It is clear that the rental of a property must be declared because it implies a capital gain, but what happens with the properties owned by a taxpayer that are empty or not rented? Do they also have to pay Personal Income Tax (IRPF)?

Yes. This is known as allocation of real estate income. This is defined in article 85 of Law 35/2006 on Personal Income Tax. I

How this type of property is declared in the income tax return

The allocated income is a sort of wealth tax justified by the correlation between owning a property and generating some sort of income, though not specified. It also a way to cover the transfer of property use to family members at zero cost. 

The ING blog explains that “simply because you have the possibility of earning this money, you must pay a minimum amount in your tax return”. In the tax return, the taxpayer has to go to Section C of real estate not affected by economic activities.

The properties subject to this allocation, according to the Tax Agency are

1- Urban properties that are not used for economic activities.
2-Rural properties with buildings that are not essential for the development of agricultural, forestry or livestock activities, not used for economic activities.
3- properties which do not generate capital yields as a result of the leasing of real estate, businesses or mines or of the constitution or transfer of rights or powers of use or enjoyment over real estate.
4- Properties that they do not constitute the taxpayer’s habitual residence. (For these purposes, parking spaces acquired jointly with the property, up to a maximum of two, are understood to form part of the taxpayer’s habitual residence.)

They must not be unbuilt land, properties under construction or real estate which, for urban planning reasons, are not eligible.

How is it calculated?

The persons who must include this income in the tax return are the owners of real estate who have it at their disposal all or part of the year. Usufructuaries or persons with a real right of use or enjoyment over the property must include the corresponding income in their tax return in the same amount as that which would correspond to the owner. In addition, holders of a time-sharing accommodation right must include it according to the duration of the period of use.

* How much personal income tax is paid in the 2022-23 income tax return depending on your community 

The Tax Agency specifies that, in general, 2% will be applied on the cadastral value of the property that appears in the IBI receipt. However, 1.1% will be applied to properties whose cadastral values have been revised.

For properties which on the date of accrual of the tax (normally 31 December) had no cadastral value or which had not been notified, the amount is calculated as 1.1% of the purchasing value.