Boston Properties is considering transforming a parking garage in Boston’s Back Bay into a new office tower, which would help recreate a major city intersection that sits at a crossroads of the Massachusetts Turnpike and the MBTA.

The local developer has started down the long road to building a new skyscraper on the site of the 2,000-car garage at 100 Clarendon St. in the city’s Back Bay neighborhood.

While it is still early in the process, some facts are known about any potential development at the site. About a third of the garage’s footprint is atop actual terra firma; the remainder sits above the Massachusetts Turnpike. If a project were built above the Pike, the developer would need to build very expensive decking strong enough to support a big office building.

But "big" would likely be an understatement. The office tower would probably need to be upwards of 40 to 50 stories tall to house the number of tenants needed for the rent to support the project’s cost, especially if the entire garage were demolished and that revenue lost, according to one development consultant that works on similar projects in the city and asked for anonymity.

Those kinds of "air rights" projects above the Turnpike are not always welcomed with open arms, especially in the Back Bay. Just ask anyone that watched the WinnCompanies’ failed attempt to push the Columbus Center mega-development air rights project through the local review process. Columbus Center would have spanned the Turnpike adjacent to where Boston Properties wants to build the office tower.

Boston Properties has not yet filed official plans with the Boston Redevelopment Authority (BRA), the agency in charge of land use within the city limits. But the property owner has discussed preliminary plans for an office tower with ground-floor retail at the site with BRA officials, according to business executives familiar with the proceedings. For their part, Melina Schuler, spokesperson for the BRA, wrote in an email that she did not "have any info" on the topic. Boston Properties did not return calls seeking comment.