The shortage of affordable housing in cities across the nation often is described as a crisis, particularly in California. But within the next 15 years or so, this likely will happen: Baby boomers are expected to put more than 20 million existing homes on the market. That could have a dramatic effect on the supply and raise long-term questions about current proposals aimed at accelerating housing construction.

The notion of having an eventual overstock of homes seems counterintuitive when these days are filled with horror stories about ridiculously high housing prices that make it hard, if not impossible, for middle-income families to live in San Diego and elsewhere.

While a bubble of available existing homes is coming, it won’t be felt evenly across the country, nor will it necessarily provide the type of housing desired.

There has been extensive research about this emerging phenomenon in the last couple of years. Zillow, the real estate database company, last month published an analysis of how the so-called “Silver Tsunami” may affect housing.

“The massive baby boomer generation already has begun aging into retirement and will begin passing away in large numbers in coming decades — releasing a flood of currently owner-occupied homes that could hit the market,” wrote Issi Romem, senior director of housing and urban economics at Zillow.

“That could help end the last few years’ inventory drought, as well as a more fundamental shortage of homes in certain places.”

He summarized a handful of key points in his research:

During the next 20 years, more than a quarter of the nation’s currently owner-occupied homes likely are to go on the market as their current owners pass away or otherwise vacate their homes.

Places expected to be most affected by this sell-off include both retirement hubs (Miami, Orlando, Tampa and Tucson) and regions where young residents have left (Cleveland, Dayton, Knoxville and Pittsburgh). The effect will vary across different areas within cities, as well.

Zillow lists San Diego in the middle of nearly 60 metropolitan areas it surveyed in terms of owner-occupied homes expected to be put on the market by seniors (60 years and older) through 2036. Tampa, Fla., has the most, at well over 30 percent of its owner-occupied housing in this category, and Salt Lake City the least, at just under 20 percent.

San Diego housing analyst and consultant Gary London cautioned that this trend will be drawn out, even more so in places like San Diego than elsewhere.

“It will be a huge and gradual transition,” he said.

Seniors who bought or built here years or decades ago have benefited from tremendous appreciation of their homes. That also may have limited their options. While the notion of downsizing might be attractive, the capital gains hit from selling a current home and the significantly higher property taxes of a potential new one has many people staying put.

The concept of “aging in place” may be desirable in order to remain in familiar surroundings, but increasingly it also is an economically advantageous one. Then there’s the evolving, active lifestyles of older people (“70 is the new ... ”). And who knows what human longevity will be in 20 years?

A study by Fannie Mae’s Economic and Strategic Research Group warned that the “beginning of a mass exodus looms on the horizon” and it may create a housing glut in some markets, resulting in lower selling prices.