This Philadelphia Inquirer article is behind a paywall, but I can tell you that it speaks to the city's increasing use of modular construction for infill apartment buildings:
Building modularly can save 20% on total construction costs, he said. Projects can be constructed in half the time, and rental revenue comes in sooner. Workers build apartments in pieces in a factory as others lay the foundation. Factory work doesn’t have to pause for inclement weather.
Alterra Property Group has found that modular construction is cost- and time-effective when it builds between 100 and 500 units and between four and six stories. Under that, building on-site is more efficient, Addimando said. Above that, builders can run up against building code restrictions.
Consider this recently completed project, called LVL North:
1.5 acre site
Site acquired in February 2020