Can I model with in a mode where quads are all planar? I have started modelling with triangles and quads.

As many novice blender user before me, it took me a while to understand that my quads were in most case not planar.

For my project, I need my quads to be planar. I understand that I can use the Mesh->Clean_up->Make_Planar_Faces to flatten quads.

However, I find this a bit messy as soon as you have a lot of quads. 

I would like to know if it is possible, when you make a face or move a vertice, to impose a "quad needs to be planar" constraint.

I mean, that if I select 3 vertices, Blender should be theorically able to put  the 4th vertice in the same plane as the 3 others.

I hope I am clear... is there a way to model quads with this constraint ?

Or if not, is there a way to make a quad planar, by specifying that 3 vertices should not move, and only the fourth should ?

Thanks in advance for your help
I never needed that, but I think this workflow could do the job for you:

Creating new planar quads:


Make sure you have F2 addon enabled (it is enabled by default in Blender) by pressing CtrlAltU and navigating to Addons. Search for "F2" and enable it if it isn't already.
Extrude one vertex from existing polygon's edge so you get 2 edges prepared for the new quad
Select the middle vertex and move your mouse to the position you want to create a quad's opposite corner vertex
Press F - now the F2 addon creates a planar quad based on these 3 vertices






To move a vertex while keeping a polygon planar (when creating new polygons)


Make sure you have Transform Orientation set to Normal


Select a vertex and press G for move and then ShiftZ 2x to move on a plane perpendicular to Z normal of the face
This of course work only for corner vertices that are not part of other faces this is going to work when you are creating new polygons.
Alternatively, you can use Vertex Slide ShiftV that move's a vertex along edges so it also keeps a face planar. You can press Ctrl while sliding to be able to extend the sliding outside.






And as you have already noted, you can always press Spacebar > Make Planar Faces to convert the existing faces to planar ones.