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How can you add space to a modest-sized house that's already had two additions?

That was the question Marty Berens faced with the Shakopee rambler where he and his family have lived for about 15 years.

"As our family grew, the house had to," he said.

Over the years, Berens, an architect and Shakopee native, had designed a kitchen/bath addition, finished the basement and added a sunroom, expanding the square footage on the main floor to 1,335.

With three teenagers in the family, it was time to expand again.

"The kids are getting older and require more private space. I had to find some," said Berens. "I knew going up was the answer."

So he designed a pop-up addition, going through the roof to create a "craft room" where the Berens kids could hang out with their friends.

But there was a problem: There wasn't enough space for a traditional staircase up to the room.

"It would take too much space out of the bedroom below," Berens said.

He came up with a creative solution: a staircase with alternating treads. Each tread is half the width of a conventional one, which reduces the run by half — as if the staircase was compressed from front to back.

Hidden from the street

Berens designed the pop-up with a high-peaked gable so that it's visible only from the back of the house, not the front.

"I didn't want to change the profile of the house," Berens said. "That was critical. I didn't want a dormer popping up. Passing by on the street, you would have no idea that there's an addition."

Getting his plan approved by the city was another hurdle, Berens said, because city officials assumed he was building the addition to be a bedroom. "There's a closet, but it's for storage," he said. The alternating-tread staircase design was new to the city, so there was initial uncertainty about the use of a non-conforming stair to access a bedroom.

Once he won approval, Berens tackled the project, doing all the work himself, except for the electrical.

The 1930s-built house had original ceiling framing that was not adequate to carry the additional weight of the addition, so Berens had to supplement with additional floor joists.

Maintaining the existing height of the floor structure allowed Berens to maximize vertical clearance within the finished space and keep the addition below the existing roofline.

The pop-up also required fully insulated walls, to keep it from getting too hot in summer.

Inside, the 173-square-foot room was finished with laminate wood flooring, budget-friendly floating shelves from Ikea ("this neighborhood doesn't warrant fancy built-ins," he said) and a maple-paneled accent wall that Berens salvaged when his office was remodeled. The sloped ceiling is low, because of the high gable, so Berens clad it in shiplap, figuring it would be more scuff-proof than drywall.

Furnishing the room was another challenge. Berens wanted to include a sleeper sofa, for sleepovers, but a conventional full-size couch was never going to make it up the alternating tread staircase. He found a convertible sofa bed by Klik-Klak that can be folded in half lengthwise. Add a smart TV, and the room also functions as a media room.

The DIY project cost about $8,300, Berens said, compared with the $16,000 to $20,000 it would have cost if he had hired out the work.

And the retreat on the roof is a hit with its intended users.

"The kids are always up there when they have friends over," Berens said. "It's unique — a separate space. It's a little more private and a better feeling than going down to the basement."

The unusual staircase adds to the appeal of the unexpected space, he said. "When kids come over for the first time, they go, 'Wow! Where did this room come from?' "