Three of the largest developments in Chicago history could be under construction by the end of this year, potentially adding more new real estate to the city than if New York’s Hudson Yards — the largest private development in U.S. history — were built and then built again in the Midwestern city.

Years in the making, the Chicago megadevelopments have broken ground or moved close to doing so during one of the trickiest times the city’s developers have ever faced.

Sterling Bay last year began construction of the first building in its $6 billion Lincoln Yards project along the Lincoln Park and Bucktown neighborhoods on the city’s North Side, while a team including Farpoint Development said it plans to kick off the $7 billion Bronzeville Lakefront project south of the Loop business district near McCormick Place, the largest U.S. convention center, later this year. Related Midwest is expected to soon follow with construction of The 78, its $7 billion vision for riverfront land between the South Loop and Chinatown.

Combined, the three projects could add as many as 27,000 apartment units and 20.7 million square feet of office space to the area in and around downtown Chicago over the next 10 to 20 years.

For the developers — and the city center as a whole — to pull it off without a major disruption to the real estate industry probably will require a massive creation of jobs and a continued residential boom downtown, where the number of apartment units already has more than doubled over the past decade.

The historically ambitious pursuit comes nearly two years into COVID-19, paired with concerns that existed in Chicago before the pandemic, including crime and increasing property taxes.

These new mini-cities are set to go up during a time of already tenuous prospects for longtime Chicago strongholds such as the Magnificent Mile shopping district and the historical financial center on and around LaSalle Street.

Chicago, known for its diverse economy, may need a major new jobs driver — two of the megadevelopments are focused on pulling in lab and office jobs from the life science industry — for demand to match the fast-rising supply of buildings.

“If Chicago doesn’t expand the labor pool, I don’t see how the city can take on another 27,000 units and all of this office stock,” said Brandon Svec, CoStar’s national director of retail analytics. “There just doesn’t seem to be enough demand for the [central business district] to heal itself while bringing on all of this new product.”

Combined, the three Chicago projects could have more than 41 million square feet of space across about 217 acres, costing about $20 billion to develop, according to information from the developers and CoStar estimates.

By comparison, New York’s ongoing Hudson Yards project is 28 acres and is expected to cost $25 billion, making it the most expensive single private real estate development in U.S. history. There will be more than 18 million square feet of new buildings when Hudson Yards is completed by 2025.

If the three Chicago projects reach their full planned size in the next one to two decades, it would balloon the amount of real estate within the area of the city between the Kennedy Expressway to the west, Lake Michigan to the east, Webster Avenue to the north, and 31st Street to the south.

The 78, Bronzeville Lakefront and Lincoln Yards could combine to create a 10.9% increase in office space compared with existing and under-construction buildings, even if no other properties were built, according to a CoStar analysis. The effects on the apartment market would be far greater, with the three projects pushing up supply 35.5%.

CoStar’s analysis only includes three multibillion-dollar projects under construction or likely to break ground within the next year, all of which are likely to take more than a decade to build.

Projects such as Wisconsin developer Bob Dunn’s proposed One Central near Soldier Field were not included in the CoStar analysis because they do not have zoning approval and could be years from breaking ground.

The analysis also does not include other unusually large — but not multibillion-dollar and multidecade — projects such as Onni Group’s plan to build 2,650 apartments on Goose Island and JDL Development’s plan to build 2,680 residential units east of there on the Near North Side.

The construction pace will probably depend on several factors, including demand from tenants and the state of the economy.

Chicago’s upcoming decision on a casino site could directly or indirectly affect two of the plans, since one proposal would use part of The 78 site and another would take some of the land eyed by Bronzeville Lakefront’s developers for an expansion of the former Michael Reese Hospital site.

Megadevelopments are taking shape more than a decade into a Chicago construction cycle that has brought waves of corporate headquarters moves from the suburbs and other cities. Highlights include Fulton Market’s transformation from meatpacking district to the fastest-growing urban office market in the United States, the $1 billion-plus redevelopment of the Old Post Office, the 101-story St. Regis Chicago’s completion as the city’s third-tallest skyscraper, a $500 million expansion to the lower floors of the 110-story Willis Tower and completion of the tallest office skyscraper in three decades, the 55-story Bank of America Tower.

