An "interior staircase" refers to a set of stairs that are located inside a building or structure, typically connecting different levels or floors within that building. Full definition
A third king bedroom / bath suite is on the villa's lower level, accessed via an L - shaped interior staircase. (paradizo.com)
Interior staircase of the Hayward Gallery, London, designed by the GLC Department of Architecture and Civic Design and completed in 1968. (apollo-magazine.com)
Renovations, which began in December 2011, included the creation of a new conference and meeting room space, a new reception area, reconfiguring office space to an open design and incorporating an existing interior staircase into the new design. (nreionline.com)