Pouring a slab next to a concrete apron.

[Treumann]

My daughter has a 3m x 5.5m building on a 100mm slab in her backyard with a 500mm concrete apron down the long side that slopes down 30mm to draw water away from building. She wants me to add a small bathroom 1.5m x 2.5m. Is it necessary to remove the apron and pour a 1.5m x 2.5m slab or could I place a 1m x 2.5m slab next to the apron and fill the apron to level with one of the self leveling slurries.