A 425-square-foot “micro-loft” on Manhattan’s Upper West Side. Weylin Seymour’s new event space at the foot of the Williamsburg Bridge. The Barbarian Group’s Chelsea office, with an undulating “superdesk” that seats the entire staff. The United Nations campus along the East River, fresh from a $2 billion, seven-year renovation.

Those are just a few of the architectural treasures that will be revealed during Open House New York (OHNY) Weekend, the annual event that gives participants a chance to tour significant buildings and cultural sites that usually are not open to the public—and in the process helps promote quality design.

Organizers say they have put together the most ambitious lineup ever for the 12th annual OHNY event, which will take place from October 10 to 12, making it a three-day event for the first time.

“The audience keeps growing,“ said Gregory Wessner, executive director of OHNY. “There’s an enormous interest on the part of the public to explore places they read about in the newspaper or online. We’re trying to keep up with that. But it’s not just a numbers game. It’s an opportunity to have a citywide conversation about architecture and development.“

New York is hardly the only city where owners are opening the doors to rarely seen landmarks.

Since the first Open House event was held in the early 1990s, more than 20 cities have become part of the network known as Open House Worldwide, which mounts what it calls a “global architectural festival.” In each city, the sponsor must be a non-profit organization, and most buildings must be open free of charge. According to the Open House Worldwide website, each program must exhibit at least 70 buildings of “outstanding quality,” at least 20 percent of the buildings must be contemporary, and each participating city must have a population of at least 250,000.

A second series of events, usually under the ‘Doors Open…’ label, has taken place for more than a decade. The Doors Open program also requires that sponsors be non-profits and that buildings be open free of charge. But it does not require as many buildings to be on exhibit in a given year, which allows smaller cities to show off their assets.

In both cases, the programs have seen steady growth. The Open House program started with Open House London, founded in 1992 by director Victoria Thornton, with the goal of fostering better understanding of architecture and the built environment outside the profession. The Open House program first had events in New York in 2003 and Chicago in 2011. The fourth annual Open House Chicago event will be held October 18 and 19 and feature 150 buildings. Other Open House cities include Dublin, Tel Aviv, Jerusalem, Helsinki, Melbourne, Galway, Barcelona, Rome, Lisbon, Perth, Adelaide, Brisbane, Thessaloniki, Limerick, and Buenos Aires. New for 2014 are Athens, Vienna, and Oslo. New for 2015, Prague, Monterrey, Nicosia, Cork, Gdynia, and Belfast.

Doors Open events have been held for more than a decade. Early participating cities were Toronto and Denver. Other Doors Open cities include Lowell, Massachusetts; Niagara, New York; Ontario, Canada; Milwaukee, Wisconsin, and, for the first time this year, Baltimore, Maryland.

Growth comes in many ways, but it is often triggered by an architect moving from one city to another. In Baltimore‘s case, the impetus came from lead organizer Chelsea Thomas, an architect who worked in Denver for six years and took part in Doors Open Denver events. When she moved east to take a job in Baltimore, she began working with the local AIA chapter and the Baltimore Architecture Foundation to plan a Doors Open Baltimore event. Scheduled for October 25 as part of Baltimore Architecture Month festivities, it will feature at least 40 buildings, all works of industrial architecture.

“We all loved the event so much that I thought Baltimore could use something like it,” said Thomas. “It makes you see your surroundings in a whole new light. It helps people get comfortable visiting new places and neighborhoods they might not otherwise go to. It helps you know more about your city.”

It all goes back to Thornton’s vision: “Experiencing architecture in the flesh,” she said on the Open House Worldwide website, “helps everyone become more knowledgeable, engage in dialogue, and make informed judgments on the buildings, places, and neighborhoods where we all live, work, and play.”