I have been to Istanbul three times and have seen extensive suburbs mostly south and west of the Theodosian Wall. I read the article (see below) about the Gecekondus and found it of great interest. I wonder are most of these suburbs Gecekondus. If they are, who owned the land before? This is the article:
In the outer suburbs of Istanbul, Neuwirth visited squatter settlements that were largely indistinguishable from the legally built communities nearby. This situation was attributable to two unusual features of Turkish law. If a person succeeds in building a home on unoccupied land, eviction requires a court hearing and can be difficult. This rule had led to the practice of gecekondu residency, by which an aspiring squatter builds a home-like structure in a single night and then asserts occupancy in the morning. Fully half the 6 million residents of Istanbul now live in gecekondu homes.

Under Turkish law, groups of 2,000 or more residents can obtain recognition as a quasi-independent municipality. Squatter settlements have used this device to form their own local governments, then passed municipal laws allowing conversion of squatter occupancy to a legal title (for a price set by the municipality). Even if this is not possible, less formal mechanisms of asserting permanent squatter occupancy have developed in the suburbs of Istanbul, and Neuwirth finds that "there are even real estate offices that specialize in selling these titleless properties." One of these settlements, Sutanbeyli, had "more mosques than schools," reflecting the waves of poor immigrants from devout rural areas of Turkey.
http://www.reason.com/news/show/33115.html