From the perspective of geomantic omen, houses should not be "integrated with the old", that is, some houses are new and some are old. Such a house is unlucky.
Therefore, if the old house is demolished, the remaining half is the "old and new one" model, and the combination of the two is unnatural. Things with different attributes are connected together, and the feng shui attributes are different.
From a scientific point of view, it is not good to tear down half of the old house and leave half. Because the durability and strength of old building materials are different from those of new building materials, their overall strength is affected by weaknesses.
Moreover, tearing down the old house in half and leaving it in half will destroy the overall aesthetic feeling. Adding new things to the old house will definitely cause a certain gap between the two, and the contrast between the old and the new will be visually abrupt and unacceptable.
If this must be done, the connection of new parts must be as small as possible. For example, in the choice of materials and construction methods, they can be as close as possible.
The above is about building a new house, I hope I can help you!