A spectre is haunting Toronto – the spectre of hyperdensity. Jennifer Keesmaat, the city's dynamic chief planner, worries about it. So does one of Toronto's smartest local politicians, city councillor Adam Vaughan.

What is this thing and why are they so frightened of it? For years, after all, city planners have been preaching the virtues of density. After decades of seeing cities sprawl, creating vast, car-dependent, expensive-to-maintain suburbs, they came to see that dense, bustling cities work better.

Transit becomes more efficient when there are enough riders to justify frequent service. Streets are safer with lots of people on them day and night. The environment wins when people can walk instead of drive and don't need to heat big suburban houses.

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With all that in mind, the city's Official Plan seeks to direct new development – office buildings, condo towers and so on – to key areas of the city, fostering the process known in planners' jargon as intensification. The aim is to put new buildings on about a quarter of the city's geographical area, keeping the three-quarters that is left – residential neighbourhoods, quiet, smaller streets – free from runaway growth.

As anyone can see from the thickets of development around nodes like Union Station or Yonge and Eglinton, it has been remarkably successful – too successful for some. "We have reached this exciting and terrifying tipping point where we are starting to question whether it could be there is something called too much density," Ms. Keesmaat said. "There are some areas of the city where we are seeing too much density – hyperdensity – and there are other areas of the city where we are seeing no growth at all."

She worries that transit, parks and other necessities of city living won't keep up with the growth. Mr. Vaughan, who represents a downtown district where new buildings are rising left and right, says he shares the concern. He drops another word for it: vertical sprawl.

In a little-noticed move in October with potential to change the future shape of the city, he persuaded city council's planning and growth-management committee to ask city staff to find a way of sounding the alarm when an area approaches hyperdensity. At that point, the city could tell developers, "This neighbourhood is at capacity. Go somewhere else," Mr. Vaughan said at the time.

"Take a look at the bones of a neighbourhood. What's the body mass it can support?" he said. "If you just add people for the sake of people or buildings for the sake of buildings, are you in fact building a livable city?"

Worries about hyperdensity may strike a chord in a city where condo hatred and fear of height is on the rise. But, as both Mr. Vaughan and Ms. Keesmaat readily concede, this is a nice problem to have. Many other cities would kill for the downtown building boom that Toronto has enjoyed.

In the second quarter of this year, 92 per cent of the office construction in Greater Toronto was in downtown Toronto – a sharp turnaround from the days when businesses fled to the cheaper rents and lower taxes of the suburbs. City planners report that downtown is living through an office-construction rush reminiscent of the late 1980s. People are thronging to live downtown, too, attracting new businesses to serve them and bringing new vitality to the streets.

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This should be cause for celebration. The city's plans for smart density are working. Since the Official Plan took effect in 2006, 82 per cent of proposed residential units have been going just where planners want them: in the downtown; in other designated growth nodes such as Yonge-Eglinton and North York Centre; in mixed-use areas; and along the so-called Avenues – big, main streets such as Bay or Queen that can absorb growth.

If you fear that Toronto is overbuilt, just ride the elevator to the top of the CN Tower and look down. The dense, high-rise parts of the city are only small woodlots in the vast plain of low-density and small buildings that is modern Toronto. Even downtown, seen from this height, still has lots of gaps – underused space taken up by parking lots or small buildings.

If the hyperdensity tag catches on, it could become a useful tool for downtown councillors who want to appease their constituents by blocking new development or for suburban councillors who want to steer more development to their wards even if there is no call for it there. It could also help kill exciting projects like the Frank Gehry-designed proposal by David Mirvish for King Street West. Ms. Keesmaat's planning staff oppose the plan for three towers of more than 80 storeys each – too tall, too dense – and city council backed her up in a vote on Dec. 18.

It is reasonable to worry that new development will cause overcrowding on transit or overtax other city infrastructure. But if that is the concern, let's build better transit to keep up with the growth, not halt the growth for fear of the future. Central Toronto is still far less dense than it could or should be. Hyperdensity should be a goal, not a thing to fear.