I have started construction of a shopping complex for rent. I have been investing money in that project for less than a year. Construction is going on. Should I pay Zakah for the amount I am investing now? If so how it is being calculated?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

You have to pay Zakah on any sum of money as long as: (1) it is still not used in you project, (2) it equals the value of 85 grams of gold (Nisab) and (3) it is in your possession of one lunar year (Hawl). These three conditions should be met.
As for the money you spent in your project, it is not subject to Zakah as long as has not been in your possession for one lunar year (Hawl).
As for the project itself, we state the following two points:
1- If your project is prepared for trade, then you have to pay the Zakah 2.5 percent as long as its value equals 85 grams of gold or more at the end of the Hawl.
2- If your project is prepared for rent, then you have to pay the Zakah of its revenue as long as (1) it equals the value of 85 grams of gold, and (2) passing of one lunar year from the time in which the rent becomes due (Hawl).
Allah knows best.