As it approaches its 100th anniversary, the adoption agency Spence-Chapin Services to Families and Children is relocating to a new building in Yorkville from its tony property in Carnegie Hill.

The agency expects to receive approval next week for a city-issued $9.8 million bond to purchase the space, part of a development by Madison Equities that includes a Courtyard by Marriott hotel.

Meanwhile, it put its home since 1956 on the market three weeks ago for $23 million. The agency plans to direct the proceeds to its $30 million endowment, so it can provide services on a sliding scale.

“Our needs have changed,” the executive director of the agency, Katharine Legg, said. “When we moved here in the ’50s, adoption was a very secret activity for both the adoptive parents and the birth parents. As a result, the agency had two separate entrances, waiting rooms, and elevators. We didn’t have our name on the door.”

Today, in domestic adoptions, adoptive parents meet the birth parents, and adoptees are generally told of their heritage. So the agency, which has not yet selected an architect, wants large, flexible, spaces that are child-friendly. In one workshop, children make “life books” explaining their origin. There are also parties throughout the year for adoptive families with children from Eastern Europe, China, and Korea.

The agency, which has a $6.5 million annual budget, completes about 200 international and domestic adoptions a year, including 25 adoptions of special needs children. It relies mostly on private donations and is preparing to launch a major fund-raising campaign in conjunction with its centennial.

“This is going to be much more efficient for us. We want to maximize our resources so we can enable as many people who can to adopt,” Ms. Legg said.

The agency’s property at 4-8 E. 94th St. will most likely be converted to residential use, a senior vice president at Prudential Douglas Elliman, George W. van der Ploeg, said. Elliman is handling the sale of the property with Massey Knakal.

The 24,463-square-foot property consists of three lots: a 40-foot-wide, fivestory mansion, built in 1936 on two of the lots, with fireplaces and some original molding, and a building the agency added in 1965, which is seven stories high, 20 feet wide, and features a roof patio. More than 30 parties have seen the space, and the agency has received three offers of more than $20 million, Mr. van der Ploeg said.

Interest is coming from developers and couples with lots of children. Some individuals are considering buying the space, living in the mansion, and selling off the newer building. Developers are interested in converting the entire structure into multifamily residences. The address is the first building off the corner, which is occupied by the co-op building, 1125 Fifth Ave. Whoever buys will be in nice company: Residents of the block include Bruce Kovner, who lives in the Willard Straight House on the northeast corner of 94th Street and Fifth Avenue, and Nicolas and Jeanne Rohatyn.

“You can see the East River, Central Park, and you have north and south views,” Mr. van der Ploeg said.

The agency plans to move into the new building at 410 E. 92nd St. early next year.