Buildings could rise to 400 feet in parts of Seattle's fast-growing South Lake Union area under a proposal unveiled Monday by Mayor Mike McGinn.

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Buildings could rise to 400 feet in parts of Seattle’s fast-growing South Lake Union area under a proposal unveiled Monday by Mayor Mike McGinn.

The long-awaited plan, which must be approved by the City Council, was released Monday after a neighborhood process that started in 2008.

Bigger buildings would bring thousands of jobs and households, McGinn said. They also might provide millions for affordable housing and park improvements under a policy that gives developers extra height if they contribute to public benefits.

“Many companies in our growing technology and public-health sectors want to be in South Lake Union. This proposal will help that growth continue in ways that bring significant public benefits,” McGinn said in a statement.

Proposed height increases are not uniform throughout the area, which is roughly defined east to west by Interstate 5 and Aurora Avenue North, and north and south by Lake Union and Denny Way.

Much of the Cascade neighborhood in the eastern part of the area would not see zoning changes under the mayor’s plan.

The tallest buildings — apartment and condo towers up to 400 feet, and office towers up to 240 feet — would be allowed on the blocks just north of Denny Way, where current zoning limits building heights to 125 feet. That strip includes undeveloped blocks owned by Vulcan Real Estate and The Seattle Times Co.

Condo towers up to 240 feet and office towers up to 160 feet tall would be allowed on much of the area north of that, between Aurora Avenue North and Fairview Avenue North and south of Lake Union.

The height limit there now is 85 feet at most, with the exception of two 160-foot towers, the tallest buildings in South Lake Union.

In three blocks between Mercer Street and the lake, McGinn also would allow 240-foot buildings. But the property owner, Paul Allen’s development firm Vulcan, could only build one tower per block in the city’s effort to preserve views of the water.

In a deal still being negotiated, Vulcan would have to provide land elsewhere in the area for affordable housing and parks, plus a cash contribution in order to take advantage of the increased height.

McGinn would let residential buildings go higher than office towers, in hopes of encouraging more housing near jobs in the neighborhood and downtown.

His proposal also would use a new state law by which developers would buy the development rights of rural land in King County to keep that land as farm or forest. In exchange, a portion of property taxes from new buildings would be plowed back into South Lake Union to pay for new infrastructure needed to support growth. The concept is called tax-increment financing for transfer development rights.

Led by Vulcan and aided by about $300 million in public funds, South Lake Union has been transformed from a warehouse district into a high-tech beehive, replete with upscale restaurants and housing and thousands of new workers.

Nearly 2,300 residential units and 4.2 million square feet of office, research, lab and hotel space have been completed in South Lake Union since 2000.

The zoning changes may not touch off a flood of new development there — because the flood already has begun.

Nearly 900 apartments and more than 700,000 square feet of office and lab space are under construction in the neighborhood, and developers have filed plans for an additional 1,600 apartments and 1.75 million square feet of offices.

A few of those planned projects were designed in anticipation of the zoning changes. Skanska Development, for instance, has proposed a 160-foot office tower on property where the height limit now is 65 feet.

Most of the development in the pipeline could be built without the proposed zoning changes, said Blaine Weber, a principal with Seattle architectural firm Weber Thompson.

“But we’ll definitely see some new towers,” Weber said. “We’ve got some clients looking at them right now … They’ve been in a bit of a holding pattern, waiting for the zoning to shake out.”

The proposed changes have the potential to add 12,000 new housing units and 22,000 jobs, according to city planning director Marshall Foster. “The neighborhood has changed so much. This will help step it up to the next level with more housing and amenities,” Foster said.

The City Council’s land-use committee hopes to finish its review of the proposed changes this summer.

Bob Young: 206-464-2174 or Eric Pryne: 206-464-2231 or