I live in a section of town that is zoned for single-family homes. However, I have a legal two family home. When I look up the value of my premises on some well-known real estate valuation Web sites, it is valued the same as other houses in my development. These sites have my home way undervalued. There is no “contact us” button on the sites to advise them of this error.

I will be selling my home in a few years, and this error could hurt my selling price. Do I have recourse against those sites for their generalizations? I see it as similar to a libel situation: It will penalize me by having to sell at a reduced price because of their laziness. This will financially hurt me, the real estate agent and the taxing authorities. I tried to get my property taxes reduced but the court refused to allow these Web site valuations as evidence.

Can you walk me through the process to contact those Web sites to correct their postings?

We think you are too obsessed with what certain Web sites say about your home. While the algorithms those sites use to determine valuations for homes across the country are accurate in some cases, there are serious discrepancies between what these sites say a home is worth and what they will sell for.

What are the best uses for these Web sites? Many people go to them for general information about homes and neighborhoods.

But as with the Internet in general, you should not rely entirely on these sites for a final determination on what your home is worth or whether a neighborhood has all of the amenities you desire. A good real estate agent who works in your neighborhood should give you a more informed estimate of what your home is worth and what listing price you might try.

Real estate listing and information Web sites rely on public information supplemented, in some cases, by subjective information posted by homeowners. Some of the information used to estimate the value of a particular property comes from court records, property records, listings in the area and other factors. Given that your home is a two-family home in a neighborhood of single-family homes, these Web sites clearly haven’t picked up on that information.

What you have to understand is that a national (or international) Web site doesn’t drive across the country looking at and assessing the value of each home. If your local property records office doesn’t release property records to these sites, the Web site won’t know that your home is a two-family home. (That’s why your local tax authority won’t allow you to use the information to contest your property taxes — it’s not considered definitive, and the methodology is anything but transparent. More on this in a moment.)

The good news is that these Web sites won’t matter much when you sell your home. If the property is listed in your local multiple listing service as a two-family home, these same sites will then use the information posted on your listing sheet to update their records. The listing price alone may be used by the sites to then change their evaluation of what your home is worth.

We don’t know of any governmental body that’s willing to accept the numbers given by these sites as valuations of homes. Your local government was right to not use that information.

If you really want to change the information on your home now, these sites generally allow you to create an account, claim “ownership” of the property, and then edit the information on your home.

Having said that, changing that information won’t give you an advantage when it comes time to sell. Many buyers start their search for a home on the Internet, and when they do, they usually look at active listings. We suspect that when you list your home, the information will “magically” change as well.

Finally, when you fight your real estate taxes, your best bet is to use local information for properties that are comparable to yours. You might find people willing to fight for you and take a piece of the action by billing you on the basis of the amount of money they save for you. In other instances, local governmental bodies have information you might be able to use to compare your home with other comparable homes.

Ilyce R. Glink ’s latest book is “Buy, Close, Move In! Samuel J. Tamkin is a Chicago-based real estate lawyer. If you have questions, you can call Glink’s radio show (800-972-8255) any Sunday from 11 a.m. to 1 p.m. Contact Glink and Tamkin through the Web site thinkglink.com.