The diverse residential neighborhoods of San Francisco are a simple source of pleasure for Sunday strollers and amateur architects who appreciate the myriad possibilities of urban habitation. Alamo Square is a Victorian delight, and the cuckoo clocks clinging to Telegraph Hill have an élan all their own, but when you're in a high-cholesterol state of mind, the shacks of Pacific Heights are a must. The 2000-2100 blocks of Washington Street are one damned mansion after another. Although each is shielded from the street by monolithic walls, gates, and hedges, glimpses of the architecture are possible here and there. One end features a vaguely Asian edifice of brick and pagoda, a trim white Shaker manor house, and a red brick-and-wrought iron spitting image of Tara. At the other end sits a rose-hued Mediterranean villa apartment house with a curving tree-lined walkway and a positively Tuscan panorama of bay, island, and inlet. And smack in the middle of all this habitable lettuce is the overwhelming Spreckels Mansion, a massive French wedding cake of vaulted windows, Greco-Roman folderol, and a bas-relief cherub or two. Each house has a choice view of Lafayette Park to the south and the Golden Gate Bridge to the north, and a graceful brick-lined stretch of Octavia Street neatly divides the blocks in two. It's especially enjoyable to check out the houses when you're dressed in jeans and a scruffy jacket, and you take notes while you peer through the hedges.