An innovative Houston developer has been granted still another extention of exclusive rights to develop key downtown Washington properties at the intersection of 12th and G Streets NW.

But the Washington firm of Oliver T. Carr Co. has expressed the first firm interest in the sites by local investors and is prepared to go forward with a phased development that would require no federal or local government subsidies.

Long a centerpiece of city planner's hopes for rebuilding the Federal City's old downtown business core, the 12th and G sites have been earmarked for office, hotel and retail buildings that would be located on top of the Metro Center subway station. One plan, several years ago, included a canopy over the intersection and elimination of most vehicular traffic to create a shopping and entertainment gallery.

Gerald D. Hines, a Houston firm, was the only company to express an interest in downtown development sites when the city asked for bids in March 1976. Another proposed developer pulled out earlier, having been unable to develop financing.

Hines won exclusive rights to develop three parcels - the blocks facing G Street between 11th and 13th and a third parcel at 13th and G. The company said the overall 12th & G development would cost up to $150 million.

Since 1976, Hines has won several extensions of its development rights, inpart because of continuing negotiations with the city government on the possibility that the city itself would lease office space in the project, which Hines said was essential to attract financial commitments.

Last September, the House Appropriations committee delivered a blow to the Hines hopes, by establishing guidelines generally viewed as discouraging long-term city government leases - such as that sought by Hines. Subsequently, Mayor Walter Washington said talks with Hines would continue and that a long-term lease still was possible.

This week, Hines sought and received an extension until March 1 of the exclusive rights from the Redevelopment Land Agency board for development of the two key sites. But now Hines is seeking government aid through an Urban Development Grant Action program of the U.S. Department of Housing and Urban Development, a competitive program involving proposals by communities across the country to improve rundown areas.

In the wake of this development, local developer Oliver T. Carr Jr. revealed that he has written to the responsible city agencies, offering to begin immediate work on development at the 12th and G sites.

In an interview yesterday, Carr said his firm is development a formal proposal to the District government that would include totally local investment in the undertaking.

Carr said a phased development of the 12th and G site is "feasible and practical, and there is no need for federal or city subsidies." Nor, he added, is Carr and its proposed investment partners requiring a city government office space lease.

"We can proceed much more rapidly" than the Hines company has estimated, Carr added. "I'm bullish on downtown."

A city government official said last night that Carr's proposals to date have been "exploratory" with little definite information. As long as Hines is interested and there is no formal alternative to consider, the city will keep its commitment to the Houston firm, he said.

The Carr company already is at work on a major downtown project nearby - in the block bounded by F, G, 14th and 15th Streets, which will include offices and retail stores tied to the existing main Garfinckel's location. In addition, Carr said his firm will seek development rights for the old Willard Hotel and it is constructing the second phase of a three-part International Square office and retail complex at 18th and K Streets.

If government agencies approve the Garfinckel block project, Carr said he hopes to break ground there late this year. He said he would hope to begin work at 12th and G at about the same time, following a cooperative planning effort with city officials.

"We want to integrate the existing downtown retail center," such as the F Street Mall, with the Willard and Garfinckel sites into an overall sector type development, Carr added.

They all link together very intelligently . . . the market exists . . . the market has got to the point where [such a plan] is feasible," the developed stated.

Carr declined to provide specific ideas of his firm for the parcels involved, saying that he preferred to consult with city planners about their ideas first. He also declined to identify the local investors who would finance the project, at least until the Hines company no longer has exclusive rights for the land involved.

Carr indicated that initial building could involve retail space and offices, over which additional structures could be built as markets develop for additinoal space or hotel use, if a convention center is approved ultimately for the nearby Mt. Vernon Square area.