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Facebook founder Mark Zuckerberg is reportedly doing a stealth upgrade of his new San Francisco home in the quiet neighborhood of Noe Valley. And the fallout on neighbors could be significant.
Reports surfaced this month that the Facebook founder bought a pied-à-terre, a 5,542-square foot home in the Noe Valley neighborhood, 35 miles away from Zuckerberg's Palo Alto residence. In documents secured by CNBC, the home at Fair Oaks off of 21st Street and Dolores went for $9.999 million, and now the property is undergoing an extensive renovation.
It appears Zuckerberg overpaid for the home. Donald Van de Mark of Sotheby's International said the property was worth only as much as $3 million. And now the remodeling may cost millions more.
"This house looks as if it needs remodeling now, but that's because it's been torn apart. It actually was not in that bad a shape when it was bought," said Van de Mark. He talked about the property outside the house, as contractors were fixing it up.
(Read more: Facebook to turn teens into virtual Don Drapers)
Van de Mark said Zuckerberg "has submitted permits for about $1.6 million worth of upgrades, repairs, remodeling. But that isn't the real number. Everyone always goes in and submits numbers that are going to be less than what the real cost is, because then you keep your tax rate down."
According to Van de Mark, the remodeling could cost more than $4 million to $5 million.
A Multiple Listing Service of the property said it was sold for $1,200,000 on Jan. 9, 1997. In that MLS document, the assessed value of the home and 7,980 square foot lot in 2012 was $1,559,572. That's far, far less than the latest price tag of $9,999,999.
The Dolores Heights home was purchased through a legal entity known as SFRP LLC. The main beneficiary is Cary S. Collins, according to public records.
Neither Zuckerberg nor a personal representative could be located for comment by CNBC. A Facebook representative said: "We don't comment on Mark's personal life."
(Read more: Facebook pushes both ways on privacy)
By any measure, Zuckerberg is on a roll. He was America's highest-compensated CEO last year bringing in $2.3 billion, and just in the last three months Facebook shares have more than doubled.
And if history is any guide, the 28-year old Zuckerberg and his wife may take interest in the neighboring properties.
As CNBC reported a couple of weeks ago, the Zuckerbergs purchased four properties adjoining their Palo Alto home to expand their backyard and to create a security buffer.
Zuckerberg has been seen walking his dog and eating in neighborhood restaurants in Noe Valley, according to SocketSite, a local social media news blog. The property was never openly listed for sale, and Zuckeberg's name is in no way attached to the house.
—By CNBC's Mark Berniker; follow him on Twitter .