Toronto mayoral challenger Jennifer Keesmaat is pledging to build 100,000 new affordable housing units over the next decade, arguing Mayor John Tory has failed to do enough to keep rents in the city from skyrocketing.

While she promised her plan would end the city’s affordability problem within a generation, the first formal campaign announcement from the city’s outspoken former chief planner was scant on details. Ms. Keesmaat said rents had shot up while vacancy rates sank to a 15-year low on Mr. Tory’s watch. She said the mayor had a “timid and reactive” approach and had failed to “use the tools available” to create more purpose-built affordable rental housing. She said Mr. Tory “sat on one of North America’s largest real estate portfolios” instead of using city-owned land for affordable housing.

“In the last four years, under John Tory, has housing affordability gotten better for you and your family, or has it gotten worse?” Ms. Keesmaat asked. “Let’s look at the record: Rental housing in Toronto is at a crisis point, both in supply and in price.”

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Mr. Tory’s campaign was quick to punch back, saying that during her five years as chief planner, Ms. Keesmaat never raised her goal of 100,000 new affordable homes over 10 years. In an e-mailed statement, the Tory campaign also said the city hadn’t received a single application from her Creative Housing Society, an affordable housing project where she was named chief executive officer after leaving the chief planner’s job last year. (The program’s founder, Vancouver developer Ian Gillespie, points out the initiative was still in its early stages.)

The Tory campaign boasted that last year the city met and exceeded its long-standing goal of approving more than 1,000 affordable-housing units a year for the first time. At city council, another 1,650 units were approved for this year, beating the target for the second year in a row.

Ms. Keesmaat said more details on her plan would be released later this week. But she said her vision includes making faster use of city-owned land. She also said she would aim to create new units whose designation as affordable does not expire after 25 or 30 years, as is the case with many developments.

Mr. Tory has also made affordable housing a focus for his campaign, pledging to accelerate efforts and build 40,000 units over the next 12 years – or about 3,500 a year, compared with Ms. Keesmaat’s promised 10,000 a year.

City councillor Ana Bailao, Mr. Tory’s housing advocate, called his plan more realistic: “These are complex issues. I think it is definitely the most important issue in the city, to create a healthy city. But the tools [Ms. Keesmaat] presented are being used.”

Ms. Bailao said money from the federal government’s national housing strategy will allow the city to do more. But Toronto’s Open Door incentive program already waives the fees normally charged to developers while putting up city lands for affordable housing projects, she said.

Ms. Bailao also points out that in total, the entire building industry in Toronto only completes an average of about 18,000 residential units a year, affordable or otherwise. (Last year, the industry built only 8,725 units.) Under Ms. Keesmaat’s plan, more than half of all residential construction – 10,000 units – would have to be affordable housing.

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Affordable housing generally refers to offering rental units at 80 per cent of the market rate, a level critics say remains out of reach for many in Toronto. It is distinct from social housing, which is more heavily subsidized by the government.

With a report from Alex Bozikovic