Legitimation of the annex should start with the local architectural authorities. To do this, you must submit drawings and a construction plan there. We will have to pay a state fee. When legitimizing the already built premises, the owner will have to pay a rather serious fine for the lack of timely executed documents.
If the owner is not a professional builder, it makes sense to hire a team of specialists to build an extension. They will take into account all the nuances of such an event, gather the necessary permits and buy building materials at the best prices.
More articles on this topic:
How to make an extension to the wooden house with your own hands?
Construction of the extension to the house from the frame
Contribute a better translation
Comfortable frame sauna with your own hands: step by step instructions