As nouns the difference between legitimacy and allowableness
is that legitimacy is the quality of being legitimate or valid; validity while allowableness is the state of being allowable; legitimacy; permissibleness.
legitimacy
English
Noun
(head)
the quality of being legitimate or valid; validity
Synonyms
* legitimateness (much less common)
allowableness
English
Noun
(-)
The state of being allowable; legitimacy; permissibleness.