Here is a good rundown on performance, including discussion on level streaming. Personally I do not make a level for each house, because then you end up with literally thousands of levels. But if there is enough content for a region, a special location, then why not https://www.youtube.com/watch?v=ZRaeiVAM4LI
Okay, so there is level composition, which I tried around 4.16 and just didn’t like it. With level streaming I have one persistent level which is loaded after the lobby level, and inside this I place level streaming volumes, with the respective sub levels of the persistent level. Landscapes are also part of sub levels in my game, instead of one big one, I add a new landscape with a dedicated level, then use a landscape material which shares some materials to blend them better. Imho, this works very good. So watch the video, an start small, you can always add more smaller tiles with their respective levels. There is really no need to have one huge landscape.