One of the challenges in designing housing is finding a middle ground between monotonous repetition and arbitrary variety. For Torre Júlia, a 17-story municipal apartment building for senior citizens on the northern edge of Barcelona, a team of architects fresh out of architecture school—Ricard Galiana, Sergi Pons, and Pau Vidal—uses singular elements such as social spaces and circulation to cleverly navigate between these extremes. Developed after the trio won a 2007 competition for another project on the site that was subsequently canceled, their formal strategy is part of a larger aim to make the building into a community. “This is, after all, basically a social container,” explains Pons.

The design for the $9.7 million, 90,000-square-foot tower starts from a premise as boring as that for a speculative office block: a reinforced-concrete structure with a square footprint and repetitive horizontal ribbons of openings and spandrels finished in white corrugated aluminum. But the architects treat this basic volume as a neutral field over which they deploy a series of variations. The 77 rental units, which overlook the northwest and southeast exposures, are pulled back behind continuous terraces, converting the facades into a sun screen. Outdoor stairs run down the other two facades, selectively interrupting the spandrel bands with dramatic, Constructivist-looking diagonals that reveal planes of yellow and green behind them-part of a color-coding system that groups several floors together into communities around corner common rooms.

These public living rooms are double-story, with full-height glazing, each with a 16-foot cantilever above supported by continuous concrete walls. Crowning the composition is a roof deck, open to the sky but surrounded by a ribbon of framed openings slightly taller than those below.

Inside, the architects push against the limits of subsidized-housing standards to encourage interaction among residents. Corridors are short and more than 7 feet wide (4 feet is the norm), with glazed ends opening to the exterior stairs, inviting visits to nearby floors. The 430-square-foot apartments, each with a single bedroom and a kitchen bar in the living area, have louvered windows facing into the corridors that residents can open to create cross-breezes and allow an auditory connection to the hall (fold-down seats in front of each unit were under-detailed, however, and can't support the weight of a sitter, though they're good for parking groceries). A midmorning trip through the building found residents' potted plants enlivening some of these bright spaces, sounds of a radio playing from one apartment, aromas of baking from another, and a woman crossing over to call out to a neighbor. More social mixing takes place on the entry floor, which has a full-time social-services staff and a meeting room with a small stage. Here residents and staff interact in front of the elevators and the mailboxes.

Although the tower's site is something of a leftover space, it benefits from Barcelona's decades-old city-planning programs. It is situated at the end of Via Júlia, a commercial street where, in the 1980s, the city planted trees and created wide sidewalks, now filled with pleasant outdoor cafés. It stands beside the Ronda de Dalt ring road, built for the 1992 Olympics. Largely buried under plazas and boulevards, the busy thoroughfare is an acoustic presence but not overpowering. And the tower is grouped with other city services, including a nursing home, a public market, and a planned municipal swimming pool. Although the site is rather steeply sloped (an obstacle that the entry plaza, with trees and flower beds between meandering walks, handsomely overcomes), the location provides residents with panoramic views toward the Olympic Village and the Mediterranean. Such amenities, together with the skillful, committed design, make a big difference in a project built on a modest budget.

Size: 90,000 square feet

Cost: $9.7 million

Completion date: September 2011