Re: Sims 4 - build mode blue, not showing design

I'm bringing back this old post because i had the same question. This seems to happen when there's some sort of issue with the structure of what you're doing. Like maybe you've made a change and now your room doesn't have a wall.   

Or, I made a deck that was blue and it was because i needed to pick the floor and fencing.

If I had to recommend something, I would say back out if you can to the point that your action did this and see if you can do it a different way. If it's too late for that, see if your house isn't a complete wall all the way around or something like that.