Sujeito passivo (the person liable for the tax)
Sujeito passivo Person liable for the tax
The «sujeito passivo» is the individual or company liable to pay a tax and to meet the obligations that come with it. For IMT that is, as a rule, the buyer. For IMI it is whoever owns the property on the last day of the year the tax relates to, or the usufructuary or surface-right holder where such rights exist.
In practice
On a house purchase it is the buyer who files the declaration and pays IMT, even when the parties have agreed something else between themselves: the IMT Code does not let the obligation be moved by agreement.
For IMI, whoever owns the property on the last day of the year pays for that whole year, and a buyer who comes later starts the year after. Whoever appears, or should appear, in the tax register on that date is presumed to be the owner, so keeping that entry current matters as much as the land registry does. In an undivided estate the tax is owed by the estate, represented by its head.