Echo,
Study other projects in UE4 and even UDK to get a feel for modular models as Transcendence talks about. They will help you to understand the method of turning parts into buildings. Houses do get a bit more complex than sort of generically created buildings where walls and floors can be swapped out. An industrial building for instance is more generic, but a little house that you want to make out of modular parts means making the walls separate so you can reuse them and reconfigure them into different shapes, and this gets more complicated.
I would start with making a simple house in software like Maya, you can work in feet (not inches) and export as Centimeters (found under preferences > settings). UE4 expects centimeters.
All of this is complex. The reason that games like DayZ make choices about what you interact with or not, is that all of that takes work, and puts pressure on the game engine too. So a room with all dynamic objects, or objects that are hinged means someone has to set that up doing props and set dressing essentially. The doors and movable windows all have to come from a library that is hinged and ready to go. This all means modeling, texturing, testing dynamics or animating manually. This might mean thousands of objects are modeled, textured, tested, lit, animated, and developed over time to develop a library of parts that you use to make your game.
If you are someone who is curious about game making, and critical of games like DayZ, It’s not a bad thing to experience exactly what goes into making a game and why it is complex, often requires huge amounts of time, knowledge and creativity, and endless reserves of tolerance and patience. This is why companies that make games like Halo and GTA spend many millions of dollars and have huge teams to develop the art, the scripts, the animated cut scenes and animated people etc.
So look at games in UE4 and the content samples. Study how they are put together. Read articles on Modular building. Start to develop a texture library, and start learning to model a little bit at a time in software like Maya, as you develop your knowledge of UE4 at the same time. It will start to come together a little at time.