Single word to describe "something that should have been done at the beginning"

I am looking for a single word to describe something that should have been done from the beginning to describe the highlighted situation below:

Accessibility remediation after the fact is harder. Accessibility should be considered from the start as part of the design.

I know the following is not what I want, but I would like to have a sentence similar to: "Accessibility should be a first-principles consideration."

Solution 1:

A more colloquial term would be "baked in" - indicating something that must be there from the start. (You can't bake a cake, and then add an egg to the batter. The eggs are baked in, or they aren't there; it's impossible to add one after the fact.)

An antonym is "tacked on", suggesting something that is added later but not integral to the design. (Hey.. for that matter, "integral" is a good word too.)

Software should be written with security baked in; it's very difficult to retrofit.

or

Accessibility should be baked in to the design, not tacked on as an afterthought.

Another possibility is "foundational" -- suggesting something that is actually part of the foundation. https://www.merriam-webster.com/dictionary/foundational

Security is foundational to good software

Solution 2:

Fundamental might be one candidate. Referring to the foundations it indicates that anything built upon a shaky start could have bad consequences.

See for example the dictionary entry here. An example sentence might be "When casting a bell, the correct preparation of the mould is of fundamental importance. If the mould is incorrect, the cast bell will not have the correct sound."