Most people who stroll the domesticated side streets of Oakland’s Rockridge neighborhood are checking out the robust traditional architecture or the ever-shifting array of trees and shrubs.

Dan Parolek looks for side doors with their own address. Or multiple electricity meters or extra mail slots — any sign that this seemingly genteel terrain is more populous than you might expect.

“There probably are close to 100 units of housing around us, but who wouldn’t want to live on this block?” Parolek, whose Opticos Design firm is in downtown Berkeley, mused on a particularly scenic stretch of Benvenue Avenue. “If we can find a way to deliver blocks like this again, we’ll go a long way toward solving the housing crisis.”

An architect who also has a degree in urban design, Parolek is the easygoing champion of what he calls the “missing middle” — apartments and condominiums contained not in residential towers or multistory crates adorned with random bays and colors, but buildings that fit comfortably into walkable single-family neighborhoods. Duplexes, fourplexes or small, stylish townhomes lining a simple, deep courtyard.

“Most pre-1940s neighborhoods, anywhere in the country, have this kind of mix,” Parolek said. “It’s what got built before (restrictive) zoning was put into place.”

Parolek coined “missing middle” in 2011 as a way to describe the types of structures that can enlarge the range of housing options without disrupting a neighborhood’s character. And, as the need for new housing strains prosperous regions across the country, Parolek’s effort to broaden the mix — or turn back the clock — is picking up steam.

In the Bay Area, Opticos has recently finished planning work for cities such as Novato, Hayward, Richmond and San Rafael. The staff of 20 is involved in missing-middle studies for Iowa City and a 40-acre project emphasizing the concept now under construction outside Omaha, Neb.

AARP hired Opticos to prepare educational materials. A zoning study for the National Association of Home Builders should be released this summer. The Urban Land Institute, a think tank for developers, has singled out Parolek’s approach as a fresh way to fit modest, relatively affordable new homes into established communities.

“That missing-middle term has taken on a life of its own,” said Christopher Ptomey, executive director of ULI’s Terwilliger Center for Housing. “It allows you to talk about the deeper housing crisis in a way that’s more accessible.”

It’s also a way to explore how to add density to existing communities without stirring up NIMBY opposition.

“The second you use the ‘wrong’ term in a conversation, like ‘density’ or ‘upzoning,’ it triggers a really negative response in a lot of people,” Parolek said he has found.

How would he describe his vision? “House-scaled buildings that happen to have more units in them.”

In other words, what you find threaded through Rockridge, or much of Berkeley, or those San Francisco districts where statuesque apartment buildings coexist with single-family homes, or Victorians split into small flats decades ago.

That mix, with smaller apartments priced on the low end of the market by virtue of their size or location, allows for a fluid economic diversity. Elders on a fixed income might live near people in their 20s starting post-collegiate life in a basement studio, not far from settled families. All this blended into a landscape that isn’t segmented into rigid residential categories.

The type of neighborhood once built as a matter of course, but now rarely seen.

This is one reason Parolek is happy to walk older streets with an eye for the signals of discreet density — to demonstrate that a concept deemed frightening by opponents of growth in fact can feel like home.

“We can get creative within a two, 2½-story limit,” Parolek said.

Or higher.

At one point in our stroll, Parolek paused on 63rd Street. On one side of the block was a handsome apartment building from before World War II, four stories with a vaguely English air. Across the way: a bleak line of garage doors beneath two levels of featureless stucco and glass.

“This is what freaked everybody out in the 1960s and ’70s,” Parolek said, pointing to the later building. “Then you look across the street, that one’s much bigger, and I’m sure that nobody complained.”

John King is The San Francisco Chronicle’s urban design critic. Email: jking@sfchronicle.com Twitter: @johnkingsfchron