When does a 676-square-foot, $435,000 condo make sense? In a truly rational world, never. In the breathless stratosphere of Seattle real estate, it might be imaginable -- particularly if there were enough intelligent design in the building and living units to add substantial value and versatility.

You'll ponder these issues during a visit to Lumen, the new low-rise luxury-condo complex that replaces the old Tower Records hulk at Fifth Avenue and Mercer Street. It isn't a beautiful building, but it offers a lot of engaging and thoughtful design details, and it proposes some ideas about urban living that Seattle should consider as it rapidly grows more dense and vertical.

Let's start the tour inside out, within that 676-square-foot unit, the complex's smallest, and in several ways its most intriguing. How can an architect and an interior designer -- David Hewitt and Garrett Cord Werner, respectively -- make such a tightly squeezed space both livable and interesting?

It's a basic, boxcar-shaped enclosure with the kitchen appliances and counter aligned on one side of the living room, a bedroom defined by sliding frosted-glass walls in the middle, and a bathroom at the back. Clutter can have a deadly effect in such close quarters, so a generous bank of four closets takes what could otherwise be wasted space along the wall opposite the bedroom and bath.

Several strategies conspire to enlarge the apparent space. A German-made glass Nana Wall forms the front wall, which folds back to extend the living area outdoors onto the unit's private patio. The disappearing bedroom walls are a terrific idea. The bedroom becomes a private cocoon when in use, but during waking hours the room can be added to the apparent volume of the living area, making it feel more expansive. Quality details -- which had better be tossed in for $643 a square foot -- mean that the critical eye finds satisfaction as it roams. There's a razor-sharp line, for example, wherever the sheetrock meets the concrete ceiling -- no sloppy or fuzzy edges. It's a small issue, but in such a compact residence every surface and detail needs to be carefully thought out and executed.

Not quite everything was. Ceilings, floors and occasional structural members are raw concrete, expressing the building's bones. It's honest, it conveys strength and durability, it suggests an edgy contemporary 'tude -- and it's something that enthuses architects about 700 percent more than it does consumers of architecture. Raw concrete is a drab, ugly material, and it doesn't grace a residence any more than a Jersey barrier landscapes a highway.

A more serious problem is that the boxcar admits daylight through just one wall, which is also the case with most of the building's 98 units. In some, the one outside wall substitutes translucent panels for some of the available window area. It's an unsatisfying solution to an unfair problem that's bedeviling designers of multifamily housing. The Seattle Energy Code uses complicated formulas to regulate the glass area allowed in a building's envelope, and Hewitt says he could increase the total amount of daylight capture with the translucent panels because there's less heat loss than with clear glass.

But humane, joyful living spaces in buildings like this need much more daylight and more views, even at the expense of some energy efficiency. It's hardly unreasonable, since modern condo buildings are already more efficient than single-family houses. Ideally, the building envelope should provide windows on two or more sides of every residence. It's going to take more creativity and out-of-box thinking, both by architects and regulators.

There's a 52-by-150-foot open courtyard on the building's second floor. Several residences open directly onto it, and it's accessible to everyone in the complex. It's the kind of informal, unprogrammed space every neighborhood needs, although it's impossible to predict in advance how residents will use it. Hewitt appears to have done thoughtful work, however, in providing a large variety of spaces in the complex where residents can interact, in planned and unplanned encounters.

"Cities are about interaction and communication and dynamics," he says, and he has concentrated exactly those qualities in this building.

Downstairs and outside, it's a mixed and complicated bag. A new QFC store claims the southwest corner of the building, and patrons enjoy a sophisticated and tempting view down into it from a mezzanine-like platform at the entrance. Curiously, it's a thoroughly suburban store form -- wide aisles, vast wasted space, 40,000 square feet in all -- shuffled into an extremely urban context. There's a disconnect between the feel of the residential building and the supermarket, and you're not entirely sure where in the urban universe you've landed.

The building's exterior has a different treatment on each side, reflecting the complexity of its program. Its best face is on the west, where the QFC entrance takes a dramatic wedgelike slice off the box and the condo bays on the upper floors form rhythmic sawtooth projections. The London plane trees, left over from the 1962 World's Fair, remain in place, although they've suffered lopsided pruning to make room for the building. On the north side, Hewitt provided a clever drainage hardscape that will become a water feature whenever it rains. On the south side along Mercer Avenue, the illuminated community rooms above the QFC provide a lantern effect at night -- hence the name Lumen.

What this building illuminates more than anything else, however, is the enormous and widening cultural chasm between urban and suburban Seattle. The two might have something to learn from each other in terms of architecture and lifestyle, but they're not talking -- they're as polarized as gun-toting evangelicals and Camus-quoting intellectuals. Suburban houses could be much improved by a more contemporary style and efficiencyenhancing devices like those disappearing bedroom walls, and urban condo designers might think more daylight, softer edges and more organic surroundings. As it seems now, it's as if each type were being designed in appalled reaction to the other.

Like it or not, Lumen's 676-square-foot, $435,000 condo represents something closer to the future of housing on a crowded planet than does the 2,400-square-foot neo-Victorian house in the 'burbs.

It's worth a hard look, unless we decide it would make more sense to quit growing the population.