With its first phase 91 percent leased, the developers of Uptown’s MoZaic office-retail complex are busy finalizing plans for a $40 million to $50 million addition fronting the Midtown Greenway bike trail.

The new building from the Ackerberg Group, to be dubbed MoZaic East, could be underway on what is now a surface parking lot behind the Lagoon Theater as early as spring should anchor office tenants sign up, said JoAnna Hicks, senior vice president for Ackerberg.

“We’re encouraged about our chances of success” in luring such tenants, Hicks said of the new effort, which would significantly expand the supply of Class A, large-floor-plan office inventory in Uptown.

When the initial building was proposed in 2011, the idea of constructing new multitenant office space in Uptown was a risky one. Then — as now — the Twin Cities office market was generally stagnant, with little growth evident in occupancy or lease rates. Most new office construction remains of the “build-to-suit” variety for single owner-occupiers.

But Ackerberg CEO Stuart Ackerberg said then that he believed Uptown was an exception because of its attraction as an entertainment destination, its quickly growing stock of new housing for young professionals and its status as a mass transit and bicycling hub.

Also, there was very little competition in terms of top-tier office space nearby — the closest such building is Ackerberg’s own Lake Calhoun Center, more than a mile away.

City leaders backed it because it answered a need to bring more daytime population to the neighborhood.

The first phase, which opened last year, quickly attracted anchor tenants such as digital marketing firm JohnRyan and advertising firm Mono, which responded to the developer’s pitch that MoZaic would be the “office of the future” for adventurous and up-and-coming companies.

The first building has 65,000 square feet of office space, but MoZaic East would greatly up the ante with 170,000 square feet available, perhaps making it a test case for just how far the Twin Cities office market has recovered from the recession.

Hicks said her talks with prospective tenants have given her reasons for optimism.

“I think we’ve proven out the concept — we have a pretty good chance of 100 percent occupancy [at the existing MoZaic] in the not-too-distant future,” Hicks said. “We feel the demand is there.”

As with the first building, MoZaic East will tap an architectural design meant to combine high density with maximized “daylighting” and opportunities for views of the Uptown neighborhood, Lake Calhoun and downtown.

The plans submitted to the city by Perkins + Will Architects are distinctly different from the first phase. They show rectangular tiers of three, six and eight stories stepping back from the Greenway, designed to avoid casting shadows onto the bike trail and MoZaic’s public art plaza, which would be expanded.

The plans also envision 15,000 square feet of first-floor retail space that could house a restaurant or other commercial tenants.

Architect Tony Layne said the goal was to come up with a design that would “be sensitive” to Uptown’s small-building context by breaking it up into what at first glance appears to be three separate elements.

“What we tried to do was really optimize the amount of square footage on that site in an economical way without necessarily putting a big box on the corner,” he said. “We didn’t feel that was the right answer.”

Instead, he said, Perkins + Will worked to develop a floor plan that was functional but also perforated with “daylight atriums” and organized into distinct linear bars.

The design not only results in more leasable space and maximized views of downtown, but also puts a premium on environmental sustainability and energy efficiency, he said.

“We included as much open space on the north and south sides as we could to allow in daylight,” which would reduce the need for electric lighting. Ackerberg has indicated it will be seeking a “gold” or higher Leadership in Energy & Environmental Design certification from the U.S. Green Building Council.

Don Jacobson is a freelance writer in St. Paul and former editor of the Minnesota Real Estate Journal. He has covered Twin Cities commercial real estate for about 10 years.