Say what you will about architects, the good ones tend to be optimists. For instance, they believe that they can use design to create cozy nooks for everyday people in brash big cities.

Such faith is heartening — and often no match for how cities really work.

I was reminded of this blunt truth while lounging in the publicly accessible ground floor of 505 Brannan St. near Mission Bay, the new headquarters for Pinterest. It’s a sharp-looking building, six stories of large, clear windows within a taut dark grid. The spacious lobby wins points as well: 26 feet high, on full view from the sidewalk, with chic lounge chairs and sofas as well as a small cafe with to-go offerings like goat cheese and walnut salad.

The catch is, nobody walking by would know it’s OK to come in and kick back.

Yes, the architecture firm Heller Manus included glass panels at the northeast corner that can slide back to create an obvious connection to the sidewalk and a linear plaza on the lobby’s east side.

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“We’re trying to give a more expansive indoor-outdoor dynamic to the project by doing this,” Eric Lundquist of Heller Manus told the city’s Planning Commission when the six-story office building was approved in 2014.

But the corner was closed on my visits, with the sliding panels locked in place. That leaves two traditional doors that open and shut facing Brannan Street — so much for the supposed flow between the indoor space and the linear plaza.

This wouldn’t be so bad if there were prominent signs on the building letting passersby know they’re welcome inside.

No such luck.

Unlike the privately owned, publicly accessible spaces sprinkled through the Financial District — mandated by the city and known colloquially as POPOS — semi-public spaces in this part of town aren’t required to have signage. Outdoor signs are called for in the district plan being done for the Central SoMa area, which contains 505 Brannan, but that long-delayed document hasn’t been approved.

The lack of signage stirred concern at the 2014 approval hearing: “I’m not sure how you will manage the perception ... that this is a private lobby, unless you put a sign out there” saying the public indeed was welcome, said one planning commissioner, Kathrin Moore.

“We’d be happy to do that if you felt that was needed,” responded John Kevlin, the attorney for developer TMG Partners. “We really are trying to make this as welcoming as possible.”

When time came for a vote, though, outdoor signs weren’t added to the conditions of approval. TMG brought in Alexandria Real Estate as a development partner. Before construction began, Pinterest signed a lease for the entire building.

Fast-forward to 2018.

If you pause to look inside from the sidewalk, the most obvious feature of the “welcoming” lobby is Pinterest branding. The wall above the security desk features a company logo made from customized push pins. The ebullient red logo also is the sole feature of the blade sign that pops out above the main doorway.

The cafe is called The Point. It offers, yes, “pin-worthy drinks and eats.”

Perhaps all this branding is par for the course in our hyper-commercialized culture. But it’s confusing: Can I go inside if I’m not here on business? And when there’s confusion, people tend to stay away.

Which is too bad. The seating is plush and the sophisticated interior by IwamotoScott Architecture feels smartly of the moment, complete with a taut staircase that leads from the glassed-in mezzanine to the third floor, hovering above the open lobby like an enormous angled Slinky. Nobody hassled me for hanging out. When I asked to use the bathroom, a security guard wearing Pinterest red cheerfully pointed the way.

In other words, 505 Brannan’s “public” space functioned as intended. But I’m not someone who looks like they might be “trouble,” however security guards might define that inherently subjective term.

If the details here are unique, the larger problem is all too common.

San Francisco is lucky to have a range of unusual spaces in commercial buildings that are open for public use during business hours, from rooftop terraces to enclosed retreats. Too often, though, the good design intentions are negated by purposely off-putting management.

The barrier can be a sign-in process that filters out anyone who lacks a photo ID (or doesn’t want to be bothered). Or signage so discreet as to be laughable. Did you know that the Millennium Tower has a public room off Mission Street? It does — a dimly lit space announced by a brushed metal plaque that is 6 inches by 6 inches.

Trust me: The weight of that sign is not why our infamous 58-story tower is tilting.

Across the way, the tower fully leased by Salesforce at 350 Mission St. has movable corner panels just like the eight at 505 Brannan St. They were conceived as the entrance to an “urban room ... blurring the threshold between the public and private realms,” according to a description by the architecture firm Skidmore Owings & Merrill.

Nice idea — but good luck finding them open. Even when they are, the combined clutter of sidewalk barriers and Salesforce’s faux-national-park decor mean they might as well be closed.

The easy answer is to place such spaces outdoors. But urban rooms have advantages, especially in poor weather, if certain elements are included:

•A clear distinction between public room and private lobby.

•Doors that pivot or slide open — and stay that way.

•Large signs that let you know you can come in.

•A variety of seating spaces, so that an individual who wants to relax feels as relaxed as a small group settling in for a meeting.

It’s also good to have an eye-catching but non-branded hook, such as the three vast Frank Stella murals in the atrium of the LinkedIn tower at Second and Howard streets.

As for such spaces as 505 Brannan St. — hidden in plain sight — at least they exist. With luck, the management at some point will give the architecture a chance to deliver on the promise of an inviting public space.

In the meantime? If you’re in the neighborhood on a weekday, stop by. Like I said, the chairs and couches are great.

John King is The San Francisco Chronicle’s urban design critic. Email: jking@sfchronicle.com Twitter: @johnkingsfchron