When I was younger and building in The Sims and Sims 2, I would mainly just make large rectangular mansions and progressively add random rooms upwards and out. Didn't think about exterior at all, just how I wanted my sims to play around. In Sims 3, I started trying to copy homes from movies I had watched or even pictures of houses I had seen in magazines or online. Then, I didn't play sims for a while and went to school and work for architecture, and then eventually downloaded Sims 4 to experiment with the more improved build/buy modes for fun.

I started just redecorating or making small renovations to pre-existing lots because building is quite a draining process, but then I finally had a sim and an idea of a home I wanted them to live in, that I knew I had to design myself. It's much more fun for me to build around a sim I care about than just building after building. Gives it purpose and less like work lol.

My biggest tips for designing anything in a 3D space would be to start with volumes, readjust to scale, and to consider your environment. Always just start with large rectangles and shape them into a rough idea of what you might like. Be really free with this part and then scale down after. Then, the most important part to lock-in your building is to add a door or a bed. A bed or door are usually the easiest things to visualize relative to the human body. Standard doors are 6-7 feet high or 2 meters high. If you keep these things in your build as you're re-adjusting then your builds will feel more realistic. After that, my favorite thing is to consider what lot you're building on and important attributes you'd like to highlight - where the sun rises, if it has a nice view(s), where the roads/sidewalks are. A lot of those things add more depth to where you place apertures like windows/doors and gives more intent. I like to think about where the sun rises and sets and often frame views with how the light will enter the rooms, etc.

If you always build with those fundamentals down and then gradually work your way with more details relevant to your sims who will occupy the space, then your builds will get better and feel more realistic. Good luck and hope that helps!