Left side menu with vertical sub items

Hi all,

Lately there is a demand for left sidebar menus. I guess a lot of Enfolders would like to implement a left sidebar menu, with nice subitem boxes arranged vertically.

The example is: https://www.tamtam.nl/werk/

The hard work: the sub items need to fit in between the main items and the content.
The solution could be: the main content left margin changes the moment a sub item is hover or opened.
See the screenshot of my work in progress http://snag.gy/KdOb2.jpg

Curious to read some ideas, work arounds, css about this.

Thanks