1. The lower court acknowledged the facts as indicated in its reasoning based on the evidence, and found it difficult to view that there is a qualitative difference between the instant apartment construction built by the public law 1, including the balcony size in the area for exclusive use by apartment buildings, and the practical practice of using the area for exclusive use by public registers as taxation data was established, and accordingly, such practice seems to have been generally accepted by taxpayers. ② As long as the taxation practice of excluding the area for balcony portion was established when calculating the area for exclusive use by apartment buildings, it can be seen that the legal principles clearly indicate that the area for balcony portion cannot be included in the calculation of the area for exclusive use by apartment buildings, and it is difficult to view that there is no room for dispute over the interpretation thereof among the instant apartment built by the public law 10th of the general apartment and the high-class apartment construction 10th of the instant apartment construction 20th of the Seoul Special Metropolitan City, which does not constitute the criteria for imposition of capital gains tax on the area for exclusive use by the Defendant in front of the 20th of the instant apartment complex construction 160th of the Seoul Special Metropolitan City.

2. However, the lower court’s determination is difficult to accept for the following reasons.

A. In order for a tax disposition to be deemed null and void as a matter of course, the mere fact that there is an unlawful ground for the disposition is insufficient. The defect must be objectively obvious as it seriously violates the important part of the law. In determining whether there is a significant and apparent defect, the objective, significance, function, etc. of the law that serves as the basis for the said tax disposition should be examined in a teleological context and the specific nature of the case itself should be reasonably considered at the same time. In addition, in a case where a tax disposition is made by applying a provision of a certain law to a certain legal relation or factual relationship, the legal principle that the relevant provision cannot be applied to the legal relation or factual relations is clearly stated and there is no room for dispute over the interpretation, and thus, if a tax disposition is made by applying the provision of the law, it is obvious that the defect is significant and obvious, but if there is room for dispute over the interpretation of the law, it cannot be said that even if the tax office erred by interpreting it, it is merely a mistake in the fact requiring taxation (see, e.g., Supreme Court Decision 2006Da138375.

B. Examining the above legal principles, even if the practices of national tax administration excluding balcony areas have already been established in calculating the floor area of the balcony prior to the Defendant’s imposition of capital gains tax, the principle of retroactive taxation prohibition by a new interpretation or practice under Article 18(3) of the Framework Act on National Taxes applies only to cases where there are special circumstances deemed that the protection of taxpayer’s trust is consistent with the concept of justice even if the principle of legality is made, the burden of proving the existence of “an interpretation or practice of tax law” under the above provision is clearly accepted by taxpayers (see, e.g., Supreme Court Decisions 2005Du2858, Jun. 29, 2006; 91Nu13670, Sept. 8, 1992). It is difficult to readily conclude that the above provision was established as an practice that is subject to the application of Article 18(3) of the Framework Act on National Taxes with respect to the establishment of an apartment building’s exclusive use area under Article 18(2) of the former Enforcement Decree of the Income Tax Act.

3. Nevertheless, on the grounds indicated in its reasoning, the lower court determined that the disposition imposing the transfer income tax of this case was null and void due to its significant and apparent defect. In so doing, the lower court erred by misapprehending the legal doctrine on the invalidation of a taxation disposition, which affected the conclusion of the judgment.

4. Therefore, the lower judgment is reversed, and the case is remanded to the lower court for further proceedings consistent with this Opinion. It is so decided as per Disposition by the assent of all participating Justices on the bench.