A four-story gleaming silver ellipse will feature new condominiums over a two-story café space at the corner of 888 Seventh St. in San Francisco. The unique structure pushes the limits of contemporary design for affordable housing.

Developed by Oakland, Calif.- based AF Evans Development, Inc., the community also solves the problem of how to pay for this sort of flashy design by fitting a tremendous number of apartments onto the project’s site, keeping the project’s development cost to a reported $50 million. The development is expected to be finished by the end of the year.

Project architect David Baker + Partners based in San Francisco plans to squeeze 170 affordable apartments and 54 market-rate condos, plus more than 7,000 square feet of retail space, onto a 2.1-acre site in a fast-growing section of the Potrero Hill neighborhood known as Showplace Square. That works out to an average density of more than 100 units of housing per acre, even though most of the development only rises five or six stories high.

The project’s actual density will be even higher than that in the end. That’s because about a quarter of the site will be left as green space traversed by the new Mission Creek Bikeway and Greenbelt, a bike path that will run from the Mission neighborhood all the way to downtown.

Mark Humphreys has good news for the growing number of affordable housing developers building mid-rise apartments. “We figured it out,” said Humphreys, CEO of Humphreys & Partners Architects, L.P., based in Dallas. “Problem solved.” The problem is that developers building more than 30 apartments on an acre typically have to include an expensive concrete parking garage.

Humphreys can make that kind of density work without a garage. His company’s new “e-Urban” mid-rise design will be more than 20 percent cheaper to develop than conventional construction, Humphreys said.

The plan gets rid of the long hotelstyle hallways that form the backbone of most mid-rise apartment buildings, leaving only 65 percent of the total space available to be sold, rented, or offered to residents as an amenity, according to Humphreys. In contrast, the e-Urban design allows 87 percent of the space to be rented or sold by effectively trading long hallways for elevators.

On a two-acre site, the design puts four elevators into a five-story wood-frame building with 96 apartments. Each elevator opens onto an antechamber ringed with four or five apartment doors.

That’s close to the 110 units and 170 parking spaces a conventional Dallas apartment design with a concrete parking garage includes. And the hard cost to build an e-Urban complex is just $75 per square foot, compared with $105 for the conventional design, according to Humphreys.

At press time, just a few weeks after the plans were released, the firm was working with six developers to build e-Urban projects in markets ranging from Dallas to Phoenix.