The Alexandria City Council, despite concerns by the city Planning Commission, yesterday unanimously approved amending plans for a $75.6 million federal courthouse to allow for greater height and substantially more square footage.

Under the new guidelines, the maximum height of the courthouse tower would be boosted from 155 feet to 225 feet and the overall design would allow for a 400,000 square feet rather than 295,000 square feet previously approved by the council.

The new complex, which will be dominated by the 10-story courthouse tower and flanked by a five-story wing for the U.S. Attorney's Office and a three-story wing for offices, will provide an anchor for the first phase of a mixed-used project being developed in the Eisenhower Valley by Oliver Carr Co. and Norfolk Southern Corp.

Scheduled for completion in late 1993, the new facility would be on 2.6 acres just south of Duke Street between the King Street and Eisenhower Metro stops. It will replace the 65,000-square-foot courthouse at 200 S. Washington St., which has grown increasingly cramped in recent years as criminal and civil caseloads have mushroomed.

In other action, the council adopted development guidelines for the Eisenhower Valley that would cut in half the amount of commercial space allowed in a 44-acre tract near the Eisenhower Metro station.