Though, there is possibility of large earth quake in Tokyo, there are a lot of densed old houses in Tokyo.
Owner wants to rebuild new house on the lot.
And government wants the owner to rebuild the house.
But, when site under the house does not connect with public road with more than 4 meters width, owner can not rebuild the house in principle.
Because, when there is a fire in such houses, fire engines can not go to the site and can not prevent from speading a fire.
So, Japanese government wants owner to demolish such kind of houses.
Japanese government thinks in the case a house gets old and it is no longer available, owner will demolish the house.
But, actually even if a house gets old, many owners do not intend to demolish their houses.
And now some city hall has partly changed their management.
This diagram below is explanation of this change.
( Quoted from Shinjuku City hall, Tokyo. http://www.city.shinjuku.lg.jp/whatsnew/pub/2015/0325-01.html )