If you have income from properties rented to relatives, these are subject to special income tax scrutiny. The tax office particularly checks whether the property was rented at a reduced rate. There is an important change from 1 January 2021:
If this total surplus forecast check is positive, the intention to generate income is assumed for the provision of reduced-rate housing, and the full deduction of advertising costs is possible. If the total surplus forecast check is negative, the intention to generate income is only assumed for the paid part. Advertising costs can be partially deducted for the paid part.
The total surplus forecast check for income from renting and leasing is carried out according to long-standing and established BFH case law. The BMF letter of 8 October 2004 (BStBl 2004 I p. 933) remains applicable.
When renting furnished or partially furnished apartments, it may be necessary to include a surcharge for the furnishings to determine the local market rent. Such a furnishing surcharge must be considered according to the Federal Fiscal Court ruling of 6 February 2018 (IX R 14/17) if it can be determined from a local rent index or marketable surcharges. Other methods of determination are not permitted. In particular, it is not permissible to derive a furnishing surcharge from the monthly amount of the linear depreciation for the furniture and furnishings provided. Nor is it permissible to apply a percentage rental yield surcharge.
The 50% or 66% threshold applies only to the rental of apartments, not to commercially or professionally used premises.