Developers who say they have purchased the air rights over the very downtown rail corridor where Mayor John Tory is planning to build his massive $1-billion "rail-deck" park are warning they won't roll over without a fight.

Architect Dermot Sweeny and a firm called Craft Development Corp. want to line the site with condos, offices and retail, and include a park that would be less than half the size of the one in Mr. Tory's plan. But they say their vision would also mean a much cheaper price tag for the city.

Craft chief executive officer Peter Griffis said he plans to have a development application "formalized and ready to go" for the site by the end of November, despite the city's intention to rezone the entire area for a park. He warned the battle would end up at the Ontario Municipal Board.

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"I am quite comfortable … with my position. The city is going to do what they want. This is political, that you can see," Mr. Griffis said in an interview.

Toronto City Council, which is meeting on Wednesday, is set to debate whether to spend $2.4-million studying how to pull off Mr. Tory's plan to bring 8.5 hectares of parkland to park-starved downtown condo-dwellers by building a platform over the railways that cut across the city's centre. Craft's alternative plan is expected to gain some traction with some councillors who question the expensive rail-deck plan.

The city's chief planner, Jennifer Keesmaat, has previously dismissed the alternative plans, saying a win for the city in any OMB battle would be a "no-brainer." Ms. Keesmaat also told The Globe last month that the city had no confirmation the developers had even purchased the air rights from the railways.

To address this claim, Mr. Griffis instructed his lawyers to send The Globe two short letters provided to city planners and dated Sept. 15. One is from a lawyer for Craft, the other from a lawyer for Canadian National Railway Co. and the Toronto Terminals Railway Co. Ltd., the company co-owned by CN and Canadian Pacific Railway Ltd. that controls the tracks around Union Station.

The letters say that Craft entered into a "binding and enforceable" agreement of purchase and sale in 2013 for "air space above the rail, south of Front Street West, between Bathurst Street and Blue Jays Way."

The letters do not say if the deal has closed. The letters also do not say whether the deal is conditional on the land actually being zoned for development or for a certain density – a common clause in such deals. Mr. Griffis would not reveal any other details on Tuesday. Mr. Sweeny said the deal had not yet closed.

Local city councillor Joe Cressy, a booster of the city's rail-deck park concept, said he and city staff had seen those letters but that they lack necessary details. The developers themselves, he said, told him when they met earlier this year that the deal was "conditional." Regardless, he said, the city will still need to discuss terms with whoever actually holds the air rights as it proceeds with its plans to build the park.

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"There are developers who have different visions of the rail corridor than Mayor Tory and I," Mr. Cressy said. "Rather than a park, they would like to see more condo towers. However, the mayor and I are determined to build a new park."

A spokeswoman for the mayor, Amanda Galbraith, said the city is within its rights to rezone the corridor for parkland, regardless of any air-rights deal: "It is important to understand that it's fully within the city's jurisdiction to determine the appropriate use of land. And the appropriate use of that land as recommended by city staff with the full support of the Mayor is a park."

According to its website, Craft has not attempted a project approaching this magnitude in the past: It has only nine smaller developments in Toronto, but has built plazas and retail developments in smaller cities around Southern Ontario, including Kitchener and Keswick.

Mr. Sweeny, an architect whose buildings include the Queen Richmond Centre West near Queen Street West and Spadina Avenue, says allowing developers into the mix might actually see some sort of rail-deck park become a reality, rather than languish without funding.

"We could achieve eight acres [3.2 ha] of fantastic downtown fabulous parkland, at a tiny fraction of the cost," Mr. Sweeny said.