Content stating that the surface-level Class E (or D) extensions ("arrival extensions") "become part of" the adjoined core surface-level controlled airspace ("primary core surface area") whenever the core surface-level controlled airspace is in effect, first appeared in the issues of the Airport / Facilities Directory effective beginning May 25, 1995.
The exact text in the volume for the Northeastern region for this date reads "NOTE: AIRSPACE SVC EFF TIMES INCLUDE ALL ASSOCIATED EXTENSIONS. Arrival extensions for instrument approach procedures become part of the primary core surface area. These extensions may be either Class D or Class E airspace and are effective concurrent with the times of the primary core surface area. See CLASS AIRSPACE in the Airman's Information Manual for further details"
The last sentence apparently contains a typo (missing letter/s) and was likely intended to read "See CLASS E AIRSPACE..." or "See CLASS D/E AIRSPACE..." (Note that this apparent typo was still present as late as 2001 -- see https://aviation.stackexchange.com/a/91352/34686 )
The previous volumes of the A/FD, effective beginning March 30, 1995, contain no content pertaining to these surface-level Class E (or D) "extensions".
Scans of the relevant pages of the Northeastern region volumes effective beginning March 30 1995 and May 25 1995 are attached below:
This resource may be viewed at the National Archives in College Park MD -- see https://catalog.archives.gov/id/2068505
The fact that this note appeared in the Airport/ Facility directory less than two years after the September 16 1993 "alphabet" airspace re-designation -- when the surface-level Class E "extensions" were first introduced-- would seem to shed some light on the FAA's original intentions as to whether any FARs should ever be considered to apply to the surface-level Class E "extensions" but not the core "surface areas", or vice versa. This would seem to have some bearing on the following ASE questions:
Does FAR 91.155c apply to class E surface extensions? (FAR 91.155c prohibits VFR operations below a 1000' cloud ceiling in certain airspace, unless conducted under a Special VFR (SVFR) clearance.)
Does an SVFR clearance extend to Echo surface extensions?
In the US, in actual practice, workload permitting, will ARTC facilities grant SVFR clearance for surface-level Class E "extensions" (E4 airspace)?
Which parts of class E airspace can an ultralight (part 103) fly in without prior ATC authorization?
What is the primary reason that the non-towered airports ACV, TVL, SGU, BIH, and SIT/PASI have E4 extensions to E2 airspace?
What indication has the FAA given that phrases like "surface area of Class E airspace designated for an airport" do or don't include E4 "extensions"?
This content in the AF/D (now Chart Supplement) would also appear to bear on the question of whether or not a Small Unmanned Aircraft System (e.g. model airplane, quadcopter, "drone") may be operated in the surface-level Class E "extensions" with no prior authorization, under the terms of either FAR Part 107 (specifically 107.41), or 49 U.S. Code § 44809 - "Exception for limited recreational operations of unmanned aircraft". Note that the "extensions" are not currently included in the "gridded airspace" indicating the areas where prior authorization is required for SUAS operations on the LAANC map for automated on-line authorization of SUAS operations. Several FAA issuances including the Gardner memo (scroll down to "Text of the FAA Memorandum Clarifying Class E Airspace"), and this power-point document from an FAA Aviation Safety Inspector, take the viewpoint that prior authorization is indeed not required to operate a SUAS in surface-level Class E "extensions", under the terms of either FAR Part 107 or the "Recreational Exception", even though such authorization is unquestionably required in the adjoined core Class E or Class D "surface areas" that include the airport itself. This would seem to be inconsistent with the note in the A/F D (now Chart Supplement) that is the subject of the present answer.
The best (and most widely accepted) interpretation of the regulations appears to be that whenever the "extensions" are in effect, then from a regulatory standpoint they should be treated just like a core Class E "surface area" that actually includes the airport for which it is designated. In the specific context of a Special VFR clearance, the most conservative approach would be to assume that FAR 91.155c's prohibition on flying VFR below a cloud ceiling that is lower than 1000' AGL (without a Special VFR clearance) does include surface-level Class E "extensions", but at the same time always get explicit confirmation from the issuing controller before assuming that any given Special VFR clearance should in fact be understood to include a surface-level Class E "extension".1
- The lack of a universally shared viewpoint on this issue throughout the FAA, and the resulting need for pilots to clarify with the issuing controller or controlling agency whether or not a Special VFR clearance should be understood to include surface-level Class E "extensions", was emphasized by a high-level FAA staffer during a recent (October 2021) meeting of the FAA's Aeronautical Charting Group, in discussion around a peripherally-related agenda item. His comment included the statement "Pilots have failed checkrides due to the ambiguity around this issue."