Building a mixed city on the city

The scarcity of land, the ecological, climatic and social crisis... and the Climate and Resilience Act's objective of zero net artificialisation of land by 2050, which aims to combat land concretization and the renaturation of spaces... To face up to these challenges, real estate and urban players are today asserting the need to rebuild the city on the city, since 80% of tomorrow's city already exists. Building the city on the city, yes, but a mixed city, as some urban planners, developers and project managers are calling for. But why? How?