Proper serving of notices of assessment from the Bureau of Internal Revenue (BIR) is crucial. If done improperly, the assessment can be invalidated because of the lack of due process.

Section 228 of the National Internal Revenue Code provides that the taxpayer should be informed in writing of the law and facts on which the assessment is made. Otherwise, the assessment will be void. This rule has basis in the 1987 Constitution, which provides that no person should be deprived of life, liberty, or property without the due process of law.