You are the owner of a property located in Spain and a Belgian taxpayer. 
How should you declare your real estate income? 
We will be happy to provide you with more information to correctly complete your tax return. 

Is the property rented out?

- If so, state the gross rental price of this property in your tax return after deducting the foreign taxes on this income.

    • When you rent out the house, you must state 22.5% of the rental value.

- If not, state the gross rental value (*) of this property after deduction of the foreign taxes on this property in your tax return.

    • (*) The rental value is the average annual gross rent that you could have obtained in the event of a rental during the taxable period.
    • There is no legislation yet to determine this amount. 
      But the Belgian tax authorities accept to declare 2% of the Valor Catastral if you only use your home yourself. 
      If the value was determined in Spain after 1994, you must declare even 1.1% to the tax authorities, so this is the case when you buy a new building.

You may deduct the IBI (Impuesto sobre los Bienes Inmeubles) that you paid in Spain from the gross rental value or gross rental price. 
You state this amount in your tax return.

Where should I declare the real estate income?

- In your tax return, state the gross rent ⁄ the gross rental value in boxes III, B, 1

    • next to the code 1130/2130 for a building;
    • next to the code 1131/2131 for a ground.

- If you have a loan for your second residence in Spain, you may declare the interest paid in box IX

    • in addition to code 1146/2146.

Am I taxed twice?

Theoretically not, but in practice it is. 
You will not be taxed on any income from Spanish rental income, but it will be added to your total Belgian income. 
This may allow you to enter a higher rate bracket so that you still pay extra taxes.

Source:  https://financien.belgium.be