One of the best luxury apartment buildings built in Manhattan in recent years is also quite small. HL23, designed by Los Angeles-based Neil Denari, achieves more with a tiny lot and small overall square footage than Jean Nouvel nearby in Chelsea or Frank Gehry in his play for prominence on the skyline at Spruce Street in Lower Manhattan.

Nouvel, on a site next to a women’s jail along an inhospitable stretch of the West Side Highway, and Gehry, building on a generic base in a warren of congested downtown streets, both overcompensate for the poor sites of their file-cabinets for the super-rich with elaborate, even fussy, facades: Nouvel with layers and piles of windows and empty frames, and Gehry with folds and waves of metal. Both buildings appear to run out of steam on their flat “rear” facades (though both are so tall that they are seen in the round above their bases), revealing a shallow facadism.

At HL23, Denari, guided by the savvy developer Alf Naman, was able to capitalize on his tiny site—3,900 square feet—rearing up alongside the High Line. The building bulges out over the old elevated freight tracks, so that inside, through the floor to ceiling windows, residents of the lower floors will be able to see every button on the crowds promenading below. What Pierre Koenig captured of LA’s expansive, auto-centric urbanism in his Case Study House Number 22, Denari does for Manhattan’s thrilling throngs of pedestrians. Many have written about New Yorkers’ increasing willingness to put their lives on display in glass walled apartments (privacy appears to be the first casualty of the digital age), but the fascination of people watching on the High Line harkens back at least to 19th century Parisian café culture. Turn your rattan chair to the street and the action will entertain you for hours on end.

The effect evaporates even a few floors up, and those in the more expensive higher units will enjoy more privacy but will have more conventional views and quarters. Layouts include tasteful kitchens, well-proportioned rooms, super-luxe bathrooms each covered in distinctively veined marble, and a view of the High Line that reveals its tiny, ribbon-like pathway through the neighborhood. The penthouse features a glass box with three sides of floor to ceiling window walls opening out onto a wraparound balcony, surely one of the best trophy party rooms in the city. Most New Yorkers will never see beyond the picture window vignettes visible on the exterior, but no matter. The building has plenty to offer the passerby.

Much of its visual interest comes from the structural tension in holding up that bulge, tension Denari dramatizes by putting the diagonal bracing just behind the exterior glass. In his bow to facadism, he underscores the bracing with a white ceramic frit on the glazing. The shadow frit is a bit of a cheat, but it is an effective trick, making the building appear softer and more fluid, with curved joints rather than angled ones. The frit also begs the question, is Denari’s vision one of an evolved modernist, using structure and technology to achieve a high level of function and maximize sellable space, or is he a digital formalist pushing the limits of the buildable and expanding the visual language of design? At HL23, the architect appears to have it both ways, rendering these concerns beside the point.

The curves are echoed on the east side of the building, which is clad in metal panels with a subtle raised ripple. These curves are more than decorative. They prevent the panels from appearing bowed or pocked like so many of their contemporaries, including an unfortunate imitator by Delle Valle Bernheimer a couple of blocks up and also on the High Line, where the dotted panels look dented and dirty.

Denari’s muscular little building is formally dynamic without feeling frivolous. Architect and developer have managed to add something unique to the crowded luxury market and to Manhattan’s delightfully jumbled streets. For those of us strolling by on the newly opened second phase of the High Line—so tightly woven into the fabric of the far West Side—it’s a building to puzzle over, and a delight to voyeurs and architecture lovers alike.