In my experience it would be better to avoid these areas marked in red as much as you can. They are marked that way for a reason and as much as I agree that you should be able to build anywhere you want in single player, even if you’re not building on the red but near it…things don’t always go smoothly.
Example I can give is, I like to build in this one area and I thought I was far enough away from a NPC camp that was near by. Things went fine for the first couple months but as time went on the NPC’s stopped spawning and I was also getting horrible lag near the red zone and if I went down to where the NPC camp was the lag was even worse. Even though there were no NPC’s there anymore. Ripping out a section of my base that was close to the red “no build here” didn’t work. So I ended up wiping the whole thing.
This is not always a bad thing. I’ve rebuilt my base 3 times so far and each time I rebuild I always come up with better Ideas and the base looks better overall. I know that time is an asset and things can get really frustrating sometimes but maybe take what I’ve said into consideration. Like with anything, the more you do the better you get at it.