That often there will be slips between cups and lips is a certainty in the practice of architecture -- when years go by between the conception and execution of a design all sorts of mischief can be done to a good one. But sometimes everything works out okay.

The 1200 block of 24th Street NW, between M and N streets in Washington's West End, is one of these happy occasions. This is a stretch of street that once was a mix of low residences and light industries that came to resemble, as speculators and developers moved in, a scene in some hapless postwar city after the rubble had been cleared.

Today, though, it's well on the way to becoming a real city street, its handsome brick sidewalks lined on both sides with young maple trees and, behind these, an ensemble of distinguished (or at least acceptable) new buildings.

Because of the height limitation and a few other regulations (governing building setbacks, open space and so on), and because of a sensitive architectural conception of the street as a whole, there's unity here, and a strong sense of place. It's a new street but certifiably Washington in character -- low, quiet, maybe a little bit boring, but green, commodious. There's a quotient, too, of architectural variety, and street life is picking up as people move into the new offices and guests visit the upscale new hotels.

What's missing are some genuine residents. The West End, 15 years ago, was envisioned as an ideal in-town location for a mix of offices, apartments and stores. City planners even drew up a zoning ordinance requiring a 50-50 split between office and residential construction. But when developers and financiers proved slow to take up the offer -- although a few office-apartment buildings have been completed -- the law was changed, allowing hotels to qualify as "residential."

Obviously, this was an opportunity missed, one of those slips that continues to haunt the West End: If you see a vacant lot there now, you can almost bet that it represents the unbuilt residential half of a project. It's a safe bet, too, that there will be a little parade of developers to city authorities with lawyers in tow, pleading for a change in the zoning of these lots from residential to commercial.

Still, the results -- three hotels on the corner of 24th and M streets, and others underway or already done in the vicinity -- are at minimum a lot better than the dreary alternative of continuing the westward march of the K Street office phalanx. The lemminglike behavior of developers clearly has produced a surfeit of hotels -- business seems sparse in the Grand, the Park Hyatt and the Westin on 24th Street -- but the economic dislocation most likely will be temporary. With both downtown and Georgetown close by, it's a good spot.

The guiding design spirit behind the flowering of 24th Street -- and it's literally flowering, for the owners of the new buildings are keeping their narrow yards and even the tree plots well stocked with pansies, marigolds and the like -- is David Childs, a partner in the big, prestigious firm of Skidmore, Owings and Merrill. Childs, who founded SOM's Washington office in the early 1970s, has moved on to head the firm's New York office, but he left a lot of good work behind. This street is an excellent example of his skills as both architect and urban designer.

Urban design basically has to do with the spaces in between buildings -- streets, sidewalks, alleys, yards, parks -- and the objects in these spaces. But it also has to do with architecture itself, for the character of the in-between spaces is determined in large part by the character of the individual buildings -- their forms, materials, colors, textures. Childs' pervasive influence upon this street is due mainly to the fact that he was employed by a single company, Boston Properties, which owned all of the land on the eastern side of the block and also a site on the other side of the street.

Childs and his SOM colleagues thus were able to design the U.S. News & World Report building, completed in 1984 at 24th and N streets, the office building opposite U.S. News at 2300 N St. and the Park Hyatt Hotel at 24th and M Streets (both completed last year). That the firm also designed the Grand Hotel, which opened in 1984 on the southeast corner of 24th and M, is a lagniappe. The remaining buildings on the block -- the Westin Hotel at 24th and M and the office building at 1250 24th -- were designed by Vlastimil Koubek and Don M. Hisaka, respectively.

The U.S. News building was the key, and for a couple of years it remained a grand, isolated gesture. With its concave, quarter-circle fac ade facing the corner and its brick surfaces with stonelike concrete banding, the building is a curious mix of strong baroque form and quasi-industrial style, but it works -- it's impressive yet modest, powerful yet likable. Most importantly, it established a visual pattern and a rhythm of forms that SOM was able to echo in the office building across the street.

This structure is an adept variation on the theme. It too faces the corner with a concave curve and continues the banded brick motif, but so many things are subtly changed, inside and outside, that it's more like a cousin than a twin. The result is a pair that gives extraordinary form to the northern end of this ordinary street. The recall of Washington's monumental core (the hemicycle in the Federal Triangle) is distanced by the absence of classical detailing, but it's there, like a pleasant afterimage.

The Park Hyatt Hotel is more of a break in the theme. The brick-and-concrete materials match those to the north, but the building plays mainly to the Grand Hotel to the south, across M Street. The contrast between the two is a deft piece of work, the Grand all sculpted solidity, the Park Hyatt an essay in glass, metal and brick. It's an urbane building through and through (although the lobby, because of the narrow site, is none too interesting).

The corner itself is a special treat -- the building is set back generously here, to make room for a fountain and outdoor cafe', and the details are economical but first rate. The ample use of glass and picturesque pieces such as the green-metal pinnacle at the top gives the place a park-in-the-city kind of feel. As a result, instead of the customary build-to-the-property-line anonymity, this block is framed north and south by spaces with different, though equally strong and pleasing, characters.

Koubek's Westin Hotel is odd -- the evidence here suggests that this is a firm arriving late at the postmodernist banquet and not digesting it well. The pedimented windows and the division of the building into base, shaft and crown may have been intended as polite gestures to history, but they appear more as afterthoughts applied to a standard Washington box -- the pediments and cornices look as if they were flattened by a giant iron.

But tripartite midlevel windows and projecting bays are welcome deviations from Washington's customary ribbons, and there is good news inside -- the lobby, despite mindlessly eclectic decor, is a genuinely active, light-filled space. Though it centers on a courtyard restaurant that's a sort of Disneyland transplant, its silliness is fetching if one doesn't look too carefully at things. The serious underpinning is a great floor plan. There are lots of entrances and lots of choices -- up, down, left, right, through, double-back -- so it's a wonderful place to walk around in.

Hisaka's office building, discussed here last week, is an exciting midblock break -- too light in color to mediate authoritatively between the light-toned Westin and red-bricked U.S. News, but a strong piece. That long, bowed, glass curtain wall anchored between two vertical piers is a gesture and a half -- it gives this block a surprising lift. The block as a whole is a surprise, a brand new piece of city with a distinctive sense of itself. It will age well, I think. Thanks to all.