Today’s phrase is an Idiomatic Aphorism which is also most-commonly referred to as a “Saying” (because it is something that people “Say”) — AAAAAND it can also be used as an Interjection (as in the heading above).
It also implies that one should start with the basic necessities and/or “first steps” before doing other things which (though they may be more interesting) can only be beneficial after the foundational elements (which go in “the ground”) are firmly in place.
Also — it is basically impossible (at least in this version of reality) to Build A House From The Roof Down. (Although, Metaphorically Speaking, a lot of people seem to try doing that quite a lot of the time.)
This phrase is also good to know if you are preparing for The TOEFL Exam, as it is a rather common phrase, and may end up being in one of the reading transcripts or audio files on the exam. But you still need to “Build From The Ground Up”, by preparing for the exam with The Best TOEFL Preparation System In The Universe!
Have An Excellent Day!