Hello Barbara and thanks for your post.
Typically, the local MLS will auto-fill certain aspects of any property listing from County or township records, such as the APN or Assessor039;s Parcel Number, the square footage of the home and lot, and the age of the property. Sometimes, owners make changes to the home's square footage without the approval of the local building departments, and the increased size is not noted in the government records and does not get updated into the MLS's database. As a result, there is difference between what the government database will show for the home and what the owner believes is the actual characteristic of the home.
As a result, some (not all) MLS services do allow us--as Realtors--to insert different square footages or property information into the listing, but will provide a note on the listing that the updated information is "from the owner" or a "representation from the owner," so anyone who sees these differences will see that the amounts will differ from the information on file with the local government. If the MLS system in your area pulls data only from the government database and does not allow for changes as I've noted earlier, the differences in the property's characteristics will remain on the MLS and can be explained by the Realtor when someone calls about the home.
If you feel that your listing is incorrect, talk with your Realtor to see if any "notes" or comments can be placed on the MLS listing that can explain differences between square footage, age or lot size. To make a change like this permanent, however, it will require changes to the government's database,and that is a procedure best explored with your local building or assessor's office. Good luck!!
Grace Morioka, SRES, e-Pro
Area Pro Realty
San Jose, CA