All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
As long as you have bought the plot of land in installments, these installments have become a debt that you have to the state. Hence, you can deduct from this saved money the amount of debt that you have – if you do not have other surplus funds that you do not need to make it in exchange for the debt – so you deduct the amount of debt from the money you have saved. Then, you pay Zakat on what is left, if it is not less than the Nisab (minimum amount liable for Zakat), and then pay out of it 2.5%.
As for the value of the construction, you do not deduct it from the money saved, unless you have purchased building materials on credit, and have not yet paid for them, or you have an amount due to the construction workers or contractor, and you have not paid it. In this case, you can deduct it from the saved money.
Allah knows best.