William Lyon Homes, a homebuilder in the Western U.S., announced Monday it is acquiring Bellevue-based Polygon Northwest for $520 million in cash.

Polygon has built nearly 16,000 homes in the Seattle and Portland areas since 1991. The new owners said they will keep the Polygon brand name and its 130 employees.

After the sale goes through sometime in the third quarter, William Lyon will control more than 18,000 lots in the West, 2,169 of them in the Seattle area.

Based in Southern California, William Lyon Homes has been looking to get into the Northwest market for a long time, said Chief Executive Bill Lyon.

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“We’ve been looking for the right cultural fit,” said Lyon during a conference call with investors and media members. “The addition of Polygon’s residential homebuilding business to the William Lyon Homes family enables us to realize our goal of becoming the premier Western U.S. regional homebuilder.”

In 2012, William Lyon entered the Colorado market by purchasing Village Homes. It still operates in Colorado under the Village Homes name. It also builds homes in Arizona, Nevada and California.

Polygon CEO Jeff Gow, who will leave the company, said in a statement he is excited about the combination.

“We share many of the same core values, and William Lyon Homes has consistently demonstrated a commitment toward supporting their employees to deliver customers both outstanding quality homes and a truly great home buying experience.”

The acquisition should not mean any changes for anyone currently building a home with Polygon, said a William Lyon Homes spokeswoman Lisa Mueller.

In 2013, Polygon brought in $292 million in revenue. It is expected to build 850 to 900 new homes in 2014 and 1,100 to 1,200 in 2015.

Polygon sells new homes in 12 communities and expects to grow to 16 new home communities by mid-2015.

According to SEC documents, Polygon executives Gary Young, Chris Eadon, Natalie Schmidt, Paulette Murchison, Andy Whisenant and Eric Evans will also leave the company once the sale is finalized.

William Lyon expects the acquisition of Polygon to generate significant returns in 24 to 36 months.

William Lyon became a publicly traded company in 2013.

Its stock price closed Monday at $27.00, up 4.85 percent.

Brandon Brown: 206-464-2164 or brbrown@seattletimes.com