Reconstruct house sump to increase storage capacity

We have a small sump in our building with storage of about 3000ltrs. There are two houses, ground floor and first floor dependent on this sump. Also water supply in our area is not good so we need to fill it up with tanker on a need basis. In order to increase capacity of sump so that we can make full use of water supply and tanker water we plan to demolish and rebuild the sump to have a storage of about 9000ltrs.

The sump is located in the parking space near the main gate but it does not spread across the whole vacant space till the main door. Is it alright if we take all the space from the gate till the door ? When we demolish the sump-walls adjacent to the house-walls will there be a huge impact on the house-walls. Is there a way to do the reconstruction carefully without damaging the house walls/foundation.

The house does not have pillars. Also we have overhead Syntex tank of capacity 1500 ltrs. Can we add more here?

Pl let me know.

Thank You