Look up into the skies of many North American great metropolises and you'll see towers that soar 70, 80, even more than 90 storeys above their sprawling cities.

A residential high-rise, for example, will stretch a staggering 96 storeys into the sky in New York at 432 Park Ave., making it possible to live (almost) in the clouds. In Toronto, at least four mixed-use skyscraper projects of more than 75 floors are under way or have been proposed.

Take the 78-storey Aura condominium. Under construction at Toronto's busy Yonge and Gerrard intersection, it incorporates a major retail component in its podium, as will the soaring 75-floor tower at One Bloor to the north. Or just a short subway ride away, theatre impresario David Mirvish's proposed reimagining of a bustling strip of King Street West that would include three towers of 82, 84 and 86 storeys – designed by world-renowned architect Frank Gehry – to replace low-rise office buildings and the Princess of Wales Theatre.

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Steps from Lake Ontario, developer Pinnacle International is proposing a dramatic new complex at the foot of Toronto's iconic Yonge Street. If approved, the six-building complex designed by Hariri Pontarini Architects will feature a retail and hotel component, an office tower and four residential buildings, the tallest of which would stand 88 storeys.

Ultra-tall, mixed-use towers that offer breathtaking views to attract residents and investors alike allow developers to maximize land use and boost profit margins. It's an opportunity many are embracing wholeheartedly.

But that hasn't always been the case, says Ross Moore, the Vancouver-based director of research for commercial real estate services firm CBRE Canada. Traditionally, developers have been far more comfortable building single-use properties where financing is easier to acquire and prospective tenant groups are more clearly defined.

What has changed?

As Mr. Moore explains, developers became more receptive to adding retail or residential into their office projects as cities such as Toronto amended zoning bylaws to allow for a mix of uses in former single-use blocks, and young professionals began their great urban migration in search of a downtown lifestyle allowing them to work and play just steps from home – all while eliminating their commute.

"A lot of office developers have become way more receptive to throwing retail and residential into the mix," Mr. Moore says. "I think the industry has realized … the better the retail, the more attractive the office space, and then the residential works, as well. Hotels are part of the equation, too."

The push to build higher was a natural next step. But along the way, developers have been forced to navigate a host of practical challenges – everything from designing multiple entrances to accommodate residents, hotel guests, office tenants and retail customers, as well as incorporating mechanical systems to suit each group's divergent needs.

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Then they've had to convince the city that their plans will improve neighbourhoods, and not compromise the often fragile balance of commercial and residential needs that can make or break a downtown district.

According to Toronto's chief planner Jennifer Keesmaat, it's the reason why building bigger isn't always better. While supertall towers have the potential to boost density to maximize transit usage or strengthen a city's property tax base, Ms. Keesmaat points out they can also put huge strains on its infrastructure – from energy and water consumption, to green space and sidewalk loads.

"Tall buildings have a greater civic obligation as they have greater potential for good and harm," Ms. Keesmaat says. "The sites where we can accommodate these developments are rare. We shouldn't plunk an 80-storey building into a background site because the architect shows up with a fancy drawing."

In Toronto, as in many other Canadian cities, developers are required to contribute funds to pay for upgrades to infrastructure such as water or hydroelectric power to accommodate their projects, and are encouraged to produce designs focused on meshing their buildings with the surrounding streetscape to boost their chances of gaining approval. That would explain the current drive to incorporate retail-driven podiums at the base of many new commercial or residential buildings across the city.

"We want to create mixed-use podiums so there's liveliness and activity at all times of the day," Ms. Keesmaat says. "In an urban environment it's always better to have more people on the street. It makes it safer and more vibrant."

She adds that the city has also placed an increasing emphasis on ensuring ample green space around taller towers, which typically house thousands of residents, while enforcing a strict performance-based assessment approach to measure a project's impact on sidewalk space and even the shadowing effect on neighbouring buildings.

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"Getting that infrastructure right is a fundamental part of the assessment and review process any time we're shown plans for one of these big, sexy projects," Ms. Keesmaat stresses. "The rendering might look great, but if people can't get on transit or the toilets don't flush, they won't work."

Toronto's official planning has also focused on constructing mid-rise buildings of 12 storeys or less on its avenues, which Ms. Keesmaat says place far less strain on critical city infrastructure, while still delivering the increased densification the city demands.

Paul Bedford agrees with the push for increased mixed-use construction, but isn't convinced that Toronto has the infrastructure in place to support towers the size of the proposed Mirvish project or One Yonge.

The city has yet to make necessary investments to make mixed-use megaprojects feasible in the short-term across most neighbourhoods, says the city's former chief planner and current adjunct professor of city and regional planning at the University of Toronto and Ryerson University.

"I'm not saying it's wrong to embrace the supertowers in key locations, but if you want to go down that road, you need the public infrastructure in place or planned to make them work, and I think Toronto still has to bite that bullet," Mr. Bedford says.

Despite the challenges, David Pontarini, the lead architect on the proposed One Yonge project, feels that as Toronto's downtown continues to gain density, city officials, residents and even other developers will come to appreciate how large-scale towers can improve the urban fabric.

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He predicts the push to build much of that necessary infrastructure, particularly public transit, will become a top priority as many of these projects near completion.

"You need the political will [to build infrastructure,]" Mr. Pontarini says. "I think the city will continue to approve these projects, they'll get built and I think the transit will follow. I think there's already pressure on transit even without these buildings because the city hasn't found political will to transition away from the car. The pressure needs to be put in place to make the push from cars to transit."

Tall and taller

A selection of some of the world's tallest mixed-use towers that have either recently opened or are currently under construction.

Burj Khalifa: Located in Dubai, the world's tallest tower soars a whopping 829.8 metres in the air and boasts more than 160 storeys.

432 Park Ave.: Manhattan's newest skyscraper, now under construction, will feature 96 storeys and is already being touted as the world's tallest residential building.

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The Shard: The 72-floor London skyscraper – Western Europe's tallest building – has redefined the city's skyline with its dynamic design housing hotel, office, residential and retail space.

Shanghai World Financial Center: Completed in 2008, the 492-metre, 101-storey tower features office and hotel space.

Trump International Hotel and Tower: Gracing the Chicago skyline since 2009, the 92-floor mixed-use tower is home to a hotel and exclusive residences.