I have been working at the tax office for over 13 years and a fictitious rent does not exist. No one will find this point in the Income Tax Act in §21.
"The facts of § 21 Income Tax Act are fulfilled by anyone who, in particular, lets a developed property to another person (partly) for payment. Gratuitous use, on the other hand, does not fulfill the facts of § 21 Income Tax Act. Whoever lets an apartment to another person completely free of charge cannot (in this respect) claim any advertising expenses."
That should be clarification enough now.
Regards