These are strange days in San Francisco, where the clamor to build needed housing — especially at affordable levels — is matched only by the self-righteous vigor with which actual proposals for that housing tend to be opposed.

So I shouldn’t be surprised that the 11-month-old proposal for a 400-foot tower at Folsom and Spear streets suddenly is under attack, with a small band of opponents unfurling a long list of reasons why the idea of 391 condominiums — 140 of them priced at below-market levels — should go back to square one.

But if we want a well-planned city with distinctive new buildings for all its citizens, projects like this show that good design and good policy can go hand in hand.

The project in question would rise one block inland from the Embarcadero behind the Gap headquarters, with a design that would add a literal twist to the skyline: Chicago architect Jeanne Gang, best known for the 82-story Aqua tower in her hometown, has conceived a narrow shaft with angled bays that ripple down like a textured corkscrew, a gentle twist repeating itself every 11 stories. Instead of brick or glass, it would be clad in panels of either metal or (preferably) terra-cotta.

Alongside the 160 Folsom St. tower would be an eight-story package of storefronts and housing, with town houses facing the alley to the north.

From a design point of view, the tower would be a great addition to the blocks between Market Street and the Bay Bridge, the only part of town where construction cranes outnumber food trucks. Gang is the rare high-rise architect who seeks to strike a balance between attention-getting moves and fine-grain details; the tower here would be snaky but subtle, a serious work of contemporary architecture that also has a rhythmic swing. The storefront along Folsom would be 20 feet high, tall enough to be inviting.

Opponents’ leverage

But the design isn’t the only aspect of the project with a twist, and that’s what gives leverage to the opponents.

Part of the site is owned by the city and part by developer Tishman Speyer, and the block is zoned for a 300-foot building. Tishman Speyer wants to climb another 10 stories, so it offered to raise its bid for the city land from $14.7 million to $19.2 million if the rules are changed. That’s enough to subsidize nearly all low-income condominiums on the block, while Tishman Speyer would include 64 condos for middle-income buyers to bring the number of affordable units to 36 percent.

All this was spelled out when the project was unveiled in July, and the goal is to round up all necessary approvals this fall. But the zoning change must be approved by the Planning Commission and the Board of Supervisors, and now a group billing itself as Save Rincon Park is billing the proposed 40-story tower as the latest example of developers wreaking vengeance on San Francisco and its waterfront.

‘Greedy,’ ‘outrageous’

The opponents include former Mayor Art Agnos, who led the charge to tear down the Embarcadero Freeway after the 1989 earthquake.

“Greedy” and “outrageous” are the least of Agnos’ objections. He brushes aside the 36 percent affordability as irrelevant, saying that if lower-income residents don’t have access to upper-floor units then Tishman Speyer is guilty of “economic segregation.” He also depicts the height as an absolute evil unto itself: “This would cast a shadow on the only park between the Ferry Building and Bay Bridge ... the paltry number of affordable units being offered isn’t worth the loss in (neighborhood) livability.”

But Rincon Park isn’t a sylvan glade; basically, it’s the green punctuation on the stroll from the Ferry Building toward the Bay Bridge — a distance, by the way, of barely half a mile.

This stretch of the Embarcadero is already developed; on the inland side of Rincon Park is the Gap HQ with its 275-foot crown, next to the block-long Hills Plaza complex, a combination of an old coffee factory and a squat tower that tops off at 240 feet. Behind Hills Plaza, the curvy green-glass towers of the Infinity condominium complex are 350 and 400 feet. They’re also where Agnos and other opponents held their kickoff meeting last week.

I don’t question the former mayor’s sincerity. Agnos is as responsible as anyone for clearing the way for the Embarcadero to become an irresistable destination for strollers.

With other opponents, though, the whiff of cynical opportunism is strong. Such as nearby residents of the Infinity who might not want another tower to crowd their precious view (and I do mean precious: a two-bedroom condo on the 23rd floor changed hands in March for $2.4 million). Or folks who never miss a chance to say that Mayor Ed Lee and his administration are in the pocket of developers, even though they couldn’t manage to find anyone to, you know, run against Lee this fall.

Visual distinction

Funny thing is, the contrast between the Infinity’s sleek glass and Hills Plaza’s stocky brick forms shows the virtue of making a clean break between the Embarcadero and the inland city. There’s a bayside realm and then 21st century towers, a visual split that accentuates the urban waterfront. And that distinction will grow more profound in coming years: The Transbay district allows for at least a half-dozen towers that will reach beyond 550 feet, two of them more than twice as high as Gang’s faceted high-rise. What’s proposed at Folsom and Spear streets would be an accent and a transition, not some brooding hulk.

Yes, this means a zoning change. Such changes should be the exception, not the rule. But they make sense in cases where the result would be good urban planning and a deft way to create 140 affordable homes with relatively little fuss. We’ll see what happens next.

John King is The San Francisco Chronicle’s urban design critic, and Place appears on Wednesdays. E-mail: jking@sfchronicle.com Twitter: @johnkingsfchron