# 3. Main Section: Building Bottoms (optional)

Buildings defined using the section explained above all start at ground surface (z=0) as houses normally do.
If you want to define buildings beginning at a different height, an additional Building Bottom Section is needed.
This section begins with the keyword \BOTTOM followed by a structure similar to the main section shown before.
If you don't need non-zero bottoms, the \BOTTOM keyword must be replaced with a single remark line “%….”.