The only solution i'm thinking of is to create a whole new Panel for the plans. Do you think is possible at all?

You could create a new panel that will propose a list with icons of the different plans, or an Open from sample menu that would list these plans in a sub menu. Then it would copy the sh3d file of the sample home from a resource file to a temporary file and open it with existing classes of Sweet Home 3D.

Has anyone worked on this?

I did, but never programmed it yet, mainly because of the time required to create a library of sample plans based only on default models (to make them as small as possible).

The other question i have is a lot trickier for what i managed to see in the code. Does anyone tried to add a button to make the dimensions of walls show in the PlanPane. Any hints in this?

You can do that with a plug-in action by setting its SMALL_ICON and TOOL_BAR properties. Studying existing plug-ins (like the Advanced plan editing plug-in) should help you. If you prefer to work on Sweet Home 3D code, study how menu items and tool bar buttons are created in the HomePane class.

Finally, i read in another post that the UML diagrams posted are not updated, is there any diagram updated

No yet. But there weren't so many classes added to the model layer (mainly the Level class which is bound to many other classes).

By the way, it would be nice to present yourself and your goal to the community.