A Question About Sims 4

You can create a new game and use that save specifically for building. The new save is completely separate from your other one and won't affect anything on that save. Switching back to your other save is super easy and quick. Just click load game and choose the save you want to continue playing.

To create a building save on the main menu just click new game. It will force you to create a sim to start the save but if you just want to practice building houses you can just randomize them since you won't actually be playing with that sim.