How to stop unauthorised/unlicensed extension of the semidetached house

Dear All,

What can I do to stop non-authorized by authorities construction of extension (additional room) on the solarium of the adjoining property? The president of community convinces me that no permission is required which I don't believe in. Should the extension of the house to be approved at the AGE? How I can contact the authority, the town hall Iin order to get the construction work to be suspended until the proper permits have been granted?
What actions should I take?
Thank you in advance for all your advices.
Tanya