This is indeed a B&B with a difference, as a few of the house rules make clear:
t Do not carry your whips on your hips - put toys in a bag.
t No loud screaming anywhere, anytime.
t Do not touch another person, or their toys, or their slaves, unless invited.
As the establishment's owner, a nice-looking middle-aged lady called Elizabeth, explains to the curious or the baffled, B&B stands not for bed and breakfast - there is no breakfast - but for bed and bondage. This is certainly a hotel, of sorts, but one that caters exclusively to sado- masochist fetishists.
The guests, who come from as far afield as Germany, Switzerland and South Africa, aren't interested so much in a good base from which to explore the West Coast's most beautiful city (although some of the rooms do boast good views); rather, they come for the Gothic blood-red and purple drapes, the S&M artwork on the walls, the regular "scene" parties and - most of all - the house's basement dungeon with its chains, crosses, medieval torture devices and cupboard full of sex toys.
"I may look like your grandmother but I'm also kinky. I'm a sadist. I enjoy inflicting strong sensations on people," says Elizabeth with a big friendly smile. "Pain is not really bad. It's interesting."
Even in a city as tolerant as San Francisco, that credo has to be followed with a certain discretion. Hence the house rules - which also forbid alcohol or drugs on the premises, nudity on the patio or anything else that might upset the neighbours or lead to a criminal prosecution.
Elizabeth loves what she does, but she is also determined that nobody should be coerced into doing anything with which they feel uncomfortable. In her house, rule number one is that No Means No.
"My message is: we are not threatening your lifestyle, but we also don't want you to threaten ours. This is not for everyone. But it hasn't been declared illegal or taxable by Uncle Sam, or not yet."
The guesthouse, which for legal purposes operates as a private club, has four bedrooms decked out in a variety of styles, from The Chamber, with its dark wooden panelling, to The Penthouse, which has a king-sized waterbed and hooks all over the ceiling beams.
A narrow wooden staircase leads down to the Dungeon, which not only looks like the set of a kinky porn movie but frequently serves as the set for kinky porn movies. A deep-blue circular sheet is thrown over a Catherine Wheel suspended from the ceiling by thick black chains. On the other side of the room, the cross of St Andrew sports a selection of hooks and fleece-lined leath-er handcuffs.
This is where the twice-monthly parties take place (complete with a discount scheme for regular visitors known as the Frequent Flogger Program). It is also where Elizabeth keeps her prized collection of custom-made whips, canes and a whole host of household and industrial objects, such as a paddle-ball bat, a wooden pizza tray and an outsize pair of photographic developing tongs.
Elizabeth got into the "scene" through her second husband more than 20 years ago (in her first marriage, she played the role of the traditional God-fearing, dish-doing housewife and hated it). After holding parties for years, she bought the house on Caine Street in 1992 and opened for business the following year. The Internet, she says, has vastly expanded the reach of the S&M world; her own website, www.differences.com, has 3,000 subscribers. She herself "plays" with a number of partners, including a Swiss doctor and a policeman from Wyoming.
Although hers is the world's first known S&M guesthouse, the phenomenon is spreading. A fetishist couple in Seattle have opened a room in their house, as has a professional dominant from Massachusetts with a sideline interest in cooking gourmet breakfasts. "The scene is getting positively upper-middle class now," Elizabeth muses. "We're getting gentrified, like everything else."