The update of the location coefficient (coeficiente de localização), an element used to calculate the Tax Asset Value (VPT, or Valor Patrimonial Tributário), which can be seen on the caderneta predial document of properties that serves as the basis for the Municipal Property Tax (IMI, or Imposto Municipal sobre Imóveis), will lead to a “brutal” increase in this tax applied to properties that have increased in real value in recent years without the tax value registered with Finanças increasing accordingly.

The review of zoning for the purposes of IMI should be completed in 2024, leading to an increase in tax the following year, especially in properties located on the country’s coastal strip, such as in Lisbon and Porto, where prices have increased more in recent years, without being reflected in the tax value of properties.

This means that AL owners will be affected by the increase in the age coefficient (vetustez) due to the Mais Habitação law and all owners will be affected by the location coefficient. Which means AL owners will be affected twice.

Taken from this article, in Portuguese:
https://jornaleconomico.sapo.pt/noticias/je-diario-revisao-do-coeficiente-de-localizacao-vai-levar-a-um-aumento-brutal-do-imi/