Well, you can't double dip, so the fact that you deducted them already means you can't add them to your cost basis, even if the deduction was improper. (Unless you want to go back and amend every tax return you filed since owning the home and paying extra tax that results from the lower deduction.)
For future reference, special assessments are deductible as schedule A property taxes if they apply to all similarly situated homes in the community. Contrary to some instructions, they don't have to be a percentage of the property value–they can be a flat fee–as long as they apply equally to all homes in the area.
Special assessments are not deductible, but are added to the cost basis, if they provide a benefit or improvement only to your specific home or only some homes in the area.
For example, if every home in the country pays the same library tax, which provides a benefit to the entire community, then the library tax is a deductible property tax on schedule A. The same would be true for any other services that are of general community benefit. However, if the sewer assessment only paid for improvements to the sewers under your neighborhood, and other properties don't pay the assessment because their sewers have not been upgraded, then that is a cost basis adjustment and not a schedule A deduction.
