A Houston development firm has abandoned its previous pessimsim and decided instead to wait six more months before making a final decision on whether to build a $150 million office and retail complex that many consider a key to the revitalization of downtown-Washington.

For more than a year. Gerald D. Hines Interests of Houston has benn trying to complete plans for the project, to be at 12th and G Streets NW. The plans have been stalled by the lack of a firm commitment from the city to sign a 20-year, $60 million lease to use 300,000 square feet of space in the proposed office building.

Hines had contended in the past that without the city as an anchor tennant, the project could not be built. The city signed a tentative commitment but had not made a binding one. The city already plans to build a new $35 million municipal office to complex, and some congressional leaders who oversee city spending have expressed concerns about the city leasing space at the same time it is constructing a new building of its own.

Early last month, when the House Appropriations Committee approved the city's 1973 budget, it established guideline that were generally viewed as discouraging long-term leases such as that sought by Hines. The Washington lawyer for Hines said then that that action made it "more doubtful than ever" that the project would continue.

Yesterday however, lawyer R.Robert Linowes reversed that stand and said that Hines was willing to wait at least six more months - until mid-March of next year - before it makes a final determination.

Linowes said the new thinking on the part of the Texas firm followed a meeting two weeks ago with Mayor Walter E. Washington in which the mayor assured the firm's owner.Gerald D. Hines, that the city was still committed to the project and that the necessary lease could possibly be appraised.

Linowes said the project, which also would include a hotel and a "major" department store, could even be built without the city as an anchor tenant if Congress approves plans for a proposed $110 convention center - to be built less than a mile from the complex.

The convention center proposal is now before a House-Senate conference committee, where its fate is uncertain.