Committed prowlers of Chicago real estate may have noticed the abundance of new listings at Ludwig Mies Van Der Rohe’s iconic 860/880 Lake Shore Drive—four in February alone—and wondered about precipitating events. I’m here to tell you it’s nothing ominous, just a couple of coinciding circumstances: sales took off late in 2014 and have not abated, giving great incentive for those biding their time to act; and, a lot of residents are reaching pivotal moments in their lives, whether growing or shrinking a family, prompting an exodus.

“Last year was record-breaking [for sales at 860/860 Lake Shore Drive],” says 20-year resident and real estate agent Andrea Tucker. There have been 11 sales in the two co-op buildings since September (six in November-December alone) after just 10 in all of 2013. That’s a brisk pace for this legendary modernist duo, with roughly 240 units combined, and for co-ops generally, an ownership model known for slowing turnover. Tucker brokered a couple of recent sales, was a buyer’s agent for another sale that closed last week, and carries three of the nine current listings, two tidy one-beds in the 880 building, priced at $165,000 and $325,000 reflecting floor and finish, and one $549,000 two-bed in the 860 building. There was a bidding war for one of her closed units and another sold in a day. “Everyone I know who is selling is making a life change,” she says. “A couple of owners held units as in-towns for many years and are now shedding them.”

Despite the co-op obstacle, people want to live in these 64-year-old buildings. They are pioneering works of modern architecture setting the standard for residential high-rises for generations to come, easy to mimic but hard to equal. Once people find their way into the buildings, flexible floor plans and an absence of interior load-bearing walls makes transforming and combining units very doable thus enhancing the temptation to stay put.

860 was conceived with a single three-bedroom layout, four to a floor. The figure below illustrates how, working within this floor plan, owners have morphed many of these tight three-beds into spacious one- and two-beds. The building is now a total mix of unit types.

880, on the other hand, originated as a collection of one-bed units, eight to a floor. Even with an 800-square-foot space, owners have managed many permutations. Where combinations have happened is where you’re most likely to find three-bed units today (and some four-beds)—commonly twice the size of Mies’ standard layouts. Full-floor units exist as well.

Everything doesn’t always get larger in accordance with consumer needs; unit combos are occasionally reversed. Debra Kaden of @properties has a sixth-floor listing in 860 that is half of a former 3,200-square-foot, full-floor unit. On the market at $439,000, it has an updated galley kitchen and clean finishes. Last February Kaden helped sell a full-floor spread for $764,000, and assisted in the purchase of another unit. “The previous owner moved out because they had a child and wanted to expand their family even more,” she says. She believes a demographic fluke is behind so many sales and new listings in one season, that people are simply moving forward with their lives.

These buildings are also immensely attractive at the moment thanks in no small part to the exterior renovation that wrapped up in 2010, repainting and weatherizing the structural steel and rebuilding the elegant lobby and travertine plaza. Despite this roughly $10 million project, Tucker assures me the building’s finances are in great shape.

The quick flip from sales to listing activity at 860/880 in mid-winter points to one more straightforward explanation—no one wants to spend their summer going through a closing.