Prime Minister Justin Trudeau released a $40-billion national housing program on Wednesday, but the plan counts on the provinces to contribute billions and key elements won't begin until after the next federal election.

The 10-year program assumes the provinces will be willing to match federal spending plans in some areas, meaning further negotiations will be needed before the details are worked out. Quebec in particular says it wants to hammer out its own agreement with Ottawa.

The main new initiative announced on Wednesday is a $4-billion Canada Housing Benefit, which would provide rent support for about 300,000 low-income households and would begin in 2020. Ottawa expects the provinces to cover half of the cost.

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Ottawa is also responding to one of the most pressing concerns raised by Canada's cities, offering $4.8-billion to address the fact that many long-standing social-housing agreements with Ottawa were scheduled to expire over the coming years.

Standing in front a construction site where work is under way on a new mixed-income redevelopment of the Lawrence Heights public-housing complex in northwest Toronto, the Prime Minister said he was confident the provinces would endorse and help pay for what he called his "once-in-a-generation" housing plan.

"What we have heard from our provincial partners here in Ontario and across the country, our municipal partners, is a level of excitement and a level of commitment to getting this done," Mr. Trudeau told reporters.

But even Ontario Housing Minister Peter Milczyn, who was invited to the same podium to praise Mr. Tru‎deau's announcement, would not firmly commit to new spending on Wednesday.

"There is more money here on the table from them, so obviously, now that there is a partnership to be had, we'll be working with them," Mr. Milczyn said in an interview.

The Federation of Canadian Municipalities praised the announcement as a "breakthrough" for Canadian cities pushing for help in dealing with a shortage of affordable housing.

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The housing plan provides details as to how Ottawa will spend the $11.2-billion over 11 years that was announced in the March budget. The budget had pledged to address concerns over expiring social-housing contracts, but the government had not announced a dollar figure for that pledge until Wednesday. Other parts of the strategy are paid for out of funding for housing that had previously been in place.

Some observers questioned why the program was limited to low-income Canadians and does not address the fact that many other Canadians are also struggling to find an affordable home.

"I thought it would be much more encompassing," Conservative MP Karen Vecchio said. "We hear this government talk a lot about the middle class. What is this doing to help the middle class? All it's doing is providing government subsidies, but it's not providing a plan for the future."

Jeremy Kronick, a senior policy analyst with the C.D. Howe Institute, expressed similar concern that the program does not address the broader housing pressures in Canada's largest cities.

"This is about targeting low-income and homelessness," he said. "These are noble goals and ones to be proud of, but these measures do not deal with middle-class affordability issues in our major cities."

Some provinces provided a positive initial reaction, but cautioned that many details need to be worked out and understood.

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B.C. Premier John Horgan said federal support for social housing will help his province focus on broader affordability issues. He promised more provincial action on that front when his government releases its February budget.

"We need to bring on more housing and it needs to be not just one-bedroom apartments in the sky," he said. "We need to build houses and homes for families and that means two- and three-bedroom units. That means building density around transportation corridors."

Alberta Minister of Seniors and Housing Lori Sigurdson said in an interview that she welcomed the announcement and was not surprised by the focus on cost-sharing between governments.

"There's lots more work to be done to understand it all, but I think it's a step in the right direction," she said in an interview.

The Quebec government saluted the federal commitment of funds but wants to negotiate a funding agreement outside the national program so the province can continue to run housing, said Lise Thériault, the Quebec minister in charge of housing.

She said her province will want a "bilateral and asymmetric agreement to support our programs and objectives. We're ready to sit down now."

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"We want to remain the project manager for housing on our territory," Ms. Thériault said in an interview. "The federal government won't dictate priorities to Quebec."

Federal documents released on Wednesday state that the goal is to promote diverse communities with a mix of incomes and uses that are near transit, work, grocery stores and public services.

Brent Toderian, a consultant and former chief planner for the city of Vancouver, said those are laudable goals, but that it is not immediately clear how Ottawa can influence decisions that are ultimately in the hands of provinces and municipalities.

Mr. Toderian said the plan could be positive if it connects with other federal programs such as transit and climate-change funding to encourage more neighbourhood density and reduce car dependence.

"When you start to put these programs together, you start to see a city-building momentum. And that's a good thing," he said. "I think there's a series of moves, including this housing strategy, that show the federal government understands the challenges of cities, and that's a nice change."

Many housing advocates welcomed the federal government's commitment to a new portable-housing benefit. B.C. already has similar program. So does Ontario, but it is currently quite small, funded by the provincial and federal governments and administered by municipalities.

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In Toronto, for example, about 4,000 people now receive housing supplements that range from $250 to $400 a month to help them find a home amid the city's skyrocketing private market rents.

"It gives people a little more purchasing power, a few more options in where they choose their housing," said Greg Suttor, a former adviser to Ontario's Housing Ministry who is now a researcher with the Wellesley Institute, an urban-health think tank in Toronto. "I am not saying it is transformative, but it helps."

Housing and anti-poverty advocates have criticized the lack of a large federal role in the construction of new social housing since the early 1990s – a hole the federal Liberals hoped to fill with their long-awaited plan.

But activists in several cities across the country, including those with the Ontario Coalition Against Poverty in Toronto, staged demonstrations and marches on Wednesday to warn that federal money is spread too thin over too many years to address urgent housing needs.

With report from Justine Hunter in Victoria and Les Perreaux in Montreal