The YWCA Board of Directors has approved a complicated real estate swap that will allow the YWCA to build its new headquarters at 9th and G streets NW.

Under the deal, the YWCA board will buy the Julius Lansburgh furniture store, a landmark building at 9th and F streets, which will be torn down and paved over for a parking lot.

That parking lot, along with about $500,000 in cash, will be turned over to a partnership controlled by Dominic F. Antonelli Jr., in exchange for a 12,900-square-foot parking lot Antonelli owns at 9th and G streets, adjacent to the Lansburgh site. The Y will construct its new headquarters on the 9th and G site.

The agreement was approved Thursday when the YWCA board ratified two tentative agreements of its real estate committee. The first was an agreement between the committee and Jerome Golub to buy Golub's option on the Lansburgh building and purchase the building for a total cost of $975,000. The board also approved a tentative contract the committee had made to exchange real estate with the Antonelli-controlled 900 G Street Limited partnership.

Golub, informed of the board action by a reporter, confirmed that he had a tentative agreement with the YWCA. Antonelli refused to comment.

In June, the YWCA agreed to sell its 50-years-old building at 17th and K streets NW to the John Akridge Company for $6.8 million. The cost of the new site - excluding construction costs - will be about $1.6 million, including $100,000 in expected demolition costs and a maximum of $48.000 to be paid to ANtonelli for loss of parking lot revenue if the demolition of the building is delayed. YWCA officials estimate that cost of building a new facility will be $3 million.

A demolition delay on the Lansburgh building of at least six months is almost certain since the building is listed on the National Register of Historic Places and thus protected by the District delay-in-demolition laws.

Mildred Savacool, executive director of the YWCA of the National Capital Area, said in a telephone interview that the Y was making the swap rather than renovating the Lansburgh building or constructing a new facility on the 9th and F streets site because that property "is too small for us to have the kind of facility we want."

According to city records, the Lansburgh site contains about 8,000 square feet.

The present parking lot at 9th and G is on the former site of the McGill building, a landmark structure that was torn down in 1973. The razing of the McGill building provided the impetus for the delay-in-demolition laws that were passed later that year. The YWCA building at 17th and K streets is also slated to be razed. A landmark application for that 50-year-old building is currently being prepared by Don't Tear It Down Inc., a local preservation organization.

According to YWCA sources, the contract with Antonelli specifies that if the demolition of the Lansburgh building is delayed beyond January 1980, the problem will be Antonelli's and the YWCA can be charged no more than $48,000 in compensation parking revenue losses. A delay beyond that date is possible, especially, if a stronger landmark law currently before the City Council is passed.

The Julius Lansburgh furniture store, now closed, is a French Renaissance-style building designed by architect Adolph Cluss in the 1860s as a Masonic Temple. The Masons moved out of the building in 1908, leasing it to a variety of tenants, including the operators of a dance hall. The furniture firm moved in in 1921 and altered the ground floor.

YWCA officials emphasize that the 17th and K street building will remain operational until the organization relocates in the new downtown facility. The contract with Akridge gives the YWCA until March 1981 to vacate the 17th and K building.

According to a statement issued by the Y's board of directors, "the increasing obsolescence of the (present) . . . building and inflationary costs have contributed to the YWCA's decision to relocate to a new building."