At the mayor’s request, the Seattle City Council on Monday delayed a scheduled vote on the city’s universal preschool initiative so council members can put a price tag on a different plan that also would be on the November ballot.

That plan is backed by unions representing child-care workers.

The one-week delay means that next Monday will be the council’s last chance to place a $58 million property-tax levy on the November ballot to make high-quality preschool free, or at least more affordable, for all of Seattle’s families.

Mayor Ed Murray said the results of that financial analysis will inform how — and perhaps even if — the city takes its own levy proposal to voters this fall.

“We could choose not to put ours on the ballot at all,” Murray said. “It’s why I asked the council to pause for a week. I want to get pre-K right. I want to understand what the implications are here.”

The city has made the estimated costs of its plan public — about $43 a year for the owner of a Seattle home valued at $400,000.

The other plan is backed by Service Employees International Union (SEIU) Local 925 and the American Federation of Teachers-Washington (AFT). If approved, it would set city policy on how much parents should pay and how much child-care workers should earn, but it doesn’t specify how the city would pay for that policy.

“It’s going to be constrained by budget, and we understand that, but it’s a step in the right direction,” said Heather Weiner, spokeswoman for Yes for Early Success, a coalition funded primarily by the two unions.

In March, the coalition began collecting signatures for its initiative, in a bid to make sure its voices were heard in the city’s yearlong planning process for universal preschool. But the city and the coalition couldn’t come to an agreement, and the coalition is going forward with its plan.

The coalition’s plan, which will appear on the ballot as Initiative Measure No. 107, would establish a city policy that Seattle families should pay no more than 10 percent of their gross income on “early education and child care.”

Council President Tim Burgess said Monday that the union-backed initiative included provisions for parents to sue the city, one of the many reasons he recommended the council take a position opposing it.

“It establishes a right of private action whereby the city could be sued by a family if their costs exceed 10 percent of their gross household income,” Burgess said.

Measure 107 also would establish a $15-an-hour minimum wage for child-care workers, but under different terms than the agreement the city worked out with business and labor groups earlier this month.

That ordinance requires businesses with more than 500 workers to pay them at least $15 an hour by 2017. Businesses with fewer than 500 will be required to pay $15 an hour in 2019.

The coalition’s initiative would require businesses with more than 250 employees to start paying child-care workers the minimum $15 wage starting Jan. 1, with smaller businesses allowed three years to phase it in.

Child-care workers and prekindergarten teachers in King County now make, on average, $13.93 an hour, and assistant teachers make $11.35, according to the coalition.

“I’m very, very concerned that a proposal that changes the $15 minimum wage that we just agreed on will set off a whole other slew of initiatives and referendums in the next several years because it looks like we negotiated in bad faith,” Murray said Monday.

While the coalition is hoping voters would approve both measures and work out areas of conflict afterward, Murray and Burgess have argued that the differences are significant.

Burgess led the council’s yearlong effort to add Seattle to a growing list of cities and states that provide universal high-quality preschool, spurred by evidence that it improves kindergarten readiness and pays off later in life with fewer dropouts and teen pregnancies and less crime.

Murray has called it the most important work he’ll do as mayor.

“It could very well go forward,” Murray said about the city’s levy proposal. “There are different ways that you could put the two ballot measures on at the same time.”

But that will depend on what he learns this week about the potential costs of the union-backed initiative.

John Higgins: 206-464-3145 or jhiggins@seattletimes.com On Twitter @jhigginsST