Light areas in watercolor (or white) can be confusing to paint - where do you start? It's good to make a plan of where these light shapes will be. Reflections, wet paint, and modified edges are covered in this full demo.
Try painting pastel shades or white objects with the plan to support the objects later with dark. Notice as you create the painting that edges can be manipulated. Because trees are forgiving in shape they are relatively easy to do. A building poses a challenge - but it's easier than you think! Keep to the main blob/shape and break apart in small bits. Don't over describe the architecture.