general terms as of the date shown. It is not legal, tax or immigration advice for your situation ― please confirm with a licensed professional in Japan. Japanese terms are kept in parentheses so you can search for them. Read the Japanese original

At the explanation of important matters, the licensed real estate transaction specialist (宅建士) spread out a copy of the hazard map. "This property is in a flood inundation assumed area, in the zone with a flood depth of 3 to 5 metres." The buying couple looked at each other, and the husband asked, "With how much rain is that?" The specialist paused for a moment and answered, "That's the maximum assumed scale." The husband asked further, "And what about the planned scale?" The specialist did not have that map. This article is about that second question.

In the article on landslide warning areas I wrote about the dangers of cliffs and valleys. This article is about the dangers of rivers and rain. In August 2020, an amendment to the Real Estate Brokerage Act made it mandatory, in the explanation of important matters for sales and rentals, to show and explain the property's location on the flood hazard map produced by the municipality. An explanation that had been optional became required. If you know how to read the map, you can turn that explanation from something you "listen to" into something you "check."

Flood hazard maps (水害ハザードマップ) are produced by municipalities and come in three types: river flooding (洪水), inland flooding (内水) and storm surge (高潮). River flooding is water spreading when a river's embankment breaks or is overtopped. Inland flooding is the town being flooded, regardless of rivers, by rain exceeding the capacity of the sewers and drainage channels. Storm surge is coastal flooding caused by the sea level rising in a typhoon or similar. Which map is relevant depends on the property: low ground away from rivers, inland flooding; river mouths and the seashore, storm surge; near large rivers, river flooding.

The colours of a flood inundation assumed area (洪水浸水想定区域) show the depth of flooding. The colour coding differs a little between municipalities, but following national guidelines it uses bands such as under 0.5 m, 0.5 to 3 m, 3 to 5 m, 5 to 10 m and 10 to 20 m, with deeper colours for greater depths. 0.5 m is an adult's knee, 3 m the floor of the first floor up, 5 m the ceiling of the first floor up. Read an area marked "3 m or more" as one where evacuating upstairs may not be enough.

Know too that there are two premises: the "maximum assumed scale" (想定最大規模) and the "planned scale" (計画規模). The planned scale is the rain that embankments and other works are designed for (once in several decades to once in a hundred and something years), and the maximum assumed scale is the largest conceivable rain (once-in-a-thousand-years class). The 2015 amendment of the Flood Control Act (水防法) required flooding assumptions at the maximum assumed scale, and many maps show the maximum assumed scale. A maximum-scale map has a wide area coloured in, so it looks as if "everywhere floods," but that is on the premise of "rain of a once-in-a-thousand-years class." Compare it with the planned-scale map and you can see the extent of more frequent flooding. Where a local authority publishes both, look at both.

Apart from the colours, look at two indications. One: the "assumed area for house collapse due to flooding" (家屋倒壊等氾濫想定区域). This is an area where houses may be swept away by the force of the water when an embankment breaks, or where riverbanks may be eroded, shown by hatching or similar. This is not about "depth of flooding" but about "whether the house will be destroyed," and the principle is to evacuate out of the area, not to an upper floor of the building. Two: the "duration of flooding" (浸水継続時間). This is the time until the water recedes; read an area marked as several days or more as one where you may be cut off even if you evacuate upstairs.

How to read a flood hazard map. Depth of colour: depth of flooding (under 0.5 m / 0.5 to 3 m / 3 to 5 m / 5 m or more and so on); 3 m or more may be too much even for the first floor up. Maximum assumed scale: flooding in rain of a once-in-a-thousand-years class; a wide area is coloured, but the frequency is low. Planned scale: the rain targeted by embankment works (once in several decades to a hundred and something years); the extent of more frequent flooding. Assumed area for house collapse due to flooding: risk of the house being destroyed by the force of the water if an embankment breaks; evacuate out of the area rather than upstairs. Duration of flooding: time until the water recedes; several days or more means a risk of being cut off.

