Question: What percentage of the Mecca's historic buildings have been demolished?
About the article "Saudi Arabia", section "Islamic heritage sites".

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Under Saudi rule, it has been estimated that since 1985 about 95% of Mecca's historic buildings, most over a thousand years old, have been demolished.
Historic sites of religious importance which have been destroyed by the Saudis include five of the renowned "Seven Mosques" initially built by Muhammad's daughter and four of his "greatest Companions": Masjid Abu Bakr, Masjid Salman al-Farsi, Masjid Umar ibn al-Khattab, Masjid Sayyida Fatima bint Rasulullah and Masjid Ali ibn Abu Talib.
It has been reported that there now are fewer than 20 structures remaining in Mecca that date back to the time of Muhammad.
Other buildings that have been destroyed include the house of Khadijah, the wife of Muhammad, demolished to make way for public lavatories; the house of Abu Bakr, Muhammad's companion, now the site of the local Hilton hotel; the house of Muhammad's grandson Ali-Oraid and the Mosque of abu-Qubais, now the location of the King's palace in Mecca; Muhammad's birthplace, demolished to make way for a library; and the Ottoman-era Ajyad Fortress, demolished for construction of the Abraj Al Bait Towers.
The reason for much of the destruction of historic buildings has been for the construction of hotels, apartments, parking lots, and other infrastructure facilities for Hajj pilgrims.
However, many have been destroyed without any such reason.
For example, when the house of Ali-Oraid was discovered, King Fahd himself ordered that it be bulldozed lest it should become a pilgrimage site. 
Saudi Arabia, and specifically the Hejaz, as the cradle of Islam, has many of the most significant historic Muslim sites, including the two holiest sites of Mecca and Medina.
One of the King's titles is Custodian of the Two Holy Mosques, the two mosques being Masjid al-Haram in Mecca, which contains Islam's most sacred place, the Kaaba, and Al-Masjid al-Nabawi in Medina which contains Muhammad's tomb.
However, Saudi Wahhabism is hostile to any reverence given to historical or religious places of significance for fear that it may give rise to 'shirk' (that is, idolatry).
As a consequence, under Saudi rule, the Hejaz cities have suffered from considerable destruction of their physical heritage and, for example, it has been estimated that about 95% of Mecca's historic buildings, most over a thousand years old, have been demolished.
These include the mosque originally built by Muhammad's daughter Fatima, and other mosques founded by Abu Bakr (Muhammad's father-in-law and the first Caliph), Umar (the second Caliph), Ali (Muhammad's son-in-law and the fourth Caliph), and Salman al-Farsi (another of Muhammad's companions).
Other historic buildings that have been destroyed include the house of Khadijah, the wife of the Prophet, the house of Abu Bakr, now the site of the local Hilton hotel; the house of Ali-Oraid, the grandson of the Prophet, and the Mosque of abu-Qubais, now the location of the King's palace in Mecca.
Critics have described this as "Saudi vandalism" and claim that over the last 50 years, 300 historic sites linked to Muhammad, his family or companions have been lost.
It has been reported that there now are fewer than 20 structures remaining in Mecca that date back to the time of Muhammad.
Trains still pass the site on what is now known as the Caldervale Line, but all of the structures (platforms & buildings) have been demolished. 
The Longley area had a number of large country houses before it became part of Sheffield, most of these have now been demolished. 
Many of Elmer Feig's designs have been demolished, but the following list from the Oregon Historic Sites Database and other sources includes at least most of those left standing.