Seattle Mayor Ed Murray believes that making preschool free or at least affordable for all families in Seattle would be his most important work in office. But he doesn’t want to rush it.

On Thursday he proposed placing a four-year, $58 million property-tax levy on the November ballot that would focus first on boosting the quality of existing programs, then on ramping up enrollment.

The “demonstration phase” would fund preschool for 2,000 children in 100 classrooms by 2018, according to the plan he submitted to the City Council on Thursday.

“Giving all of our children a fair and equal chance to thrive in school, to live productive and prosperous lives is, again, the most important thing that I will ever do as mayor,” Murray said at a news conference.

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The cost would be about $43 a year for the owner of a Seattle home valued at $400,000, according to the city.

The money would go to pre-K providers that meet the city’s quality requirements, to be used to improve their services and provide free or subsidized slots

to families of all income levels.

The city would cover the entire cost — estimated at almost $11,000 a year per child — for families making up to twice the federal poverty level (four a family of four, that would amount to $47,700 a year). Families making more would pay a share of the cost determined on a sliding scale.

A family of four making almost $167,000 a year, for example, would get a 50 percent break on tuition. Even families making more than $238,500 would get a 10 percent discount.

Supporters of universal preschool argue that children from low-income homes do better in classrooms that include families with higher incomes, and the partial tuition those families contribute would help stretch taxpayer dollars.

Seattle had about 12,000 children ages 3 and 4 in 2012, and 37 percent were not enrolled in any formal child care or preschool program, according to the city.

“For far too many, it is just simply too expensive to be in preschool in the city of Seattle,” Murray said. ”This cost is too high, not just in dollars, but in terms of the future of these children’s lives.”

The Seattle City Council unanimously agreed last fall to pursue the idea, spurred by evidence that high-quality preschool can improve the social and academic prospects of all children, especially those from low-income families, and reduce government spending on crime and social programs in the long run.

Council President Tim Burgess, who has long championed preschool for all, heads a committee that will take up the proposal Friday. A public hearing will be held May 29 at 5:30 p.m.

The council has until June 23 to adopt legislation that would put the tax measure on the November ballot.

Although the city’s long-term goal for the program wouldn’t be reached for at least 15 years, the levy would pay $14.5 million per year for the first four years to make sure a high-quality system is in place before enrollment is expanded. Then the city would have to return to voters for more money.

Among the key features of the proposal:

• In the fall of 2015, the first year of the program, the city would serve 280 children, ages 3 and 4, in 14 classrooms, adding slots and classrooms each year after that.

• Children would be in preschool five days a week, six hours a day during a typical week, 180 days a year.

• The maximum classroom size would be 20, with one adult for every 10 children, typically a lead teacher and an instructional assistant.

• The program would have a zero expulsion policy, meaning behavior problems would have to be resolved in the classroom using methods in keeping with research on children’s social and emotional development.

To be eligible for the city’s funding, preschool providers would have to receive at least a three-star rating from the state’s new, voluntary Early Achievers evaluation system, which was developed by the University of Washington.

Ratings of between three stars and five stars indicate a provider uses a proven curricula and well-trained teachers who frequently engage students in rich, focused discussions that encourage kids to think out loud.

So far, 28 providers in the Seattle area have received a three- to five-star rating, and 28 more have completed the initial requirements and are ready to be rated, according to the state Department of Early Learning.

The city also would require that lead teachers have a bachelor’s degree with appropriate specialization in early-childhood education and get paid on par with public-school teachers.

John Higgins: 206-464-3145 or On Twitter @jhigginsST