They have strict building regulations but lax zoning regulations. I.e. you have to make sure that whatever you build conforms to an exacting set of regulations to ensure its physical resiliency in the face of disaster. But you are not so restricted in terms of use and form. So, you can tear down a house and put up a three-storey set of apartments. You can have the ground floor be a small business. You can build an extra housing unit in place of a yard, you can subdivide, you can add an extra floor, a mansard, and so on and so on. All of this is by-right -- you don't have to ask the council if you can do it, you just do it.
Contrast that with many Western countries: USA, Canada, New Zealand, the UK to an extent (although the situation is somewhat different here). Generally, landowners are not allowed to do these things by right. There is a strict set of rules governing precisely what may and may not be built in each zone, with emphasis on "may not". Whole areas where you are only allowed to build detached single family houses, and even something as trivial as a corner store is effectively illegal. To deviate from the zoning rules, you have to apply for a variance from the city government or local authority, which is in most cases onerous and time-consuming for the applicant, and discretionary on the part of a bureaucrat, which inhibits housebuilding, small business formation and is a vehicle for corruption. It retards the natural forces that result in improvements to land, and encourages zero-sum rent-seeking behaviour.
So, in Japan, cities and neighbourhoods can respond rapidly to changing housing demand. But in Anglosphere countries, places stay legally trapped in the development form established decades ago, unable to build more or build different as the world changes around them. Hence the housing affordability crises: more and more people try to rent in a relatively fixed housing stock, causing prices to skyrocket. Incumbent landlords benefit in a zero-sum fashion from price speculation, while everyone else suffers high rents and reduced labour productivity.
There is a pervasive misconception that abolishing zoning rules means getting rid of building codes. They have almost nothing in common. I recommend reading the book Arbitrary Lines by Nolan Gray on the topic.
Oh I forgot to mention, regarding the situation in the UK, we don't really have "zoning" in the North American sense. But there are analogous problems in the planning system. There's a brief overview of the situation in this Policy Exchange paper:
https://nitter.net/AntBreach/status/1121362679367598080
Contrast that with many Western countries: USA, Canada, New Zealand, the UK to an extent (although the situation is somewhat different here). Generally, landowners are not allowed to do these things by right. There is a strict set of rules governing precisely what may and may not be built in each zone, with emphasis on "may not". Whole areas where you are only allowed to build detached single family houses, and even something as trivial as a corner store is effectively illegal. To deviate from the zoning rules, you have to apply for a variance from the city government or local authority, which is in most cases onerous and time-consuming for the applicant, and discretionary on the part of a bureaucrat, which inhibits housebuilding, small business formation and is a vehicle for corruption. It retards the natural forces that result in improvements to land, and encourages zero-sum rent-seeking behaviour.
So, in Japan, cities and neighbourhoods can respond rapidly to changing housing demand. But in Anglosphere countries, places stay legally trapped in the development form established decades ago, unable to build more or build different as the world changes around them. Hence the housing affordability crises: more and more people try to rent in a relatively fixed housing stock, causing prices to skyrocket. Incumbent landlords benefit in a zero-sum fashion from price speculation, while everyone else suffers high rents and reduced labour productivity.
There is a pervasive misconception that abolishing zoning rules means getting rid of building codes. They have almost nothing in common. I recommend reading the book Arbitrary Lines by Nolan Gray on the topic.