Checking is easy. Search for the address on the municipality's flood hazard map website, or on the Ministry of Land, Infrastructure, Transport and Tourism's "Overlay Hazard Map" (重ねるハザードマップ). The Overlay Hazard Map lets you see river flooding, inland flooding, storm surge and landslides on a single map. In a sale, the licensed specialist shows "it's here" in the explanation of important matters, but if you look yourself beforehand, you can ask at the explanation, "Is that the maximum assumed scale or the planned scale?" and "Is it in an assumed area for house collapse due to flooding?" It is one way of not leaving it at "listen and that's it," as I wrote in the article on the explanation of important matters.

On the link with insurance premiums. Since October 2024, the premium for flood cover in fire insurance has been divided into five bands according to each municipality's flood risk (before that it was uniform nationwide). Areas with deep flooding on the hazard map may be in a band with a higher flood premium rate. Before buying a property, check the flood risk band (the premium category) for that area with an insurer. That is one way of converting the colours on the hazard map into "money."

Here are the figures checked on the spot. The property was in a residential area about 600 metres from a class A river. On the city's flood hazard map, at the maximum assumed scale (rain of a once-in-a-thousand-years class, about 600 mm in 48 hours in this river's basin) the flood depth was 3 to 5 metres. At the planned scale (once-in-a-hundred-years class, about 400 mm in 48 hours) the flood depth was 0.5 to 1 metre. Between the two maps, the difference is between water reaching the ceiling of the first floor up and water from an adult's knee to the waist. It was not in an assumed area for house collapse due to flooding, and the duration of flooding at the maximum assumed scale was "12 to 24 hours." It was a place where, if you evacuated upstairs, the water would recede within a day.

The buying couple looked at these two maps and decided to buy. There were three deciding factors. One: if the planned scale gives 0.5 to 1 metre, a design raising the ground floor 70 centimetres above the road (higher foundations) copes with frequent flooding. The builder's estimate for raising the foundations was about 1,200,000 yen. Two: since it is outside the assumed area for house collapse due to flooding, "evacuate upstairs" is enough even at the maximum assumed scale. Three: the flood cover premium. Under the five bands from October 2024, this city was in the second-highest band, and fire insurance with flood cover cost about 15,000 yen a year more than in the lowest-band areas. 450,000 yen over 30 years. They added this to "the price of buying" when thinking it over.

I will also give an example where someone walked away. In another consultation, a person went to see land near the confluence of two rivers. At the maximum assumed scale the flood depth was 5 to 10 metres, it was in an assumed area for house collapse due to flooding (flood flow), and the duration of flooding was "3 days or more." When these three come together, evacuating upstairs is not enough: the premise is fleeing out of the area, three days until the water recedes, and the possibility of the house being swept away. The land was about 30% cheaper than the surrounding market, and the person said, "The reason it was cheap was written on the map," and walked away.

For inherited land, think of it like this. If you will not live there and will sell, the colours on the hazard map are material for the buyer's decision and affect the price too. Add this factor to the decision described in the article on vacant houses. If you will live there, consider renovation (raising the foundations, making the ground floor a car park) and an evacuation plan according to the depth of colour and whether it is in an assumed area for house collapse due to flooding. Land prices already partly reflect the colours on this map. The official land price described in the article on the four land prices is the price of a standard site, and flood risk is an individual adjustment.

The second question, "and what about the planned scale?", was a question about a map the specialist did not have with him that day. The buying husband had looked at both maps on the Overlay Hazard Map beforehand. At the start of the main text I wrote, "turn that explanation from something you listen to into something you check." Having checked, this couple raised the foundations by 70 centimetres and bought the house. The colours on the map can be a reason not to buy, and also a design for how to buy.

Written by: 長澤湊（licensed real-estate transaction specialist, administrative scrivener, assistant surveyor）　Content checked on: 2026-10-24

This article is a general explanation of Japanese rules and procedures, not advice on an individual case. Figures and rules are as of the date shown. For your own situation, consult a lawyer, judicial scrivener, tax accountant or administrative scrivener in Japan.

All translated articles · Read the Japanese original

How to read flood hazard maps ― what the colours mean, the \"maximum assumed scale\", and how they are handled in the explanation of important matters

More from KuraLab in English