Another day, another construction deadline extension request for South Station Tower.

More than 20 years after first working on plans for a tower over South Station, Houston-based developer Hines is seeking Boston Planning & Development Agency approval on a change to its financing structure for the long-awaited, 678-foot skyscraper.

The developer has tapped Dutch pension fund investor APG Asset Management U.S. Inc. and New York-based Dune Real Estate Partners as co-investors. Chinese investor Gemdale Properties & Investment Corp. will no longer be involved in the 1.1M SF South Station air rights project.

BPDA owns a portion of the air rights being developed at South Station, so Hines must seek its approval for any financing changes. The developer will also need the MBTA to sign off on the arrangement.

“This is another step forward for this project and we are excited to be at the point of completing the financing for the project,” Hines Senior Managing Director David Perry said in a statement.

Along with financing approval, Hines is seeking to extend its land disposition agreement to April 30, according to its BPDA application. The developer has been given a series of 10 deadline amendments so far to finalize plans for the project.

The developer didn't state when construction would begin or the financial terms of the new development partnership. Whatever the construction start date may be, it is expected to take four years to complete the tower, Hines said in its application. The company declined to comment beyond Perry’s statement.

The discussion of an air rights development over South Station dates to the early 1980s, when foundations were placed during track renovations to allow for a future development overhead.

Tufts University development subsidiary TUDC Inc. acquired the air rights for the project in 1991 and brought Hines on as a co-developer in 1998. The tower was first approved in 2006, but Tufts rolled off the project three years later.

While the South Station project’s development costs haven’t been publicly revealed, Millennium Partners’ nearby 691-foot office-and-condo project under construction in Winthrop Square is estimated to cost $1.4B and doesn’t entail the complicated engineering of constructing over an active transit hub.

The 1.1M SF South Station office-and-condo tower is the first phase in a planned 2.75M SF three-phase remake of the area over the station's tracks. Development plans call for an eventual 279-foot office building over the adjoining bus depot and a 349-foot building that will either be a hotel or residential project.