The District of Columnia's 1979 fiscal year has not even begun, but the city already has proposed its $1.5 billion budget for fiscal 1980.

Fiscal 1979 will begin in two weeks, on Oct. 1 with city operations to be financed under an appropriations bill that was signed into law yesterday by President Carter.

Adoption of the annual D.C. budget is probably the most complex and rigorous procedure of any city in the nation. It involves intensive review byboth municipal and federal officials and ultimately presidential approval.

Despite increased local - determination under the D.C. Home Rule Charter, the deep federal involvement in the city budget results chiefly from the fact that the U.S. Treasury contributes a large annual payment to the city. This is intended to reimburse the city for lost revenues and added costs largely related to tax exemptions on federal property.

With release of the proposed budget yesteday, the city Council now has 50 days - about seven weeks - to propose increases or reductions. Then the mayor has 10 days to decided whether to accept or veto the council's changes. The council then has 30 days to act on the mayor's decisions.

Then, in early December, the final proposed document is sent to the White House , where U.S. Office of Management and Budget reviews it before sending it to Congress. The president includes his proposal for the amount of the federal payment. The city is seeking $317 million, up from the 1979 figure of $235 million.

Transmittal from the White House to Congress often has been delayed, and the House Appropriations District subcommittee has said it wants the 1980 document nolater than Feb. 1.

After hearings, both the House and Senate must adopt appropriations bill encompassing their versions of the budget. They also make the final decision on the size of the federal payment. Any differences between House and Senate versions are resolved by a joint conference committe.

From there, the bill goes to the whiteHouse for the presidential signature, where it becomes law.