When you use the term "Duplex", you mean that it is a two story house, correct ? In the USA, the term "Duplex" means two separate living units side by side......but in other parts of the world it means a two story residence. Please clarily that you are talking about a two story single familly house...... If so, the footprint that your house will have on the ground floor will be approximately 1000 square feet. Do you need to park a car on the site or is parking offsite? If the parking is offsite, then you have lots of room for a two story, 1800 plus square foot house. The design will be dependent on your personal criteria, and it would be helpful if you could hire a local architect to help you with the layout of the rooms. It should be relatively simple once all of the factors are considered.

@Cornerstone design studio: Apologies for making big post; thought that more info will help. @Rupal ji, If it sounds like I dont need any changes; my apologies for that. Let me put it in short,"I am looking for options that give modern facilities without leaving good things from old charm/ comforts. " Thanks for the straight forward comments. I shall try to edit my original post to make it simpler and that looks open to new ideas.

What about if I build pillar along with plinth beam for 6 locations and beam over that is it required?

A evaluation of the structural integrity of the structure is needed before proceeding with the construction Typically if you have 9" walls throught the base and follow the same layout of the walls on the first floor, you should be able to add rooms as per your requirement. There are also ways in which you can add a framed structure to your existing ground floor, so as to have the freedom to redesign and renovate the entire house as per modern concepts