How to Extend Window Stool Into a Bookcase?

Check the building code first. If it's upstairs and small children can climb easily on it, they can fall off the window easily.

Replace the window stool with a larger piece of wood, or plywood with an applied solid wood nosing to replicate the stool profile. As an alternative, you can cut off the stool nose, hit the cut edge with a plane and biscuit join the stool extension. Make sure it is well supported - particularly the leading edges.

it can be helpful for visualization and for dimensioning to make the piece full size from cardboard first. you can trim with scissors and assemble with tape, and see what the piece will look like before getting into wood.

What general area do you live? If this is in fact true, it's _got_ to be California.

It seems that a code would be pretty silly to regulate window stool height. The building occupant could simply place a freestanding item (hamper, low bookcase, credenza, etc...) under the window, and a child could easily climb on it and fall out the window.

Is this code where anyone else lives?

If it is, I hope my local officials never decide it's a good idea.