The splashiest news from Seattle's newly announced upzone plans is who will and won't see the growth. (TL;DR: Single family homeowners are almost completely off the hook.) But another, smaller piece of the announcement could be good news for affordable housing—if it works.

Along with rezoning in urban villages to allow taller buildings in exchange for affordable housing, city officials propose a new rule they say could encourage developers to build housing big enough for more than one person.

Here's the problem officials from Seattle's Office of Planning and Community Development say they're trying to address: Even as Seattle has grown at such a fast pace, renters in need of more than one bedroom have struggled to find housing. The city has known about this shortage for years. But, as Sarah Anne Lloyd at Curbed Seattle has reported, it hasn't gotten any better:

A whopping 52 percent of multifamily units constructed in Seattle since 2012 have been one-bedroom apartments, according to data provided to Curbed Seattle by real estate data group Costar. 29 percent of new units are studios.

Just 17.5 percent of units built since 2012 are two-bedrooms—and just over 1 percent are the ever-elusive three-bedroom.

To start to chip away at those numbers, the city proposes a new requirement on developers. The proposed rule would require developers constructing in certain areas to build one apartment that is at least 800 square feet and two bedrooms for every four apartment that is 400 square feet or smaller. Those areas, Low rise 1 zones, are basically transition neighborhoods between single family zones and commercial zones. Row houses, townhouses, and three-story apartment buildings are allowed in "Low rise 1" zones.

The rule is not just about producing two-bedroom apartments, OPCD officials said, but also a response to neighborhood griping about micro-apartments, or so-called "apodments."

The small apartments, often with no parking, have faced a flood of neighborhood backlash in recent years. The city council has passed new regulations to try to address that backlash. And developers have since largely stopped building the smallest and cheapest versions of those apartments. (Even if few of the smallest micro-units are actually being built in Seattle these days, the well of neighborhood complaints never runs dry.)

Seattle City Council member Mike O'Brien, who has worked on micro-housing regulations, said he hadn't yet taken a position on this proposed rule. He and his fellow council members will consider the proposed rule over the next year.