The eight-storey block in the Kartal district on the Asian side of the city collapsed on Wednesday but the cause is not yet clear.
The official toll had previously stood at 10 people dead and 13 injured. Media reports said three of the eight floors had been illegally built -- a relatively common practice in the metropolis of some 15 million people.
The collapse fanned criticism of a government amnesty granted last year to people accused of illegal building -- a measure announced ahead of municipal elections this March.
A four-storey structure in Istanbul crumbled last year following violent thunderstorms. In January 2017, two people died when another building collapsed in a working class part of town.
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