What does "build in" mean?

Also, build into. Construct or include as an integral part; also, make automatic, concomitant, or inherent. For example, Frank Lloyd Wright liked to build in as much furniture as possible, not just bookcases but desks, tables, and the like, or We've got to build some slack into the schedule for this project. The literal usage referring to physical objects dates from the late 1920s. The figurative arose a decade or so later. Both are frequently used in past participle form, that is, built in.