A builder is auctioning off a Water Mill mansion that had previously been listed for $5.25 million.

Concierge Auctions, in cooperation with Vanessa Mothes, Bryan Midlam and David Butland of real estate brokerage Compass, will hold the online auction for the nine-bedroom house Friday with a reserve price of $3.9 million.

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The house features a two-story great room with a floor-to-ceiling gas fireplace and a media room with built-in seating. The 1.14-acre property has a gunite pool and spa, and a cabana with a kitchen and outdoor shower.

The builder wanted to sell the home within a certain time frame, says Jacqueline Moldawer, project sales manager for Concierge Auctions. Since the house hadn’t sold by the end of the summer, the builder decided to auction it off.

“It really is a seller-elected decision for a faster, more aggressive approach,” Moldawer says.

The builder sold a Southampton property a year ago after scheduling an auction, Moldawer says. That home had been listed for $4.5 million with a reserve of $3.2 million. Within a couple weeks of the auction being announced, someone who had originally been interested in the property made an offer of $3.8 million.

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The auction will take place via the company’s online bidding platform. Bidders must prequalify for the auction by providing a bank letter with proof of funds for a 30-day close. They also need to wire $100,000 into an escrow account. If they don’t win, the money is wired back to them.

Go to conciergeauctions.com for more information.