New York has developed rapidly in only several centuries, becoming a super metropolis today. The city is growing more densified. Less land can be used for public used. However, more and more people flooding into this city call for space to contain more activities. How and where can public spaces be integrated in the future city, considering the highly densified urban environment such as New York? In future, will it be possible to design public space as an important block of city planning, instead of designing on existing city planning or building? Based on research of public space in New York, compared to traditional large opening parks, POPS, which are smaller, more flexible one will be the most important type of public space in future New York. There is an assumption that a totally new city will be built on existing city according to research of building morphology in New York in the future. But how to create POPS in such a city and how to use POPS to Implement such assumption? Can we design POPS on different layers in city and extend city ground to different layers or create new cities by these POPS? In a word, the task asks for a “plug-in typology” on existing buildings. The specific project site is an old building in Midtown called Farley Post Office. The building consists of two parts, the west part was built in 1934, while east part in 1914. The program of the project will be city hall, hotel, office and media centre.