I read about this recently.
It didn't mention price per square metre but it did say that new houses in Japan drop 10-15% in value the instant they are sold.
Valuers in Japan work on a scale that says the houses (not the and) are worth zero after 25 years.
So each year you own a house, the value of the house (not the land) drops.
It is because modern houses in Japan are not built to last (except for the steel framed ones).
With that in mind, maybe buying an old house and renovating is a good plan..