From several sources I understand that the Kazasker Abdurrahman Çelebi Mosque was lost during a great fire in 1908 known as the Golden Horn fire. Its walls remained in place but at some point were taken down because a new road was planned to pass through them. As it turned out the road missed the mosque, but that was gone anyhow.
I came across a critical article in Muquarnas 24, on this and other planning mistakes, quoting: “Paradoxically, Istanbul was to reconnect with its past by being dramatically renewed through the construction of 50-to-60-meter-wide straight roads that cut through the historical peninsula. The street, conspicuously missing in sixteenth- and seventeenth-century representations of the city, was transformed into the highly prominent boulevard. As Menderes’s advisor, Hans Högg, conceded, the new scheme was nothing but a reproduction of Haussmann’s nineteenth-century model for Paris. During the overhaul, not only were the scale and texture of the traditional city irrevocably altered but countless buildings were moved to new locations, several were chopped off, some were half buried, others had their foundations exposed, and 7,289 buildings—among them mosques, masjids, baths, fountains, sebils and cemeteries—were demolished. “
With a note concerning this mosque “The operation, personally conducted by the prime minister, did not rely on any plan. A great many losses could in fact have been avoided. One example of the randomness of the operation was the futile demolition of the Friday mosque of Kazasker Abdurrahman Çelebi, designed by Sinan. It had been repaired in 1951. Then, assumed to be on the course of Millet Caddesi, it was torn down. However, the new avenue bypassed the site of the mosque. Its plot was sold, and apartment buildings were later constructed in its place. Behçet Ünsal, then a member of the incapacitated Superior Council, later recounted the demolitions.”
Recently (I read articles from 2013-2014) work is in progress to buy the grounds of some 130 mosques in Istanbul alone that were lost for various reasons, and that work is underway to restore the mosques in “their former splendor”. The Kazasker mosque has been restored, though I doubt if its architect, Mimar Sinan, would have been pleased by the looks of it if it came out like that in 1554-5 when he designed it: a brand new building, looking as if it just got out of some factory. Still, it’s there, and these pictures show what it looks like. I understood the workmanship on the wooden pieces is by masters from Kahramanmaraş.
Do note there is another Kazasker mosque of that name near the Walls, a larger building, that however was being restored when I tried to visit it again in 2015.