Ok. We all know the “Overlapping UVs” problem when building lighting. We all know (more all less) how to fix it with a 3d software (eg 3ds Max, Unwrap UVW etc). This method is helpful for simple, lowpoly models. Here’s my question. I have an entire house with 260056 polys and 260406 vertices(meaning this is a really compicated model). How do I fix the “Overlappng UVs” problem now? Every time I use the usual method, 3ds Max gets stuck and then stops running.
You mean the entire house is one mesh?! …I wouldn’t waste my time with it: you won’t be able to assign a high enough lightmap resolution to achieve nice baking results… well ok, I don’t know what quality you’re after! 
But I would break up the mesh into smaller pieces…
You can try to flat map it (if just walls = hard surface) you should get a good result…
…or you can do it manually…
…or you can try Unreal Studio, it’s pretty good in generating lightmaps too!
It’s a house with a lot of details. Windows, small lights, doors, garage door, balcony etc. 
Actually it is broken up into smaller pieces. The creator hasn’t attached anything. And when I say anything I mean that he would create a piece of wood for the window and another one for the same window and he wouldn’t attached them. There are so may pieces. Eventually I thought of what you said too. I’m trying to attach some stuff and then try and your solution. Thanks for the reply!
You should look into Tiling textures.
If you plan to break up the house, you may want to break up the house by the general walls. if you do split the mesh up into different sections, I’d keep the origin point at the same location so that placing it in world is easier.
Thanks for the reply! I’ll give it a try.