As the skyline has changed and expanded farther from the Loop, the rate of high-profile corporate relocations has slowed in recent years, after a run of attracting headquarters such as McDonald’s, Kraft Heinz, United Airlines, Motorola Mobility, Conagra Brands, Archer Daniels Midland, Mondelez International, and Motorola Solutions.

Developers of Lincoln Yards and Bronzeville Lakefront are focused on finding jobs growth, and thus increased demand for real estate, from the life science sector. The 78’s key driver could be a university-led research center.

“To fill all this space, these developers will need to look beyond LaSalle Street and (suburban) Oak Brook,” Svec said. “Chicago will need to find its niche, and I don’t think we’ve found it yet with life sciences.

“I think a lot of that new office space will need to be lab space or some other use because downtown we already have enough traditional office space.”

Developers and local leaders have been pushing to attract life sciences companies from markets such as Boston, San Francisco and San Diego where costly and difficult-to-build lab space is in short supply.

There is a crowded field of cities trying to lure those high-paying jobs, buoyed by growth in jobs and medical research funding, an aging population, and strong interest from both tenants and investors. Because most research must be performed in labs, the sector also is viewed as relatively immune to work-from-home trends.

Among 12 major lab clusters identified for a CoStar white paper on the industry, Chicago currently is in the third and lowest tier for categories such as lab market share and funding. Because labs tend to cluster together in relatively small areas, Chicago will need to move up that list for sites such as Lincoln Yards and Bronzeville Lakefront to gain critical mass in the biotech world.

Another barrier for the likes of Sterling Bay and Farpoint is that the sector is dominated by the three largest landlords, which combined own nearly half of the total U.S. space, according to the CoStar report.

Like many markets, the Chicago area remains near historic highs for available office space, with the timeline uncertain for a full or nearly full return to pre-pandemic levels of in-person work downtown.

Another byproduct of the pandemic could confound developers of new buildings, too.

Managers of major projects may need to navigate ongoing shipping backlogs and high material costs, said Etienne Nel, Chicago managing director of construction consultancy Cumming.

“We’re going to see serious pressure on supply chains in 2022,” Nel said. “That’s the scenario these projects are starting under. If you’re doing a project of that size, you’re going to have to think outside the box.”

Availability of the three massive sites could be an advantage for Chicago in attracting new businesses, according to real estate professionals.

Former Mayor Rahm Emanuel’s administration made it a top priority to move the long-languishing sites toward development, said Steve Friedman, president of Chicago-based SB Friedman Development Advisors.

That included committing public funds for infrastructure and changing land-use policy along the Chicago River to shift the Lincoln Yards site away from industrial uses. Friedman has represented the city in creating public-private partnerships for the infrastructure to support the three new projects, including setting up tax increment financing and creating a fee structure for developers to obtain zoning changes to build high-rises in a former planned manufacturing district along the North Branch of the river.

“There’s nothing in the history of Chicago development that was so lacking in infrastructure except maybe Illinois Center in the 1960s,” Friedman said, referring to skyscrapers built on a massive platform over rail lines along Michigan Avenue.

“One of Chicago’s advantages is that downtown is surrounded by vacant land, but it was vacant land that wasn’t anywhere near ready for development because there was no infrastructure,” he said.

The rise of Fulton Market and continued growth of the downtown population also are hopeful signs for the developers. Last year there were 6,954 more apartment vacancies filled than created, according to CoStar data, shattering the yearly downtown absorption record.

Challenges of the pandemic are an opportunity for office developers, said JLL international director Meredith O’Connor, an office tenant broker and site selection expert who has worked on headquarters relocations for companies including Toyota North America, Mercedez Benz and Conagra.

“Big companies that are in the market are all considering these sites,” she said of the megadevelopments. “It’s a matter of when, not if.”

Since COVID-19, office tours have evolved to sometimes include a building’s experts on ventilation and technology to answer questions from prospective tenants, she said. Looking to differentiate themselves and attract workers back to the office, many companies are focused less on being in the center of the business district than creating a dynamic workplace, O’Connor said.

