The District of Columbia City Council completed action yesterday on the city's 1980 budget which contains added funds for public schools, under a procedurally liberalized new law.

The council also voted preliminary approval of a bill tightening rules on demolishing or altering buildings in historic district, and approval a three year extension of a law permitting mortgage lenders to charge as much as 11 percent interest on home loans.

Approval of the budget, $11 million larger than the $1.4 billion version submitted by outgoing Mayor Walter E. Washington, came on a roll-call vote of 13 to 0. It was the first unanimous adoption of a budget bill by the council since the home rule government began in 1975.

Included is a $264 million outlay for public school operations, $6 million more than was proposed by the mayor. The council urged that the added money be spent on books and classroom supplies, not on personnel.

The council had expected to give budget only preliminary approval two weeks. But Council Chairman Tucker announced that president Carter signed a congressional act over the weekend letting the council adopt the budget in one, rather than two, legislative steps.

That sent the measure back to the mayor, who has the right to veot individual items. After any vetoes are resolved, the budget goes to the White House and then to Congress for final action.

The measure signed by Carter also reduces the time during which countil-passed legislation must wait on Capitol Hill before either being voted by Congress or allowed to become law. The old rules required 30 legislative days - that is, days in which both houses of Congress are in session. The new rules require 30 calendar days.

Tucker said the change should reduce the large volume of emergency legislation the council has been forced to pass as stopgap measures while permanent bills await congressional approval.

The new bill on historic buildings sets up a procedure under which applications found to be unjustified, based on esthetic or economic considerations, should be rejected.

The provisions apply to officially designated historic buildings or, more broadly, to buildings located in officially designated historic districts, such as Dupont Circle and the Uniontown district of central Anacostia.

Council members Douglas E. Moore (D-At Large) and Willie J. Hardy D-Ward 7) criticized the bill, and cast the only votes against its approval. Moore said the bill would "make it possible for speculators to move in" and restore structures to their "old grandeur and decadent glory."

But John A. Wilson (D-Ward 2) said the bill, which he introduced and which was rewritten by Nadine P. Winter (D-Ward 6), helps preserve old homes from destruction.

The bill extending the 11 percent mortgage interest rate in a permanent version of one passed earlier as an emergency bill. It permits a 12 percent rate on cooperative apartment loans and restricts sellers from charging more than one point (1 percent of the face value of a mortgage loan). Without the bill, the city's legal interest rate would have reverted to 8 percent.

At the request of Secretary of State Cyrus Vance, the council delayed action on a bill designed to bar diplomatic chanceries from residential areas. Council members said they did so as a courtesy but plan to revive the bill later.