階段 and 段差: a staircase and a difference in surface height
Compare stairs between floors with a small entrance step, without turning every height difference into a long staircase.
A structure made of successive steps
階段, read かいだん, means stairs. 二階へ行く階段は奥にあります。 says that the stairs leading to the second floor are farther inside.
階段の途中で立ち止まりました。 says that someone stopped partway along the stairs. It does not state arrival on the upper floor.
A height difference between surfaces
段差, read だんさ, means a step or height difference between surfaces. 入口に小さな段差があります。 says that there is a small step at the entrance.
床の段差をなくしました。 says that someone removed the difference in floor level. It does not establish demolition of an entire staircase.
Different descriptions of the same structure
階段の各段には段差があります。 says that each stair step has a height difference. It describes both the staircase structure and the levels of its steps.
段差の高さを測りました。 says that someone measured the step’s height difference. It does not describe counting the number of stairs.
Does 入口に小さな段差があります。 establish that the entrance has a long staircase?
Show the answer and why
No. It specifies a small change in height at the entrance, not a long series of steps.