Dear Kathleen,

Detached is usually better. You do not hear your neighbors as easily. Most people prefer detached homes because it makes them feel they have more privacy.

If you are, however, buying in the Marina on landfill, it is better to buy in the middle of the block where all homes are attached. This way your home is braced better in case of an earthquke. I learned that from a home inspection made after the 1989 quake.

Value depends on the the homes themselves. It is not easy to quantify because of actual views, location, interioor design and floor plan and other factors. You would need to assess these factors when making a final decision on value. Chris has some good points.
Cheers,

Sally

Detached homes typically hold more value in the eyes of the consumers. Privacy, noise reduction by not having shared walls, and the ability to maintain the property as you want without shared expenses and opinions are usual reasons given by buyers. There is usually some factor that isn't comparable when the buyers really start comparing their 2-3 favorite home choices to decide where to offer.

Kathleen,
Detached is more desirable, that is the perception of the buyers. No getting around it. More buyers given the choice would choose detached so the value is that it will go first. Dollars depend on many other factors, I don't think a percentage of number will really have any bearing here.
It is very important to consider another factor though. Most lots in the city are 25" wide. If you have detached houses on a 25" lot you give up living splace to have an alley between you. Also the houses in SF neighborhoods like Miraloma Park and the Sunset are not attached. The correct term is "zero lot line". Each house has it's own foundation and there is an air gap between the exterior walls.

Kathleen, the value is, like many other variables, based on the individual characteristics of the subject property. In and of itself, detached versus attached may not be the strongest factor in owing a home in San Francisco. For instance, a block of homes that are all detached except for one home (lets say on the corner) might reflect similar values for all the attached homes, but a higher value on the one detached home. This is, of course, given that the detached home is in similar or better condition that the rest of the row of attached homes. However, again, if one of the attached homes has an unencumbered view, and has fit and finishes that clearly outshine the one detached home on the block, then the factor of attached versus detached becomes less of an important value - generally.