Lump-sum tax on private rental
Since 2023, private rental income in Poland is taxed only as a lump sum on revenue (ryczałt). The rate is 8.5% up to PLN 100,000 of revenue a year and 12.5% above that. You cannot deduct costs such as repairs or interest.
Spouses can choose for one of them to declare all the income, or split it in half, in which case each has their own PLN 100,000 threshold. The tax is due by the 20th of the following month, and the yearly PIT-28 return by the end of February.
Charges passed through to the tenant, such as metered utilities, are usually not revenue if the contract lists them separately. If in doubt, ask a tax adviser.