When a vector map is in use, buildings use the 3D style beginning at zoom level 17 and higher by default. A Footprints style is also available, which displays only the 2D outlines of buildings for a cleaner appearance. You can customize fill and stroke colors for both types.
Set the building style
To set the building style:
Open the Style Editor, then select the Buildings feature type (under "Landscape, Human-made").
Select the 3D building style to display buildings with a 3D outline:
Select the Footprints building style to display buildings with a 2D outline: