All over downtown Boston, new housing units have been coming to market in waves over the last several years, typically, luxury and upscale buildings – sometimes with doormen - all the way up to the $37 million penthouse condo on top of One Franklin Street in Downtown Crossing.

Thursday, Gov. Charlie Baker and Mayor Marty Walsh and a cast of hundreds ceremonially kicked off construction of another housing development downtown – but this time, with a major difference.

"It's the first time that I can talk about a building that's coming out of the ground in downtown Boston that I can say, '100 percent affordable,' and I want to thank you for it," Walsh said at Related Beal's groundbreaking for what it's calling Parcel 1B.

Slated to open in the spring of 2018, it's a 14-story complex at Causeway and Beverly Streets, kitty-corner to the Boston Garden, with 239 residential units.

All of the housing units will be restricted to and priced for people, couples, and families making between 30 percent and 165 percent of the government-defined Area Median Income. In rough terms, based on current Boston Redevelopment Authority standards, come 2018 that will likely mean rents and sales prices set for individuals making at most around $115,000 and couples about $130,000, with units affordable to people with lower incomes all the way down to singles and couples making just $21,000 to $24,000.

"In a great city like Boston, a moving, booming city like Boston, one of the most important things we in the public sector need to do is make sure that regular working people can afford to be part of this city's success," Baker said. "This is about as good an example of a public-private partnership to create affordable housing and workforce housing in the city of Boston as you're ever going to find."

Notably, this was a project that involved no fewer than two dozen different city, state and federal agencies, with multiple layers of subsidies, tax breaks, tax credits, tax-exempt financing, and other elements to make the numbers work. The land under the building is a site over the northbound Big Dig tunnels, and Walsh said, "Having public land brings that cost way down."

Developments nearby that are obligated to pay for or build affordable housing, including Delaware North/Boston Properties' redevelopment of the old Boston Garden site and Related Beal's Lovejoy Wharf/Converse Headquarters project, agreed to fund those housing units at Parcel 1B. Also, profits from construction and operation of a 220-room Courtyard Marriott hotel on the Causeway Street side of the parcel, next to the future homes, will help pay for keeping the apartments and condos affordably priced. CBT Architects designed the project.

Kimberly Sherman Stamler, chief operating officer of Related Beal, said in an interview: "We're so excited about everything that is going up and all the buildings that are going up here, and we are very happy that one of those buildings is our 100-percent-affordable workforce housing project."

Financially and bureaucratically, by contempoary Boston standards, all kinds of stars somewhat miraculously aligned to make an affordable housing development in one of Boston's hottest development zones get off the ground. But Walsh said he doesn’t rule out Parcel 1B becoming a model for similar success elsewhere.

"It can happen again, certainly," Walsh said, ""if you have willing partners to do it."

With videographers Kenn Tompkins and Darrell R. Smith and video editor Daniel J. Ferrigan