For the most part this is all about the UV for lightmap. The larger your object, or more complex, the higher the resolution you may need to set your Lightmap Resolution to. The default lightmap resolution for each static mesh is 32. If the object is overly large, and additionally has lots of information packed into it’s uv coordinates for the lightmap, then you need to increase this. To test the best resolution try bringing your object into a default scene, with nothing in it, to tweak your shadow settings instead of testing in your game. This way you can quickly build lighting and test different resolution without building out an entire game over and over.
The problem with really large objects is that your resolution may have to be significantly increased to get clean shadows. Double them from 32 to 64, then 128, then 256, then 512. If you keep having problems and keep increasing your resolution to the detriment of game play it may be time to break your object up. Many people still have not learned the lesson of keeping objects modular, i.e. smaller bits to build something.
So instead of having an entire house, the house may have to be broken up considerably, and then brought into the software to maximize the shadow details. Ideally, if you can build modular parts to build the same house, then you can build the house, or even many houses, and keep the resolution of lightmap fairly low.
Things to be aware of is when you trade off convenience of having one object, for loss of speed in the engine when building lightmaps or even loading them later.