Homeowners who opposed the plan said it would destroy the qualities that had first attracted them to their quiet neighborhoods, while the urbanist and racial justice groups who backed local lawmakers’ efforts argued that it would diversify those areas and create more housing options in this expensive community.

Marcia Nordgren, who sued the county over the policy along with eight other Arlington homeowners — most who testified against the zoning change during a week-long trial in July —said her lawsuit was about “following the law and following the right procedures.”

Judge David S. Schell’s ruling, she said,“shows the county can’t pull the wool over the eyes of its own residents and act as if we’re not going to stand up.”

But the ruling is probably not the end of the fight: Arlington is expected to appeal to higher courts, further extending a years-long battle that mirrors one playing out in cities and states around the countryover what suburban neighborhoods should look like.

“Arlington County is disappointed in the judge’s ruling today,” David Barrera, a spokesman for the county board, said in a statement. “The County Board remains committed to ensuring Arlington has housing options that meet our community’s diverse and growing needs.”

Schell ordered the county to stop issuing permits for “missing middle” developments that were allowed under the zoning change, though the exact details of his order and how they might apply to the county were unclear. Barrera said county board members were determining “proper next steps” to adhere to the ruling and would be “exploring potential options moving forward, including appeal.”

The Rev. Ashley Goff, a leader of Virginians Organized for Interfaith Community Engagement, which advocated for the zoning change, said she was disappointed that a judge had voided a tool meant to help young families and seniors looking to downsize, as well as other groups not well served by the housing market.

“This decision represents a textbook example of why we have the housing crisis we do,” said Goff, a pastor at Arlington Presbyterian Church. “It is a shame that a few wealthy and well-connected owners of single-family detached homes can derail a democratic process that was years in the making.”

The Arlington County Board’s unanimous vote last year on “Expanded Housing Options” (EHO) made it easier to build residential buildings with up to four — and, in most cases, six — apartments in areas that had for decades been set aside for one house with a yard on each lot. The plan made Arlington the first locality in the D.C. region — and much of the East Coast — to loosen its zoning rules for more of this missing middle housing, a contested idea that has nonetheless been adopted by at least 20 localities and five states.

Lawmakers framed their effort as one that would reverse zoning rules initially meant to keep out low-income residents and people of color, and open up a tight real estate market just a few miles of downtown D.C.(The median home price in Arlington was $718,000 last month, more than 1.5 times the national figure.)

But homeowners fighting the effort warned that it would overwhelm or even destroy their neighborhoods, clogging up their streets and storm drains and removing tree canopy.

Schell, a retired Fairfax County judge who was appointed to hear the case, said those concerns — in particular, the influx of additional sewage from more units — were serious enough that they needed to be studied ahead of time.

“The board failed to consider the localized impact of EHO developments in the neighborhoods where it would be built,” Schell told a packed courtroom Friday morning. The plaintiffs and their supporters responded to his ruling with cheers.

The lawsuit ispart of a national wave of litigation against efforts to allow more “missing middle” housing. Similar policies in Minneapolis and Montana were put on hold by judges over the past year over complaints about environmental impacts and property rights, respectively, though higher courts have since lifted those injunctions.

Another lawsuit is underway in nearby Alexandria — a case that Schell is also overseeing — while yet another is before a judge in Charlottesville. (Schell was appointed to oversee both Northern Virginia cases after all local circuit court judges recused themselves.) And a judge in California has stopped a state law from taking effect in five cities near Los Angeles that sued the state government.

In the Arlington case, many of the plaintiffs took the stand during a week-long trial held in July to describe how the missing middle effort would affect their property. They arguedthat the county had not properly studied the potential impacts or followed the policies and procedures required by Virginia state code.

The plaintiffs “all share a deep concern for preserving the character and functionality of the place they call home [and] see through this flaky veneer to what the EHO Amendment really is: a wholesale violation of the Board’s enabling authority,” their lawyers wrote in a post-trial brief.

Nordgren, who bought a $1.5 million Cape Cod about two decades ago, described how she initially thought the county’s amendment to its zoning ordinancewouldsensitivelyintroduce affordable housing to her quiet, leafy neighborhood.But after she dug into property records, Nordgren said, she came to learn it meant many of her neighbors could replace their houses with small apartment buildings. None of them have taken advantage of that ability.

Lawyers representing the county cast the plaintiffs — many of whom purchased their single-family houses in the wealthier, northern half of the county decades ago — as an obstacle to dealing with skyrocketing housing costs in the area.

Ryan J. Starks, a lawyer representing Arlington, asked each plaintiff on the witness stand the same question: Could they afford to purchase a home in Arlingtontoday? Missing middle, he said, was meant to ensure the same opportunity those plaintiffs had been able to take would exist for others now.

The case also posed questions over whether the homeowners are able to sue over the potential impact of a policy — a point that Arlington County tried to emphasize in the lawsuit.

The plaintiffs, county lawyers said in a post-trial brief, were trying to “tie-up a local government in litigation in an attempt to have the Court overturn legislative decisions by a locality’s elected officials.” “It is well established that Virginia courts should not make policy judgments — precisely what the Plaintiffs ask the Court to do.”

Schell did not directly address this matter in his ruling Friday.