I have a large house model that is composed of other models. Some of the models in the house for example doors are joined together.
How can I select entire model and separate into loose parts?
that's not going to work, not as a single operation anyway. first you need to select your doors and separate by selection, that will create a separate object of just the doors. then once all the connected objects have been separated in this way, you can then select the main model and separate by loose parts. then you need to edit the door mesh and separate the individual doors by loose parts and so on.