Rootfilled front yard...can I do this?

Hi! I bought a house with a yard that was neglected for 25 years. My little new front yard faces (from the front door) west, northwest. On edge of the southwesterly side, back about 3 feet is an enormous maple tree that I love. The yard gets between 1 (next to the house) to almost all day sun at the sidewalk. The side where the maple tree is is very very shallow soil-wise, there's roots and moss and packed down clayish soil. It slopes slightly down toward the sidewalk and the grass that grows is sparse.

I would like to enclose this small area with an openish picket fence with flowers on both sides. This is the sunny area. I would like a little patch of grass in the middle, and plant shady-garden stuff next to the house. My plan is to build up the whole yard with inches to a foot of good soil next year, building it to make it more level toward the sidewalk, put in flower beds and seed or sod the middle grass portion.

Can I do this without hurting my maple tree? How much do I need to dig up the existing soil? I am afraid of hurting that maple tree and all those roots.

Any and all suggestions, comments and help are most welcome!