Why consider applicable? It says "when necessary for credible assignment results", which implies relevant.
"necessary for credible assignment results" = relevant to assignment results.
Is the cost approach relevant to developing MV indications for new construction single family residential property? Yes. Is it necessary? No.

Sure it is, when was the last new construction assignment that you got that the Cost Approach was not required?
(necessary to get paid)
FWIW, the CA is not relevant to MV today given the External Obsolescence due to the over-supply/weak economy etc
Can be applied is not the question.