Is it permissible to rent a residential flat as an office without a government permit? Many flats there have been offices for over 10 years. The permit requires all owners to apply, but they are not interested. I was told the income is halal but I am sinful. Is this correct?

We should not belittle doing something forbidden just because it does not mean that the income is forbidden. The Salaf Assalih used to stay away from what is frowned upon. The regulation should be complied with, since the Muslim is commanded to hear and obey in what is good and this is from that.