The Standing Committee was asked whether it is permissible to spend zakaah money in refurbishing the mosque etc. They replied as follows: 

With regard to zakaah, it is only to spent in eight ways which are specified by Allaah in the aayah (interpretation of the meaning): 

“As-Sadaqaat (here it means Zakaah) are only for the Fuqaraa’ (poor), and Al-Masaakeen (the poor) and those employed to collect (the funds); and to attract the hearts of those who have been inclined (towards Islam); and to free the captives; and for those in debt; and for Allaah’s Cause (i.e. for Mujaahidoon — those fighting in a holy battle), and for the wayfarer (a traveller who is cut off from everything)”

[al-Tawbah 9:60]

Mosques are not one of the eight categories mentioned in the aayah, which  limits the spending of zakaah to these eight. And Allaah is the Source of strength.