Informational update:

I had a bug listed on my, well, bug list, which turned out to be a "not" bug. Sometimes the plots wouldn't snap to some of the base game wall elements, even if you are using Place Everywhere. This happened to me on the already existing Sanctuary foundations, and is because the base game walls use a slightly different z-axis variable in comparison to the DLC walls. The only way to fix this was to either adapt all base game walls(30+ when I stopped counting), or make the plots float like one inch above ground. The first solution is disproportionate, the second I don't like from a visual ethics point of view. But there's a trick. Build the plot first, then snap the wall against it, and use the first build plot to snap all other plots/walls etc.

Prerequisites and install?

DLC02 is mandatory. It's a loose files mesh replacer, so install at will.

What is DLC02?

I see what you did there. If you do not have Garden Plots anywhere in your workshop menu(resource/misc), you do not have DLC02.

Such a mod already exists!

Really?

So what's the difference?

Unless the creator of the other mod has fixed the clipping, then there's no clipping in the new version. This new version also does not make them float. It will also make the plots snap to all elements that have the P-Floor, or P-Floor-Dif snapping keyword enabled in the mesh file, so you can perfectly snap inside a glasshouse for example(see screenshots). If you want  specific other snapping points, you'd need to point out, since I haven't found a reason to make the Garden Plot Snap to, for example, ceiling elements.

Sometimes some flora can't be placed, but suddenly it can!?! And then sometimes it would crash!

Refer to the Wiki(bug section) to understand that this is a vanilla bug. I actually experienced that myself twice(with melons attached to the plot, and picking it up in workshop mode, while the melon was still attached) even without modding anything plot or flora related, and I've yet to find a way how to prevent its very random nature. Just save the game in before moving those plots, remove the flora from the plot before moving, or don't move the plots at all after you've placed flora on top.

All your snapping mods make me snap!

That's intentional.