“What I’m seeing across all markets but specifically Chicago is flight to quality developments like these,” O’Connor said. “Companies don’t want to make a real estate decision for the long term unless it’s something very special.”

Here are detailed summaries of the three megadevelopments planned for Chicago:

Developer Sterling Bay spent years buying former industrial properties along the Chicago River such as the former A. Finkl & Sons steel plant and eventually winning city approval for its mixed-use project in 2019.

Lincoln Yards became the first of the three projects to break ground, with work starting in October on an eight-story life science research and office building at 1229 W. Concord Place on the southern end of the site.

In December, Sterling Bay said it plans to have that structure and another office building completed by 2025, along with three residential towers, retail and entertainment space.

Zoning allows for towers as tall as 595 feet within the site, which runs along both sides of the river between Webster and North avenues.

There also are plans for parks and an extension of the Riverwalk, new roads and bridges, a relocated Metra train station, water taxi access and an extension of the 606 elevated trail into Lincoln Yards from the 2.7-mile recreational path’s existing trailhead to the west.

Sterling Bay and partners Harrison Street and J.P. Morgan Global Alternatives want to create a scientific innovation district within the mixed-use development. They are courting life sciences companies outside the Chicago area, as well as startups it hopes will develop in an existing medical lab building the developer acquired not far from the Lincoln Yards site.

Residential demand already exists in the area that is surrounded by multimillion-dollar homes, shops, restaurants and other amenities.

Related Midwest was thinking big from the start, picking a name that implies a half-mile-long riverfront parcel between the South Loop and Chinatown will one day be recognized as Chicago’s 78th neighborhood.

The developer, an affiliate of Related Cos., plans a mix of low-rise building and skyscrapers as tall as 950 feet filled with offices, apartments, hotel rooms, restaurants, retail, and the University of Illinois-led Discovery Partners Institute research center.

The project won city zoning approval in 2019, but it has been delayed both by the pandemic and the city’s expected decision on where to put a long-planned casino. One proposal, in partnership with Rush Street Gaming, would dedicate part of the site to a casino and related amenities, including a concert venue, hotel and a more than 1,000-foot-tall observation tower.

The plan also includes adding a half-mile of Riverwalk and other park space, a new CTA Red Line train station, water taxi stops, and roads, including the long-planned Wells Street extension to Chinatown that’s already under construction through the site.

Once a rail yard, the site has sat unused for decades. Part of the site is land created when a winding stretch of the river was straightened in the 1920s to accommodate barge traffic. A 7-acre park is planned for that area.

The site is bordered by Roosevelt Road, Clark Street, the river, and Ping Tom Park in Chinatown.

The 49-acre former Michael Reese Hospital site has sat vacant since the hospital closed in 2008 and the city demolished most of the buildings a year later.

The site was proposed as the Olympic Village for Chicago’s bid to land the 2016 Summer Games and for Amazon’s second headquarters, the so-called HQ2, which landed in suburban Washington, D.C.

Now, a group of developers led by Chicago’s Farpoint Development said it plans to begin building infrastructure such as roads in the next few months, with construction of a 500,000-square-foot life science innovation center to begin later this year.

Chicago ARC, a life science accelerator developed in partnership with Israel’s Sheba Medical Center and Kaleidoscope Health Ventures, will be housed in that building. After that, plans in the initial phase include senior housing and a data center.

The project’s developers plan to expand the site to about 100 acres by acquiring additional land, including potentially building over train tracks and buying marshaling yards used for setting up large conventions at neighboring McCormick Place.

The development venture has zoning approval for more than 7 million square feet of buildings. It would need further city approvals to build millions of square feet of additional space.

The site runs along Lake Shore Drive roughly between the sprawling convention center and 31st Street.

Singer Pavilion, built in the 1940s, is the last remaining structure from the hospital campus. It will be renovated during the first construction phase.

Plans also include adding retail on Cottage Grove Avenue and building a new Metra Electric line train station at the south end of the site.