A major proposal to add hundreds of housing units to an existing apartment complex won support from Mountain View's Environmental Planning Commission Wednesday, despite reservations that existing tenants would have to live through years of heavy construction and air pollution.

The project at 555 W. Middlefield Road has been kicking around since 2015, evolving after a fairly poor reception early on. But the overall strategy has always been the same: Remove the surface parking lots and squeeze in hundreds of new homes in their place, without demolishing any existing homes in the process.

The latest iteration calls for adding 323 housing units -- bringing the total on the property to 726 -- across three new buildings, with underground garages to replace the lost surface parking spaces. Unlike past versions of the project, the proposal now includes 111 rental condos, with the potential for future conversion into for-sale units.

The developer, AvalonBay Communities, is also dedicating just over 1.3 acres in the center of the complex for use as a public park.

Critics of the project, many living next door on Cypress Point Drive, say the developer is trying to cram too many homes on the property and that the drawbacks are simply too steep. In order to build the extra units, the developer would have to cut down 57 heritage trees and remove trees that act as a buffer to Highway 85. Existing tenants at 555 Middlefield Road would have to live alongside construction for up to seven years, exposing them to elevated levels of particulate matter.

"The science behind this is undisputed," said resident Daniel Shane. "Tree canopies along our highways and streets provide a buffer that mitigate noise levels and pollution levels and reduce cancer risks among residents who live in the neighborhood."

Even with these concerns, there's still a lot to like about the project, said commission member Hank Dempsey. It adds hundreds of homes to the city, including close to 50 affordable units, with infill development that doesn't displace any residents. On the balance, he said the project's benefits outweigh the problems and should move forward.

"The truth is I don't want the perfect to be the enemy of the good," Dempsey said.

Commissioner Joyce Yin acknowledged that the project has been locked up in the development process for several years, but that the extra time has paid off. Compared to the initial proposal, she said the project is "leaps and bounds" better now. The project's appearance has gone through numerous improvements, and the number of heritage trees slated for removal decreased from 117 to 57.

Still, she said AvalonBay shouldn't downplay or ignore the problems ahead, and should proactively work with tenants to disclose the environmental hazards caused by construction in the near term and potential long-term impacts of living close to Highway 85 with a limited tree buffer.

"Transparency is paramount and you need to continue to have the conversations with the neighborhood, with those who are going to remain to find out, truly, what they are going to have to deal with," Yin said.

Residents and advocacy groups alike have raised concerns that the project, in axing trees near Highway 85, could disrupt an important riparian habitat along Stevens Creek. City officials say that does not appear to be the case, referring to a consultant study that found the strip of vegetation does not meet the definition of a "habitat movement corridor." In other words, the area of trees doesn't link habitats and currently forces wildlife to retrace its steps to return to the creek.

Commission Chair Bill Cranston said the developer's strategy of adding new units to an existing property -- rather than a full demolition and redevelopment -- is a positive one that retains rent-controlled apartments. He said the city should have a solid strategy for supporting residents who live right next door to lengthy construction, including clear expectations for what the developer should do to mitigate the noise and pollution impacts.

"This is going to be one of the first 'truly' infill development opportunities where we're trying to build in the middle of something and not disturb either side of it," Cranston said. "I'd much rather do this than plow down 402 units and start over."

The commission voted unanimously to recommend the project for approval. The Mountain View City Council is set to review the project for final approval next month.