Treehouses add charm to even the most humdrum of listings. Take apartment #1 at 238 West 11th Street, a 3BR, 3BA that just hit the market. The brokerbabble just goes right along, la la la, duplex co-op, 1838 wood burning fireplace, chef's kitchen, patio, washer/dryer, basement storage and?"an adorable grandfathered treehouse above your own gardening shed!" We like the secretive charm of the thing, not to mention the fact that there's enough garden for the place to have a gardening shed. (The place was landscaped by Edwina von Gal.) We're less charmed by the asking price, $5,995,000, or the maintenance of $3,654/month.