What area of row house / Duplex is taken on property registration document?

You should include the total carpet area when you are purchasing an already constructed house. Taking only the ground floor area is not correct. You will land in problems afterward. But if you are purchasing the land and then you are getting the construction made the case is different. First, they will register the actual area of the land for registration and when you go for construction you have to apply for municipal approval with the actual plan of construction. If it is an apartment first they will register the undivided share of the land and then they will make an agreement for the construction and then get the municipal permission.
I purchased land and the actual area is registered through sale deed. Then I have taken permission from the municipality for the construction of suplex house by showing the complete plan of both the floors and got the approval.

The carpet area is defined as the net usable floor area of an apartment or duplex house. So all such usable floor areas are to be added to reach the total carpet area of the house or flat. Please note that balcony, lift, lobby, common area etc will not be added to the carpet area. If the common areas are also included then that is known as built in area. In general the carpet area is about 70% of built in area.

The assessment of the property is generally done by multiplying the built in area by the current market rate going in that locality. The market rate fluctuates from location to location depending on various factors like vicinity of schools, offices, market and bank facilities etc.

The stamp duty at the time of registration of the property is charged on the built in area and not on the carpet area. In some states the Govt has directed to sell the property based on the carpet area but that is not implemented properly so far and is in a transition state as Govt is still charging the registration charges on the built in area.

One should be careful in distinguishing between carpet area and built in area as a flat advertised as 1300 sq. feet of built in area will only be having only about 900 sq. feet of carpet area.