Though may not be obvious at first sight, the floor of the Class E "extensions" is definitely the surface. Look at any of the E3 and E4 airspace descriptions in FAA Order JO 7400 -- https://www.faa.gov/documentLibrary/...O_7400.11C.pdf
I know, it's a can of worms. If you want to read about another possible can of worms re model airplane flight, see a related discussion here. However, I need to emphasize, the apparent problem here is in the words of the authorizing congressional legislation-- not the FARs. The actual FARs have not yet been codified, or at least have not yet been released to the public. -- https://www.rcgroups.com/forums/show...er-E4-airspace
Steve