Here is an update on one event that might be coming down the road: the time when the Baby Boomers decide to sell their homes.

Nelson pointed to the affordability issue as well as the fact that about a quarter of Millennials prefer urban housing, such as condos or townhouses, over the detached suburban homes that were the Boomers’ preferred habitat. Younger buyers, he said, will also be looking for starter homes—smaller than the big Colonials and split-levels that line America’s cul-de-sacs. “We can predict the next housing crash,” he said at the time. “That’ll be in about 2020.”

Four years later, Nelson tells CityLab that that he believes the sell-off will still occur—but later, in the mid- to late 2020s. This has to do with people deciding to defer selling their homes, hoping to get a better price later than settling for a lower price now. “Home values in much of the country are still less than those before the Great Recession of 2007 to 2009,” he says. Prior to the recession, the typical homeowner would sell a house about every six years. “It was like clockwork,” says Nelson. “This drove a lot of planning and development projections.”…

Nelson predicts that the fringe areas surrounding cities will bring the biggest headaches for Boomers looking to unload their houses. Because Millennials will be looking for small homes when they finally start to buy in larger numbers, the sprawling McMansions of the exurbs won’t be desirable to many of them. “The Boomers in the exurbs are going to be in a real pickle,” says Nelson. “Even in a dynamic market like Washington, D.C. or other booming cities, the market for those homes is going to be soft.”…

But many analysts do agree on one thing: More housing will need to be built for Millennials—and it needs to be scaled to their desires, not their parents’s. “Millennials are likely to prioritize different features in their homes, such as greener materials or in-law suites,” says Molinsky. And according to the Harvard Joint Center’s projections, nearly 90 percent of those looking for homes in 2035 will be under 35 or 70 and over—and both groups tend to buy less square footage.

I suppose we’ll see what happens. I tend to think that Millennials might not be as transformative as some have suggested in regards to where they want to live or in what kinds of houses they inhabit. At the same time, there may be fewer Millennials than Baby Boomers in the market for housing – both due to different sizes of the various cohorts as well as the limited purchasing power of some Millennials  which means it could take some time for those Baby Boomer dwellings to find buyers.

It is also interesting to consider what might happen if these homes, particularly those on the metropolitan fringes, can’t be sold. Would they be demolished? Converted? The community retrofitted? Drop to a low enough price that they become very attractive to certain groups? We have plenty of history as a country of people spreading out but not much experience with any serious contraction.