The capacity of good architecture to change our perceptions of a place for the better is beautifully illustrated by a couple of medium-sized new buildings on Indiana Avenue downtown.

These buildings would be welcomed in any case as the first tangible signs of a residential neighborhood in the "eastern sector" of Pennsylvania Avenue, a goal often reiterated by the Pennsylvania Avenue Development Corp. According to leasing agents, the 150 expensive apartment units in the project are renting briskly to young lawyers and the like, most of whom can now walk to work.

But the architectural story in itself is extra special. Conditions were propitious. The PADC plan was bold and foresightful, and both the public corporation and private developers and other architects helped to set the stage. The architects assured a happy ending by taking full advantage of all openings and opportunities -- no simple task. A misstep or two or three and it easily could have been botched.

Designed by the Washington firm of Hartman-Cox in conservative, 19th-century vernacular, this winsome pair occupies the better part of a long block on the south side of the avenue, between Sixth and Seventh streets NW. The more extraordinary of the two, at the Sixth Street corner, establishes a pleasant rhythm with a sequence of red brick bays and balconies, and it has a gorgeous exclamation point of a tower. It looks like, and is, a classy apartment building. The other, adjacent structure is more modestly sheathed in soft-toned precast concrete modules. It's the spittin' image of a pre-modern, if post-industrial, downtown commercial building.

Together, the buildings contribute greatly to a new mood along this broad, but short, boulevard. Seen from afar they provide a dignified frame for the southern edge of the avenue, and the commercial structure sets an attractive backdrop for the historic bank building facing Seventh Street, a little blockhouse of a building in the Romanesque style. Up close, the buildings welcome you with their modeled limestone bases and handsome storefronts. As a result of these and other architectural moves, Indiana Avenue, one of the least conspicuous of the radiating boulevards in Pierre L'Enfant's original plan for the city (it now extends just this one block), is well on its way to becoming a downtown place with a unique identity.

To appreciate just how good this news actually is one needs simply to recollect that in the 1960s, when the plan for redeveloping the northern edges of Pennsylvania Avenue was announced, Indiana Avenue and its bordering blocks were to be obliterated altogether in favor of a sequence of mega-buildings symmetrically plopped among vast paved plazas. The revised plan, unveiled in 1974, wisely opted to preserve at least a few of the notable old buildings in the area and, equally important, to base new development upon the existing street pattern.

Things got off to a fair start in the early '80s when the twin-turreted Central Bank building at Pennsylvania and Seventh Street (better known as the Apex Building because of a liquor store that long occupied its ground floor), along with adjacent 19th-century commercial structures, were finely restored and adroitly added to by Hartman-Cox. From the point of view of urban vitality, however, it didn't help that this complex, including the exquisitely recreated ground-floor storefronts, was leased in its entirety by a Sears division for offices.

A second project, completed three years ago at 601 Pennsylvania Ave., was an aesthetic disaster -- it's that white building with those awful, dumb elevations and the facade of the old, polychrome Atlantic Coast Line building stuck to it like an errant piece of wallpaper. Originally, the site immediately to the north, upon which the new Hartman-Cox buildings now stand, portended more of the same -- it was owned by the same developer and the building on it was designed simply as a bigger look-alike. Fortunately, the site changed hands, and with the new developer came a new architecture firm.

All along, this second site was intended to satisfy the residential quotient spelled out in the PADC plan (225 units, originally). But the earlier proposal was suspect on this count too. Its residential units, to be stacked atop a hotel, were to be uniformly small and could easily have been converted to hotel suites in a pinch -- a sign of the distrust with which developers regard the notion of building apartments on prime office land.

However, the new owners (a consortium made up of the Sigal/Zuckerman Co., the Lawrence Ruben Co. and the Abernathy Group) decided to make the best of it, twice requesting the PADC board to reduce the number of required units permissible in the same amount of space. This is an enlightened approach to the numbers game; in consequence, the units are larger and more varied, greatly improving the chances that real residents, rather than overnight corporate visitors, will live in them.

(Among the 150 units there are 14 studios, 104 one-bedroom apartments of different sizes, and 32 two-bedroom units. Rents range from $735 to $3,000 a month, the latter for a swell suite with balcony views of the Capitol and the Washington Monument.)

The first critical decision the Hartman-Cox architects made was to separate the project into two buildings, one for offices, one for apartments. The advantages of this choice are several. It enabled the architects to a break up the block in a time-honored way, to alter its textures, colors and rhythms both at the street level and the tops of the buildings. It eliminated numerous technical and aesthetic problems arising from squeezing two different uses into the same package. It improved the chances for flexible apartment layouts. Most importantly from the civic point of view, it gave them the opportunity to design a couple of high-identity buildings.

Which they did forthwith. Probably the best place from which to appreciate the achievement is a block or so north of the new buildings, on the east side of Sixth Street, in order to catch the extraordinary juxtaposition of the new apartment tower with the beautiful dome of John Russell Pope's National Gallery -- it's as fetching a play of contrasting pure forms as you're likely to find in Washington.

Furthermore, the contrast says something about the nature of this city. Just as the dome inevitably heralds the presence of the official, neoclassical city to the south, so too does this peaked tower speak of the different order of activities that takes place in the "local" city, downtown. Another good view is to look from east to west along Indiana Avenue, toward the restored Fireman's Insurance building at the corner of Seventh Street -- this little stretch of boulevard is now anchored at either end by red brick buildings, each with an affecting corner tower.

Such consciousness of context is a Hartman-Cox hallmark; rarely has it been displayed so effectively. The buildings are similarly distinguished in detail. Proportions are good all around; the avenue elevation of the commercial structure is especially fine. Just as they did in the 19th century, the precast modules allow for a high percentage of glass, then as now highly prized in offices. Back then the modules would have been cast from iron, not concrete. And they still permit a highly abstract articulation of the facade without dispensing with ornament. The simple system of projections and incisions employed here adds significantly to the richness of the facade.

Because the site connects Indiana Avenue to C Street (now closed to cars), the buildings were designed in the round, each elevation different from the one around the corner. This causes some awkwardness -- the architects decided to hide the stepped-back massing of the southwestern facade as much as possible by simply sheathing it in blond brick. If one examines it carefully, the precast eastern facade takes on a shallow, wallpaper-like look. But fundamentally, this is just a wonderfully shipshape job.

A healthy attitude, as well as skill, produced it. "We looked at it as a series of urban spaces, instead of as a single building," says Lee Becker, one of two project architects on the design team. The office building lobby, for instance, is a walk-through, see-through space linking the avenue with C Street. The C Street passage itself needs work; as landscaped at present it is more of an obstacle than a passage. But with this sole, reparable exception, there is now an appealing sequence of public spaces from one end of Indiana Avenue to the other.

Credit is due to the developers; the PADC (which established the rules and which itself nicely reconfigured the plaza facing Seventh Street); CHK Architects & Planners, which did a fine job with the apartment layouts; and most of all to Hartman-Cox, with George Hartman, partner in charge, and Becker and Bill Neudorfer, project architects.