It took California lawmakers seven years, endless horsetrading and a flurry of last-minute rewrites to pass major legislation increasing homebuilding near mass transit last year.

Now comes the hard part: making the law work.

With time dwindling before the landmark law, SB 79, takes effect in July, widespread confusion over what its complex language actually requires is scrambling the path to implementation. On top of that, hardened opposition to the law, particularly in Southern California, is frustrating legislative efforts to quickly clear up the ambiguity.

The potential for a messy roll out is great enough that its author, state Sen. Scott Wiener, has not ruled out the need for a delay.

“We’re happy to have conversations with people who have good-faith feedback, including about that issue,” Wiener said of postponing when SB 79 takes effect.

Wiener, a Democrat from San Francisco, first introduced legislation to boost building near transit hubs in 2018. The effort attracted attention around the U.S. and elsewhere for challenging California’s sprawling growth as the senator and his allies argued denser construction in cities and suburbs was necessary for environmental and affordability reasons.

Multiple iterations of the bill failed amid opposition from homeowner groups concerned about neighborhood character, anti-gentrification advocates worried about displacement and the state’s longstanding deference to local governments on zoning issues.

Then last year, amid a national conversation over California’s stalled growth through the Abundance movement, SB 79 broke through. It allows mid-rise construction around mass transit. The tallest buildings — nine stories — will be permitted on land closest to heavy rail stations, such as two subway routes in Los Angeles and BART in San Francisco. Smaller structures — between five and eight stories — will be allowed further away from heavy rail or nearby less intense transit, such as stops on light rail and rapid bus lines.

To gain legislative support for SB 79, Wiener agreed to limit the law to the Bay Area, greater Los Angeles, Sacramento and San Diego. Additionally, he included numerous carveouts and restrictions that can block denser housing in certain areas, such as neighborhoods at risk of wildfire or sea-level rise, or on parcels that already have apartment buildings on them. Cities also are allowed to develop alternative plans that add housing elsewhere in exchange for preserving lower-density zones near transit. Many of these changes to the language were written in the final weeks of the legislative session in a dash to secure votes.

Figuring out the nuts and bolts of the law was left to regional governments, known as metropolitan planning organizations, which were tasked with drawing up official maps to determine which transit hubs were included and the boundaries for where denser housing could be built. The law also gives the state Department of Housing and Community Development authority over compliance with the law.

The implementation process has not been easy. Representatives of the planning organizations have met multiple times with state housing officials in an effort to reach consensus on basic aspects of the legislation.

For instance, the law calculates which parcels are eligible based on a specific distance from a transit stop’s “pedestrian access point.” But what does that mean for major transportation centers, such as LA’s Union Station, that have several? And how does this rule apply to stops on bus rapid transit lines which often consist of little more than a sign pole?

“This is probably the most challenging housing bill to implement because of all the unknowns,” said Jason Rhine, senior director of legislative affairs at the League of California Cities. “There are a lot of cooks in the kitchen.”

Representatives of the San Diego Association of Governments and the Southern California Association of Governments, which covers Los Angeles and Orange counties, said their agencies need more direction from the state before they can release their maps.

David Zisser, assistant deputy director of housing policy for the state, argued that the housing department is fulfilling its responsibilities.

“We trust that our regional partners are doing the same,” Zisser said in a statement. “We have been consistently communicating with them and providing technical support to help implement the law effectively so that we can build more housing faster.”

Wiener said the back-and-forth between the state and the regional entities is “not an acceptable state of affairs.”

“The maps need to be done as quickly as possible,” he said.

Wiener contended that some are exaggerating the law’s fogginess to oppose SB 79 altogether. Los Angeles has been the epicenter of the SB 79 resistance, with the City Council voting to oppose it, Mayor Karen Bass attempting to convince Gov. Gavin Newsom to veto it and City Attorney Hydee Feldstein Soto organizing other communities around a legal challenge. Last month, Metro, LA’s transportation agency, called for excluding the county entirely from SB 79.

“When people come forward and make proposals to exempt the largest county in the United States out of the law, that’s not going to be well received,” Wiener said.

Metro officials say their concern is not only about the confusion around SB 79’s wording, but also the effect the law is having on a $120 billion expansion the agency is currently carrying out.

Recently, representatives from the small, southeastern LA city of Paramount expressed reservations about supporting stations along a planned light-rail line over concerns the city would be forced to allow housing developments under SB 79.

The law “is causing the cities to begin to oppose our projects,” said Madeleine Moore, Metro’s deputy executive officer for government relations. “We don’t want to be in a position where we cannot deliver this high-quality transit.”

Wiener has no sympathy for such complaints, saying that billions of taxpayer dollars in transit investment must come alongside denser housing to encourage ridership.

He argued that ambiguity surrounding SB 79 is no different than what’s happened after the passage of other major legislation. Lawmakers, Wiener noted, will approve legislation to address imprecise language or unintended consequences.

That cleanup effort has faced its own haphazard process. Wiener said he’s shelving SB 677, his first attempt at clarifying definitions in SB 79, in favor of a new bill, SB 908. That bill will include a comprehensive list of changes that he and allies are continuing to draft, he said.

Ideally, Wiener said, SB 908 would be passed and signed with an urgency clause so it is in place when SB 79 takes effect on July 1. But doing so would require two-thirds support in both houses of the Legislature — a threshold that would be hard to reach. Some Democrats in the Senate didn’t vote for SB 79 and are continuing their opposition to Wiener’s cleanup efforts, citing concerns about too much density.

“Given how controversial SB 79 was, I’m not going to take a risk on losing the bill by putting an urgency (clause) in, unless I’m really confident it’s going to actually get a two-thirds vote,” Wiener said.

The result could be another confusing scenario where SB 79 takes effect in July but the law clarifying what much of it means won’t be active until January 2027.

Wiener said he’s aware of these timing concerns, which is why he’s not ruling out delaying the law’s implementation. But he said any forthcoming changes won’t be an excuse to undermine SB 79.

“I know that there are people who would like to jump in and blow the law up,” Wiener said. “I’m not going to let that happen.”