بسم الله الرحمن الرحيم
According to Shariah, it is called istisna (to get something prepared on order) to buy some flat or other products with the specification of features and with the condition of delivering it in some future date. Under istisna contract whatsoever advance amount the buyer pays in advance or in installments its sole owner is the seller (builder) and its zakah is wajib on him. Hence in the question mentioned above, if you at the time of booking the flat kept in mind the said conditions (specification of features and the condition of delivering it in future) then it shall be called istisna contract and the amount which you have paid its zakah shall not be wajib on you rather it shall be wajib on the seller (builder).
قال الشيخ تقي العثماني الاستصناع : ان يطلب المشتري من البائع ان ياتي له بشئ مصنوع بمواد من عنده موصوف فى الذمة ويلتزم البائع بذلك لقاء ثمن متفق عليه. (فقه البيوع ج 1، ص 585)
Allah knows Best!
Darul Uloom Deoband