I might sometimes see something someone else has done or a house or building that I love in real life and want to do something similar on occasion, (usually it looks nothing like the original) but mostly I just start building and change things until I like the way it looks and the layout. Its mostly just out of my own head... With elements of specific build styles. For instance, I really like the look of 50's modern and Art Deco so most things I build are in the spirit of one or both of those. That said, I've done a few victorian or contemporary inspired builds too.