You have to know that your House or flat lease taxed as real state capital performance and that you are entitled to a reduction of the 60% of the income that you earned if your tenant has it as their habitual residence .

If you enter €1000 of long term rental It is the habitual residence of your tenant and you have deductible expenses of ( Community ,conservation , taxes etc. ) €200 , you will get to pay taxes to finance €320 on your tax return . But if you do not declare and they discover it ….. lose the reduction of the 60% and you'll have a record penalties for having defrauded the tax authorities and the interests of the delay …..