it seems that many people who buys a landed house wants to rebuilt to maximise the GFA.
a landed house provides the owners the opportunity of having a private outdoor space which most apartments would not have. my personal opinion is that if the owners are not going to make use of this outdoor space, then might as well buy a condo which saves the headache of home maintenance. most older houses would have been built without the GFA being maximised so as to provide a balance between indoor and outdoor space. of cos an architect could design a new house with this in mind as well to maximise the upper storeys while still providing enough outdoor space for the 1st storey.
my suggestion is for you not to be fixated on the GFA but focus on what you plan to do while staying in the house and your lifestyle. also think about how long you are planning to stay in this house. if this is going to be your retirement home, then it might be better to just rebuilt it now and then provision for a home lift and spaces which allows for unobstructed movement. if it's just going to be short term (5 or 10 years) then maybe just do a simple renovation would be enough.