Had that problem, too - not so much with building but with getting bored with a household. That changed about two years ago when I decided to build a household based on some fan fiction I was writing, so they already had some sort of background story. I'm still playing that household but have added a few more because after a while, I felt I needed to shake things up a bit.
At this point, I have 16 households in as many worlds. Some of them are offspring from my original household, the others are households I added over time. The reason why they are spread across worlds is I want to make the most of a world's gameplay. So for the household in Evergreen Harbour, the goal is to help them change the part of town where they live. The guy living in Ravenwood is an undertaker, his divorced wife recently moved to Forgotten Hollow and will be turned into a vampire sooner or later (she's having an affair with Celeb Vatore).
What also helps keep things interesting: have every Sim work a different career. I only have two Sims in the same career because it fit both characters. All the others are in different careers, and those who went to college have to take a job that aligns with their degree because otherwise, why bother with getting a degree at all, right?
I also try to leave my comfort zone every once in a while by accepting weird things the game throws at me and that I didn't plan. Yesterday, I returned to a household I last played I think two weeks ago. The second I got there, I received a pop-up telling me "It's the kid's birthday soon!" I was like: "Huh? Kid? What kid? He's single... drat! Is that an alien baby in the UI?" Then I remembered that when I last played that household, the guy got abducted by aliens, and when he returned home, I switched to the next household in line (I always switch on Sundays at 2 a.m.). I didn't even think about whether or not he might have gotten impregnated even though I knew it was possible.
So my first reaction was "Great. Now that's ruined". Then I considered sending the baby back to Sixam. Eventually, I figured I'll just roll with it and see what happens. I would have never made an alien myself, so here's my chance to use the gameplay that comes with aliens :)