Static Project Cost Calculation (reinforced concrete + steel)

[kurcel]

Hello, I want to calculate the project cost for a project I'm currently doing, but I can't decide between 2 approaches. I wanted to get your opinion. I demolish the existing stairs of a building in the form of ground + 3 floors, in 2 different places, and add them to the floors, and I add an elevator and stairs from the outside to the building. When calculating the project cost, should I calculate the entire floor area or should I consider only the part where the changes were made, since it affects the entire structural system of the building? Since there is a problem with the foundation shoes, the system inside the building will be made of steel .....waiting for your comments...thanks