The Montgomery County Planning Board approved a $100 million retail and office mall in downtown Silver Spring yesterday under an unusual arrangement that requires the developer to help plan, build and operate a day care center to serve employees in the redeveloping downtown and nearby residents.

The 575,000-square-foot project planned for the former Hecht Co. department store and adjoining buildings is the first major retail proposal in more than 40 years for Silver Spring. County officials hope it will reverse the exodus of stores and shoppers from the once-important commercial center.

The innovative child care plan would require the developers of the new City Place project -- and possibly future developers in downtown Silver Spring -- to subsidize a 140-child facility to be built at an estimated $1 million cost on county land near downtown.

In exchange for the child care help and other public improvements, the developer will be allowed to build the project beyond the limits of standard zoning. Montgomery and other counties routinely swap the right to add density to a project in exchange for such amenities as landscaping or public artwork.

But yesterday's action marks the first time in this region that child care has been traded for extra development rights, according to child care and planning experts, who said that a similar program has been used in San Francisco.

"I think it is unprecedented in this area," said Therese Keegan, child care administrator for the Metropolitan Washington Council of Governments. "There is really a need for additional child care, especially near work sites, and we believe this benefits employees and employers as well . . . . Any incentive for developers to take action in this field is most welcome."

The arrangement involves plans by the McLean partnership of Petrie Dierman and Partners Inc. and Georgelas and Sons Inc. to build a 10-story office tower on top of a four-level retail mall at Colesville Road and Fenton Street in the heart of Silver Spring. Walter H. Petrie, the company president, said the firm hopes to break ground in September and open the retail part of the project in March 1990.

Petrie must submit a plan for the day care center to get a building permit for the office part of the project, and the center must be open before the offices can be occupied.

The idea for the day care center originated with the developers who offered to build one in the midst of the retail and office complex, according to Petrie and members of the county planning staff.

County planners, believing the highly urban location would be unsuitable for children and favoring a park for that site, countered with the plan adopted unanimously yesterday.

That plan proposes the center be built on a county-owned lot at Spring Street and Second Avenue, at the edge of the Silver Spring central business district and close to Woodside Park.

The Petrie group would be required to come up with a plan to build and operate a nonprofit center and then would be required to pay a pro rata share, based on the size of its development, of the total cost of the program. Doug Alexander, chief of the Planning Board's urban design division, estimated that Petrie's share would be about 10 percent of the total cost, or in excess of $100,000.

Alexander said it is the intention to get other developers who are interested in coming into Silver Spring to pick up the remaining costs, also on a pro rata basis depending on the size of the project. He said it also may be possible for the center to be run in conjunction with county programs or subsidized by employers.

"The details are still loose and flexible," said Planning Board member Nancy Floreen, describing the plan as "a precedent for creativity." However, she said it is clear that there is interest in involving other developers in a collective effort.

The participation of a number of developers is reminiscent of the now established practice of "road clubs," in which a number of developers contribute to traffic and road improvements to have their projects approved. The similarity prompted Planning Board Chairman Norman L. Christeller to call the day care program "kid clubs."

Key to the plan are the corporate subsidies for child care because they would reduce the tuition costs for parents, said Cheri L. Sheridan, whose firm, PAL Corporate Child Care Inc., is planning the new center for Petrie. "There is no shortage of child care, but there is a shortage of affordable child care," she said.

The goal for tuition at the new center is projected at $55 a week for after-school care and $110 for infants, rates which are roughly 30 percent to 40 percent below prevailing costs in the area, she said.

Corporations and governments have been slowly devoting money and attention to the child care issue, Sheridan said, adding that, "there's a lot of talk but no action." She said the developers in this case spent considerable time "selling" the county on the concept that child care should be considered an amenity similar to landscaping or artwork that the county requires on development projects.

Alexander said county officials didn't need to be persuaded of the need for day care but wanted to ensure that it was offered in the right form with the best chance for success. The county also was interested that other improvements be provided, such as a $2 million park that the developers will build.

Despite yesterday's key approval, Petrie and his partners still must get financing and tenants; the Planning Board must act on final plans, and the County Council must enact procedural zoning changes.

County officials, who have spent millions of dollars in an effort to renew the aging downtown, hope to accomplish that goal with Petrie's project and a proposal by developer Lloyd Moore for a $250 million regional mall.