Canada's big-city mayors are expressing optimism that the 2017 federal budget will pledge billions for affordable housing after a private meeting with Prime Minister Justin Trudeau.

The Federation of Canadian Municipalities has asked Ottawa to devote $12.6-billion of the $20-billion already pledged for "social" infrastructure to social and affordable housing.

The request was initially met with skepticism late last year from federal officials, including the head of the Canada Mortgage and Housing Corp., the federal housing authority. However, Calgary Mayor Naheed Nenshi said that the mayors received positive signals Friday morning on the request for $12.6-billion.

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"I was very, very, very happy to hear the Prime Minister say today that their investments in social housing, they think, will come in close to what the FCM is asking for. If that's actually true, that will be an unprecedented investment in affordable housing across the country," he said.

In Ontario alone, 171,360 households are waiting for rent-geared-to-income housing, according to the most recent data released by the Ontario Non-Profit Housing Association.

The Liberal Party platform promised new money for the construction of new social housing, as well as money for renovating existing buildings. The party also promised to remove the GST on new capital investments in affordable rental housing, but has not yet confirmed such a policy since winning the 2015 election. The GST clause is part of the FCM's prebudget request to Ottawa.

Several big-city mayors were in Ottawa Friday to make a final pitch before Finance Minister Bill Morneau releases his 2017 budget, which could come as early as next month. Mr. Morneau's fall update already provided the government's high-level spending plan for infrastructure, which promises $186.7-billion over 12 years.

The FCM is now trying to make the case for how the money should be allotted to various priorities and what formulas should be used for spreading the spending across the country. Municipalities want Ottawa to commit to funding 50 per cent of all major infrastructure projects, with provinces paying one-third and municipalities covering the rest.

The Liberal government had agreed to a similar formula for what it called the first phase of infrastructure spending, but has not yet committed to a formula for the much-larger second phase. Traditionally, the cost of big projects has often been split evenly between the three orders of government.

In addition to discussing issues such as transit and affordable housing, mayors spent most of Friday talking about their experiences in dealing with the growing crisis of opioid overdoses.

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"One of the things we did also discuss was that housing, for example, plays a critical role in responding to the opioid crisis," said Edmonton Mayor Don Iveson.