The King County Council has given final approval to a major update of its massive Comprehensive Plan, adding new, market-based incentives it hopes will create affordable housing in unincorporated areas including Vashon.

The council adopted the updated plan Dec. 10, concluding an effort that began two years ago.

The update extends the county’s “Inclusionary Housing” program to what the county designated decades ago as the “Vashon Rural Town,” which straddles Vashon Highway from roughly Cove Road south to Cemetery Road. The rest of the island would not be affected.

Under the program, housing developers will be allowed to build up to twice as many units as zoning ordinarily allows, or 200 percent of the “base” density, if at least some of the units are affordable to renters or buyers with incomes below the Seattle area’s median.

But the Comprehensive Plan update also increases that base density on many properties, so the new maximum densities on those parcels are more than double what was previously allowed.

Nowhere in Town, however, does the new maximum density exceed 24 units per acre. The old maximum, which applied to relatively few parcels, was 18 units per acre.

Vashon Town was included in the Inclusionary Housing program at the instigation of Councilmember Teresa Mosqueda, whose district includes the island. While she made clear her intention to add Vashon to the program months ago, details of her proposal were not made public until the day before the council’s final vote.

Mosqueda said she wanted more time to engage with islanders and get feedback before finalizing her proposal. The council adopted it Dec. 10 as an amendment to the update.

“It incorporates many ideas directly from islanders,” she said.

For instance, while Inclusionary Housing developers elsewhere in the unincorporated county will be allowed to build units for those with lower incomes on a different site than higher-priced units, developers in Vashon Town will not.

Density “bonuses” to incentivize affordable housing development aren’t new to Vashon Town; the County Council allowed them for some properties in 2017.

But those bonuses applied only to projects consisting entirely of affordable units. It didn’t work; no affordable housing utilizing that program was built.

Inclusionary Housing, in contrast, extends the promise of more density to mixed-income developments, with both lower-priced and “market-rate” units.

That alarmed some islanders, who said it would spawn luxury condos, accelerate gentrification and threaten Vashon’s rural character. Most of the written comments the council received from Vashon opposed increasing density; some even raised the specter of Vashon becoming another Seattle, although maximum densities in the city are much higher.

But at the council’s final public hearing on the update just before the final vote, the six islanders who spoke all supported the changes.

David Vogel, a former president of the Vashon Maury Community Council, called them “moderate, and a good first step.”

Coco Carson Rohe, who serves on the board of Vashon Youth & Family Services and the Interfaith Council to Prevent Homelessness, said she lives with her sister, her sister’s two children and a renter because island housing is so expensive. “It’s nice for us to at least have a roof over our head,” she said.

“I am very glad for this increased density option for Vashon. I truly wish it was more dense,” Carson Rohe added. “I think that it is a no-brainer to have density in the Town of Vashon. It would make our town more vibrant.”

Joshua Priest, who said he owns a small business and has worked for island restaurants, told the council that, while working full-time, he’s lived in a Winnebago, a cabin, a yurt, a trailer, his tent and his car during his eight years on Vashon. “This is the same story for many of my friends on the island,“ he said. “It’s really hard to watch people getting constantly moved due to just being priced out …”

The Comprehensive Plan update includes other changes aimed at encouraging more housing in Vashon Town. It drops the number of required off-street parking spaces for apartments and other multi-family developments to one per unit; previous regulations required up to two. It increases the height limit in the Town’s core from two stories to three (taller height limits elsewhere in Vashon Town remain unchanged).

There’s no guarantee, however, that the changes in the update actually will attract private, market-rate developers to the Town. Mosqueda’s chief of staff acknowledged last month that Inclusionary Housing usually requires densities of 30 to 60 units per acre to be feasible.

And zoning isn’t the only barrier developers face. Other obstacles include Vashon’s isolation, limited water availability and permitting challenges.

Inclusionary Housing density bonuses would be awarded to developers using a complicated matrix. They could get more — earn the right to build more market-rate units — if the affordable units in their projects target those with the lowest incomes, for instance, or are large enough to accommodate families.

The program’s maximum density of 200 percent of base density in Vashon Town is lower than what’s allowed in the rest of the county. And it’s available only for projects built by government agencies or non-profit developers such as Vashon Household. For private developers, the maximum is 175 percent.

The Comprehensive Plan update also changes the base residential density for Vashon Town properties zoned CB, or Community Business, from eight units per acre — which also was the maximum — to 12. With Inclusionary Housing’s density bonuses, the new maximum is 21 for private developers, 24 for non-profits.

Any housing in the CB zone must be in mixed-use projects, which usually means retail on the ground floor.

The update also rezones all 73 Town properties previously zoned R1, or one unit per acre, to R4, or four units per acre. For most of those parcels, one unit had been both the base and maximum density. With Inclusionary Housing, the maximum now is seven per acre for private developers, eight for non-profits.

Residential development continues to be prohibited on Vashon Town properties zoned Industrial.

Other changes in the comp plan

In addition to new regulations that aim to encourage more affordable housing on Vashon, the wide-ranging Comprehensive Plan update the King County Council adopted Dec. 10 makes other changes that affect the island.

Grange Hall: The update includes a provision that allows a neighborhood grocery to locate in the old Grange Hall next to the North End ferry park-and-ride lot, a use the property’s Rural zoning previously had barred. Many North End residents expressed support for owner Jennifer Potter’s plan for a store. The hall’s immediate neighbors objected, calling it “spot zoning.”

“Heritage Trail” signs: Groups in Ellisport and Burton want to install, with landowner approval, signs along walking routes they have mapped that interpret community history. The Comprehensive Plan update exempts such signs on Vashon from the county sign code, which would have prohibited them. In Ellisport, the plan is for 14 two-by-three-foot signs along a one-mile route. Opponents of the exemption called the signs intrusive and unnecessary.

Hospitals in residential zones: The update allows psychiatric and other specialty hospitals on properties countywide zoned high-density residential, R24 or R48. That includes the site of the Seattle Indian Health Board’s proposed Thunderbird substance-abuse treatment center on Vashon. The impact of the change on that project, if any, is unclear: the Indian Health Board already has applied to the county to approve the project as a “Community Residential Facility,” a use allowed before the update.

Regenerative Development Demonstration Project: That’s what the update designates 14 vacant acres behind IGA, where island developer Morgan Brown recently proposed townhouses, retail and apartments — some income-restricted — that he says would comply with the Living Building Challenge, the most rigorous green-building standard on the planet.

The designation allows county officials to modify or waive development regulations and expedite permits. Critics of the Demonstration Project have said it will draw more people to Vashon and threaten the island’s rural character.

Water Planning: The update requires county officials to update, by 2026, rules governing the comprehensive plans water systems on Vashon must submit to the county for approval. Those new rules, it says, should require, among other things, an analysis of the impacts of climate change and ways to prioritize available water shares for affordable housing.

The requirement, added to the update just before the council’s final vote Dec. 10, replaces a previous proposal that would have required the county to conduct a comprehensive analysis by 2033 of water systems on Vashon. That analysis would have addressed the same issues that the update now will eventually require island water systems to study.

Eric Pryne is a retired Seattle Times journalist.