The development sites are just 3 1/2 blocks apart in Washington's old downtown and developers at both locations are planning glistening, megabucks hotels. Yet, as yesterday's events unfolded, they quickly provided a lesson in the realities of trying to transform a deteriorated neighborhood.

In one case, at Gallery Place, a city-owned block bounded by Sixth, Seventh, F and G streets NW, development is stalled, as it has been for five years. Yesterday, the developers told the Redevelopment Land Agency, the city's urban renewal authority, that they need still more time to complete their financing.

At the other site, just south of the D.C. Convention Center in a block bounded by 10th, 11th, G and H streets NW, construction crews are rapidly pounding steel shoring into the ground for the first stage of building the $130 million Grand Hyatt Washington Hotel. City officials, bankers, developers and hotel officials celebrated yesterday's official groundbreaking with wine, shrimp and hors d'oeuvres.

Development in the city's old and somewhat tawdry downtown has steadily moved east of 15th Street NW, but Capital Landmark Associates is finding that a project at Sixth Street is still considered the frontier and financing the development has been a dicey proposition.

The group has been trying for five years to secure financing for a $96.8 million hotel and office complex. But on 10 occasions since 1980, the RLA has been forced to grant the developers more time to complete the financing before the property can be sold to them for $17 million, the price set five years ago.

Last September, Capital Landmark, a group headed by William B. Fitzgerald, president of Independence Federal Savings and Loan Association, told RLA the financing would be arranged in two weeks.

Yesterday, Capital Landmark told the RLA that the group has obtained commitments for "almost two-thirds of the necessary construction financing," it said, but not for a permanent loan.

In addition, long-vacant buildings still stand, although the company told RLA board members in a closed-door meeting that demolition is still planned.

In short, Capital Landmark's attorney, Philip M. Horowitz, told the RLA in a letter last week that it is unlikely the firm will be able to complete the purchase of the city land by the latest settlement date, May 14.

James E. Clay, RLA's chairman, said "the encouraging thing is that development" is moving into the old downtown area. But he noted that "nobody wants to be the first" to redevelop new turf.

Similarly, the Metro transit agency granted development rights for the Far East Trade Center, a hotel, shopping and housing complex immediately to the north of the Gallery Place site, and it, too, has found that the developers are having trouble securing financing.

Clay acknowledged that "five years is a long time" to allow Capital Landmark to try to secure financing, but he said that "18 months ago we made a decision that this proposal has the possibility of coming to fruition faster than if we started again with someone else."

At the Convention Center gathering heralding the Hyatt construction, there was no such hand wringing over the pace of development.

Robert Gladstone, president of Quadrangle Development Corp., the developers of that project and a newly completed shopping mall on F Street, said the 950-room Hyatt, an adjoining office building and retail stores would be completed in mid-1987 at a total cost of $200 million. The hotel is the first to be built immediately adjacent to the Convention Center.

The Hyatt, as is its trademark elsewhere, will be built around a huge atrium, a 12-story open space with a skylight and a lagoon. Part of the office building will be built when the existing north building of the Woodward & Lothrop department store is demolished, while the rest of redevelopment will encompass a refurbishing of the McLachlen Building on G Street.

John Fondersmith, the city's chief planner for downtown, described the Hyatt as "a beachhead" that may spur other development and help draw people north from Pennsylvania Avenue into the old downtown area.

Three other hotels, as well as more office buildings, are planned near the Convention Center, he said.