Regarding feasibility of 10,000 ltrs (using 5000 X 2 Triple layer tank) overhead water storage in 2nd floor(1st floor terrace)

Hello Sir,

My question is regarding feasibility of 10,000 ltrs (using 5000 X 2 Triple layer tank) overhead water storage in 2nd floor(1st floor terrace).

I have a requirement to store 1000 ltrs water in 2nd floor(1st floor terrace). Property is a Duplex House  using load bearing wall design. 

Please refer to Pic1 attached for the current tank and structure.

Please refer to Pic2 attached for the Triple layer tank dimensions.

I dont know ABCD of civil engineering, i can approach architect locally for this, but architects are not always approchable when you have very less or house alteration work. They tend to be very busy. Hence i am keeping hopes on this forum to clear my initial thoughts and then go to them with clear plan.

Also nowadays inspite of having professionals doing work, if something goes wrong they dont own responsibility, hence they always try to push what they know instead of trying new.

My Questions

1) Is my thinking practical to store 10,000 ltr water overhead using plastic tanks by constructing proper concrete structures to house these tanks ?

2)If my thinking is feasible, What advantages will i have in terms of space,money or overall building stability, if build a concrete tank instead of plastic?

Note:- i have a sump of 3,500 ltr capacity (no space left to extend it) and has a portico or sitout in front of the house with dimensions(ft) 10 X 15. 

3) House was constructed 15 years ago, we dont have any foundation plan or idea, is it safe to digup portico area for a new large under ground storage? 

I have a wierd fear that it might damage the foundation?