Is a building permit necessary for a change of use?

Hello. I am looking for some answers that I couldn’t clarify through Google. Maybe colleagues here know this topic and can help me. We built a second building on our property in a residential area during GDR times. With a footprint of 40sqm and a pitched roof, it looks like a smaller single-family house. The building consists of a double garage downstairs and a shed where small animals were kept. Hay was stored under the very steep roof for the animals, and pigeons were housed in the other part. The animals are now all gone, and for ages there has been no car in the garages. Now we have considered converting the building into a single apartment. I cannot say whether the building was assigned a specific purpose in 1988 when it was completed. Where can I find this information? Do I need a building permit if I only do interior work? What do I have to apply for in such cases? Does the building need its own house number even if it is in our fenced garden? Who has gone through this? What should I pay attention to? Oh yes – we live in Thuringia. Best regards