An emergency law that permitted owners of 57,000 oil-heated rental units in the District of Columbia to raise rents on Jan. 1 will expire next week, forcing the D.C. City Council to convene an unscheduled legislative session to vote on renewing it.

Unless the law is renewed, the continued collection of rent increases ranging from 3.5 to 7 percent to cover the rising costs of fuel oil could be subject to legal challenge, James M. Christian, the council's chief lawyer told a reporter.

The rent rise on oil-heated units is on top of a general citywide rent increase that went into effect last July. The D.C. Rental Accommodations Commission, in considering another general increase for mid 1980, has been told in a staff report that last year's general rise may have been excessive.

The commission may revise this year's general increase downward to reflect such overcharges. It also will substract the amount of the emergency oil-heat rent rise from the new general increase.

The council invoked its emergency power Oct. 23 to enact the oil-heat measure, which will expire next Thursday, 90 days after it was signed by the mayor.

The council's legal authority to pass the same emergency legislation more than one time has been challenged in a lawsuit now pending before the D.C. Court of Appeals.

On Tuesday the council voted preliminary passage of identical permanent legislation, but it must be approved as second time by the council on Feb. 5 before being signed by the mayor and submitted to Congress for a 30-day review.

Christian said he will recommend that the Council meet Monday to consider the bill.