Owners of landmark properties - or people who want to buy landmark buildings - may share more than $600,000 in historic preservation grant-in-aid funds this fiscal year (1978) under a new program announced by the D.C. Historic Preservation Office.

Applications for the program, funded under the National Preservation Act of 1966, will be available beginning tomorrow and must be filed by July 15. The money may be spent to buy or to improve any property listed on the National Register of Historic Places or any property in a historic district listed on the National Register.

Individuals, groups or the D.C. government may submit applications for projects, which will be funded for up to 50 percent of the total cost. Successful applicants must agree to attach a convenant to the property pledging to preserve it for a fixed amount of time, to be determined by the amount of the grant.

Nationwide, $45 million is available for historic-preservation grants this fiscal year, according to Peter Herrick, a spokesman for the National Register. The first allocations under the program were made in 1968 and now all states and most U.S. territories have programs for grants-in-aid.

The District, according to Herrick, previously has been allocated $621,000 under the program; $300,000 has been earmarked for surveys and planning and $321,000 for work on District-owned buildings. Of the $321,000 the Old City Hall (now the D.C. Courthouse) has received $50,000; the Eastern Market, $124,000, and the Mount Vernon Square Library building, $147,000. Unlike programs in most of the states, the District has not made grants to individual property owners.

"We were never geared up to manage a program before," said Kathleen McKay, who will manage the program for the District Department of Housing and Development. Legal problems and lack of adequate staff contributed to the delay in setting up the program, according to McKay. "But we haven't lost any of the money that's been allocated to us. It's all still there," she said.

The Carter Administration has asked that the grant-in-aid program be kept at the same level - $45 million - for fiscal year 1979. According to Herrick, the District share for the next fiscal year will be determined by a formula based on population and on how much preservation money has been spent in past fiscal years.

Applications for grants-in-aid for fiscal year 1979 must be filed by Sept. 30. To obtain an application and an instruction guide, for either fiscal year 1978 or 1979, write: Kathleen McKay, D.C. Departmentof Housing and Community Development, Room 973, 1325 G St NW, Washington, D.C. 20005.