How do you go inside a house on gta4

If a building in GTA IV has been made accessible, Niko only has to push the door to go inside, then the same can be said for the rooms inside that building. Sometimes there is a marker Niko has to enter. There are obviously many more buildings in the game that cannot be entered, so if you think a building looks accessible, then just try to get Niko to push through the door.

Regarding safehouses. These become accessible as Niko progresses through the game and he acquires them in one way or another.