At the corner of Southeast Division Street and 23rd Avenue sits 5,000 square feet of grass that could help solve Portland's housing crunch.

The vacant lot, owned by a California contractor, sits three blocks from a New Seasons, along one of Portland's rapidly changing streets—a corridor that has seen hundreds of new rental units.

That change still isn't keeping pace with a wave of new residents: The city's rents keep rising, in part because the rental vacancy rate is still just 3.1 percent and the number of homes for sale is at record lows. This patch of grass could be used to add dozens of rental units, easing the shortfall.

But it won't.

Portland city planners have designated this lot for lower-density housing, meaning the five planned attached homes are all that's allowed. And the city's comprehensive plan, which directs Portland's growth for the next 20 years, will keep it that way, failing to address a checkerboard of zoning regulations that change from one property to the next, even on the same block.

"I don't know if waste is the right word," says Doug Klotz, a Southeast Portland activist who pushes sustainable development. "But it's an under-utilization of valuable inner-city property near transit and shopping."

People are not going to stop moving to Portland, so the big question is how to meet demand.

Few subjects seem as dull as the comprehensive plan, a massive policy document. But few city documents have as much potential to shape the housing supply as advocates for affordability spar with residents who don't want change.

Housing advocates say planners are ignoring a potential solution by not attempting a large-scale change in zoning rules, which determine how large new buildings can be. (This week, the Portland City Club announced it supports rezoning single-family neighborhoods to allow duplexes and townhouses.)

"There's no reason we shouldn't be encouraging more higher-density use in this area," says Alan Kessler, a lawyer who sits on the board of the Richmond Neighborhood Association, located in a red-hot section of Southeast Portland bounded by Hawthorne and Powell boulevards and 28th and 52nd avenues. "These half measures are baking in some bad decisions."

Even as thoroughfares like Division, Hawthorne Boulevard and Belmont Street attract newcomers, development will proceed inconsistently because city planners have done little to address zoning regulations that alternate between residential and commercial. Division, Hawthorne and Belmont all have this patchwork zoning.

The effect of this is obvious at Hawthorne and 15th Avenue. To the west, a 30-unit apartment complex rose from the lot that used to contain the Langano Lounge. To the east, a low-slung Auto Body and Electric store sits next to a surface parking lot. That particular auto site will change under the comprehensive plan, but many others won't.

The alternating zones can create a desired effect, says Heather Flint Chatto, also of the Richmond association. It's what Flint Chatto, an urban planner, calls "pearls on a string," with development spaced between lower-density zones.

City officials are creating some additional commercial spots in Southeast Portland but don't want to incentivize tearing down smaller, existing structures that provide good housing. Eric Engstrom, a chief planner for the city, says the plan was always intended to look at the big picture. "We didn't set a bar for ourselves to completely relook at each zoning line," he says.

Zoning regulations along particular streets, he says, will have to be re-examined later: "We're waiting for proposals to come from local areas."

But that's not likely. So far neighbors are largely OK with the patchwork, more the result of historic accidents than deliberate policies. Some fear an oversupply of storefronts during economic downturns. "You end up with empty windows that sit there during recessions," says Linda Nettekoven, a longtime Southeast Portland activist.

Tenants' advocates say Portland leaders must reconsider zoning rules if they want to address housing affordability in the long term. It's one small component of Portland's housing dilemma, says Justin Buri, executive director of the Community Alliance of Tenants.

"Zoning is one piece of the puzzle," he says, "and it's something we need to look at as we search for long-term solutions."