Greetings Toby,

According to the "Zoning by-law of Istanbul", the substantial modifications and repairs are subject to construction permission.

A repair or modification which is capable to modify the building's status as regards both the statics and the nature as well as the number of the flats and the use of premises and the frontage view is substantial.

.IMHO, an additional room on the roof terrace may be deemed to be a substantial modification, and therefore a construction permission for such a modification should be acquired.The relevant authority is the municipality of the area where the building is located.