The Mosque of al-Sahla is a mosque in the old city of Kufa, Iraq. It is considered to be a sacred site that has functioned as a mosque since the earliest days of Islam when the followers of the fourth caliph 'Ali ibn Abi Talib (shi'at 'Ali, whence the term Shi'a) began to pray there. Other associations with the grounds include sites where Abraham, Idris, Ihud, Salih and al-Khidr either prayed or resided. Shrines dedicated to Zayn al-'Abidin and the sixth and tenth Shi'a imams, Ja'far al-Sadiq and 'Ali al-Hadi, are also enclosed within the grounds. It is also the site to which it is believed that the last Shi'a imam (Muhammad al-Mahdi) will return. The architectural history of the mosque is complex, the site having been expanded, renovated, and rebuilt numerous times, including an extensive rebuilding in the late twentieth/fourteenth century. Most recently, a thorough redesign and rebuilding project took place in the 2010s (see below).

Redesign and expansion of the mosque by Shubber Falah and Wael Ajam:

The most recent changes to take place at the Mosque of al-Sahla are the expansion of the site and the construction of an entirely new mosque designed by the architects Shubber Falah and Wael Ajam. According to the architects, the original design proposed in 2006 was rejected by pilgrims to the site. As foundations had already been laid when the controversy arose, Falah and Ajam were hired to redesign the new mosque based on the footprint of the rejected design in addition to expanding the site. The redesign was to take into account pilgrims' views about what constituted a sacred space. Falah and Ajam began to survey opinions and conduct research in 2009. The findings of this survey are summarized here, in the architects' own words: 

We discovered that most people think that sacred places should appear old, and should highlight the idea that “there is one just and almighty God for all people.” Besides that, they singled out some sacred examples for us. It turned out that all these examples have one feature: a certain kind of mystery. Most of the offered examples were shrines for the Prophet Mohammed’s descendants; Iraq has four of these shrines. One of the surprising realizations that came out of that survey was that people who revere such places do not distinguish between the architectures of those shrines and the Prophet Mohammed’s descendants themselves, who were buried underneath. We do not know why this is the case, but keep in mind that Islam does not permit the picturing or sculpturing of sacred people. As such, people have no idea what the Prophet Mohammed or his descendants looked like, and so perhaps the buildings are the closest thing to them.

These results led the architects to conclude that building in a historical architectural idiom was critical. A new design was developed based on these findings. After the design was accepted, construction began in 2011 and was slated to end by 2018. The architects' initial estimate of the cost of construction was USD $8,224,000. 

Sources: 

Falah, Shubber and Wael Ajam. Al-Sahla Mosque - Iraq.

Tabbaa, Yasser and Sabrina Mervin. Najaf, the gate of wisdom: history, heritage & significance of the holy city of the Shi'a. Paris: Unesco, 2014.