'laid out on three levels of elevation 20 feet in height' Help me understand this phrase

The single-storey building is constructed on ground of different height or elevation, and so the sections needed to be connected by stairs.

elevation
NOUN

Height above a given level, especially sea level.
Line of sight and terrain elevation is very important!

The ground level varies by 20 feet in height. Something like (but not exactly) this:

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