14 CFR part 21 contains the procedural rules for obtaining a type
certificate
and approval for changes to a type certificate

Yes.

(i.e., a Supplemental Type
Certificate),

No, that is Part 23 for under 12,500 lbs.

snip of rest without reading

Let me be fair to the FSDO guy and agree that the icing capability is
usually added after the Part 21 Type Certificate is issued. Therefore, if
you have anti-icing capability for your small airplane, it is probably a
Part 23 STC change.

Please allow me to delve into administrative minutia for a second.

I agree that, in our example, anti-icing capability added to a small airplane
after manufacture is a change to the type design, and that the airworthiness
rules to which the anti-icing equipment must be certificated are in part 23.
In addition to the equipment, the Kinds of Operations (23.1525) would need to
be changed to include flight in icing conditions.

However, the procedural rules governing the approval of a Supplemental Type
Certificate are in part 21. Subpart D of part 21 talks about changes to a type
certificate, and subpart E deals specifically with Supplemental Type
Certificates. Subpart E defines the applicable airworthiness requirements for
the STC in a similar fashion to subpart B for type certificates. In our icing
example, the applicable airworthiness requirements would be contained in part
23.

So, my bottom line is that the procedural requirements for obtaining design
approval of either a new design or changes to an existing design are contained
in part 21. The design approval, either a TC or an STC, is issued in
accordance with the procedures in part 21. The airworthiness requirements to
which the design or design change must comply are contained in the
airworthiness standards (e.g., part 23 for small airplanes), or part 21 for
unique aircraft (e.g., primary category).

Like I said at the beginning, "administrative minutia."

Cheers,

Gordon.