I build literally everything that my sims live in, i never could get happy with the ready made houses because I always feel the need to have my houses be uniquely based around each character and what they're about
I didn't start out with anything fancy mind you, but once I got used to tinkering with the way you can build rooms they got more and more extravagant.
My current favorite besides my aliens space ship (mild star wars and star trek influence in the shape) is my vampires castle that has the entrance lead to a large middle area where the dining table and everything are, with exits from that area going into an even larger square with staircases leading to different levels as well as being connected to the kitchen, with a massive library and normal living quarters up stairs and a heavily modified crypt downstairs that my vampire sleeps in, a pool in another chamber. And a bar, theater, dance, and karaoke room in the largest part of it.
I also did my best to create ny own version of the castle from Frozen forvmy Anna and Elsa sims, though my main focus was on the throne room concept art that I found.. and I made it... more extravagant once Hans took over, I took some pride in realizing that I could use that huge throne prop from the get famous dlc by stacking two rooms on top of one another and knocking out the ceiling . It's the single largest thing I've built XD... it kind of lags admittedly..