I did pretty much what you are thinking. My house came with the right side of the garage and I added on the left. The right side door is 16' the left side is about 10 or 12'. (I forget) I also brought the left side forward about 6' and backwards about 6'.
I came up with this for two reasons. The break in the front visually looks more appealing than just two huge doors staring at you. It allowed me to put in a window so it has more of a cottage look than a garage. (with a little imagination.

) I also designed around the tree to keep as much foilage as possible.
I can put 4 cars side by side, end to end in the left side but rarely do. Instead I use this as my detailing area because you can open all the doors, pull the vac around easily, etc. I could never do that on the right side.
The one drawback I have is the drain is on the right side. So if I wash the car it has to be done on the right.
I like your plan and
ANY extra space is welcome.
