Is it permissible to use a rented apartment as a personal residence after a clothing factory project, for which it was originally rented, has failed, or is this unlawful, making it obligatory to return the apartment to the owner since the purpose of the rental has ceased?

There is no harm in you using the apartment for dwelling without informing the owner, as the tenant has the right to utilize the rented property as they wish, so long as they do not cause damage to it. If the owner agreed to rent it for a factory, then it is more appropriate for him to agree to it for dwelling, as stated in the Code (Mudawwanah): "Yes, unless it causes harm to the house, or if it is a house in which such a thing would not be set up due to its beauty and elevated structure... So, whatever causes harm to the house is prohibited for the tenant, and whatever does not cause harm is permissible for the tenant."