(a) It shall be the duty of the county board of tax assessors to investigate diligently and to inquire into the property owned in the county for the purpose of ascertaining what real and personal property is subject to taxation in the county and to require the proper return of the property for taxation. The board shall make such investigation as may be necessary to determine the value of any property upon which for any reason all taxes due the state or the county have not been paid in full as required by law. In all cases where the full amount of taxes due the state or county has not been paid, the board shall assess against the owner, if known, and against the property, if the owner is not known, the full amount of taxes which has accrued and which may not have been paid at any time within the statute of limitations. In all cases where taxes are assessed against the owner of property, the board may proceed to assess the taxes against the owner of the property according to the best information obtainable; and such assessment, if otherwise lawful, shall constitute a valid lien against the property so assessed.

(b) In all cases in which unreturned personal property is assessed by the board after the time provided by law for making tax returns has expired, the board shall add to the assessment of the property a penalty of 10 percent, which shall be included as a part of the taxable value for the year.

(c) When the value of real property is reduced or is unchanged from the value on the initial annual notice of assessment or a corrected annual notice of assessment issued by the board of tax assessors and such valuation has been established as the result of an appeal decision rendered by the board of equalization, hearing officer, arbitrator, or superior court pursuant to Code Section 48-5-311 or stipulated by written agreement signed by the board of tax assessors and taxpayer or taxpayer's authorized representative, the new valuation so established by appeal decision or agreement may not be increased by the board of tax assessors during the next two successive years, unless otherwise agreed in writing by both parties, subject to the following exceptions:

(1) This subsection shall not apply to a valuation established by an appeal decision if the taxpayer or his or her authorized representative failed to attend the appeal hearing or provide the board of equalization, hearing officer, or arbitrator with some written evidence supporting the taxpayer's opinion of value;

(2) This subsection shall not apply to a valuation established by an appeal decision or agreement if the taxpayer files a return at a different valuation during the next two successive years;

(3) Unless otherwise agreed in writing by both parties, if the taxpayer files an appeal pursuant to Code Section 48-5-311 during the next two successive years, the board of tax assessors, the board of equalization, hearing officer, or arbitrator may increase or decrease the value of the real property based on the evidence presented by the taxpayer during the appeal process; and

(4) The board of tax assessors may increase or decrease the value of the real property if, after a visual on-site inspection of the property, it is found that there have been substantial additions, deletions, or improvements to such property or that there are errors in the board of tax assessors' records as to the description or characterization of the property, or the board of tax assessors finds an occurrence of other material factors that substantially affect the current fair market value of such property.

(d) When real or personal property is located within a municipality whose boundaries extend into more than one county, it shall be the duty of each board of tax assessors of a county, wherein a portion of the municipality lies, to cooperatively investigate diligently into whether the valuation of such property is uniformly assessed with other properties located within the municipality but outside the county where such property is located. Such investigation shall include, but is not limited to, an analysis of the assessment to sales ratio of properties that have recently sold within the municipality and a comparison of the average assessment level of such properties by the various counties wherein a portion of the municipality lies. The respective boards shall exchange such information as will facilitate this investigation and make any necessary adjustments to the assessment of the real and personal property that is located in their respective counties within the municipality to achieve a uniform assessment of such property throughout the municipality. Any uniformity adjustments pursuant to this subsection shall only apply to the assessment used for municipal ad valorem tax purposes within the applicable county.