(MONEY Magazine) – In building a lakefront estate estimated to cost $50 million just outside Seattle, Microsoft chairman Bill Gates is violating a classic real estate rule of thumb: Never own the biggest house on the block. In Gates' case, of course, his home is the block. Still, his dream dwelling could drop in value soon after its projected September completion date. Reason: When a house is too lavishly constructed or unique in style, there aren't many buyers who could afford--or would want--it. "You don't get a return on anything too personal or just plain wacky," says Susan Moran, a commercial appraiser in the local King County assessor's office.

Among its extras, according to an e-mail trail making its way around the Internet: a 60-foot-long pool complete with underwater music; a 20-seat movie theater; and an electronic system that will change the artwork, music and lighting in the 20-room main house as guests move from room to room.

Gates may actually hope his property will lose value. His own appraiser, in discussing the estate's fair market value at completion, has been reminding the county assessor's office of another estate just down the road that was listed for $17 million last year but recently sold for only $8 million. Why? Property taxes. On a $50 million home, Gates would pay a stunning $580,000 a year, according to the King County assessor's office--255 times the county's average property tax bill of $2,275. So, a 34% reduction in value, for example, would reduce his yearly property taxes by a full $200,000.

--Mari McQueen