I.-For each commune, for the establishment of the tax base for property tax on built-up properties, the rate of rental value allowances mentioned in articles 1518 A and 1518 A quater is equal to the ratio between:
1° On the one hand, the sum of the products, calculated respectively for the municipality and the department, of the abatement rate by the tax rate applied in 2020 on the territory of the municipality;
2° On the other hand, the sum of the property tax rates on built properties for the municipality and the department applied in 2020 on the territory of the municipality.
II.-For business premises existing on 1 January 2020 and assessed in application of article 1498, for the application of I of this article, the allowance rate is equal, for each premises, to the ratio between:
1° On the one hand, the sum of the products, calculated respectively for the commune and the department, of the rental value used to establish the property tax on built properties determined in respect of 2020 on the territory of the commune, after application of article 1518 A quinquies in its wording applicable on 1 January 2020, by the product of the abatement and tax rates applied in 2020 on the territory of the municipality ;
2° On the other hand, the product of the sum of the tax rates for property tax on built-up properties of the municipality and the department applied in 2020 on the territory of the municipality by the rental value used to establish the municipal property tax on built-up properties determined in respect of 2020 pursuant to Article 1518 A quinquies in the wording applicable on 1 January 2021.
III.-I and II cease to apply, for an allowance, as soon as the municipality deliberates to modify it pursuant to articles 1639 A bis or 1640.
IV.-I and II do not apply to the establishment of the tax bases for the taxes provided for in articles 1520 et 1530.