The 143-unit modular supportive housing project under construction on Bryant Street in San Francisco has drawn praise for the speed at which it is being built and its low construction cost.

Over a two-week period earlier this month, the boxes that make up the building were trucked over the Bay Bridge from Factory OS on Mare Island in Vallejo late at night and set on the concrete foundation at 833 Bryant St. Once completed it will be the city’s first 100% affordable modular project, an assembly-line-built project that will cost $385,000 per unit. That compares with about $525,000 a unit for a conventional “stick-built” development.

It will also be completed about 40% faster than a typical affordable development, according to the developers, a joint venture between Mercy Housing, Tipping Point and the San Francisco Housing Accelerator Fund.

But the project — dubbed “Tahanan,” a Tagalog word meaning “home” — has become the latest flash point in the fight between the city’s building trade unions and some affordable housing proponents over the extent to which San Francisco will embrace factory-built housing in an effort to save money and speed up production.