The IIVTNU taxes the rise in urban land value when a property is transferred.
Plusvalía municipal, formally the Tax on the Increase in Value of Urban Land (IIVTNU), is a local tax paid by anyone selling, gifting or inheriting an urban property in Spain. It is computed only on the land portion, not the building, and collected by the town hall where the property sits.
After the Constitutional Court struck down the previous system in 2021 (STC 182/2021), the Government published RDL 26/2021 which introduced two coexisting methods. The taxpayer can choose whichever yields the lower amount, which translates into a real tax saving when the sale didn't generate much actual gain.
If the sale price equals or is below the purchase price, there is no taxable event: the operation is exempt. In that case it's still wise to file the self-assessment at zero to avoid friction with the town hall.