I recently found myself looking at school buses, for potential conversion into an RV. While there are many relevant criteria to be satisfied when making such a purchase, probably the most important is "will I be able to drive it?" I am not so concerned about physically being able to operate a bus, only legally. Most school buses in which I am interested have a GVWR over 26,000 pounds, which normally requires an upgraded, "Class B" license in California (and similar licensing in most other states.) But as it turns out, this is simpler than it seems at first.
GVWR stands for gross vehicle weight rating, and it is the maximum loaded weight that your vehicle can "carry", including itself. This value is based on a number of factors, including axle weight rating, braking distance when laden, and specified tire load ratings. You can load the difference of the GVWR and the actual "unladen weight" (or "curb weight") into your vehicle and transport it legally.
In some states, you can simply register a vehicle for a lower GVWR than the original rating, but in most (including California) they go by what the manufacturer stamps on the chassis information tag. A licensed coach builder can re-rate the GVWR, but this is a fiddly process involving retesting and a VIN change, because one of the elements of the VIN is the GVWR.
The Class B License
A Class B license entitles the bearer to drive vehicles over 26,000 pounds, or to tow trailers over 10,000 pounds. It also permits them to drive "house cars" up to 45 feet in length. The combination of vehicle plus trailer may not exceed 65 feet.
Getting a Class B license is not a major ordeal, but it does add complication and expense to the process. There is a filing fee, and you have to take a driving test in an applicable vehicle. This test is broadly the same as the test for a commercial Class B license, except that the driver is not required to do the full air brake portion of the test even if their vehicle features air brakes.
The most relevant piece of the puzzle is found in California vehicle code §12804.10. which states:
(a)Notwithstanding any other provision of law, a person issued a class C license under paragraph (3) of subdivision (b) of Section 12804.9 may drive any house car of 40 feet in length or less without obtaining a noncommercial class B driver’s license with house car endorsement as described in subdivision (b).
(b)Any person seeking to drive any house car over 40 feet in length, excluding safety devices and safety bumpers, shall obtain a noncommercial class B driver’s license with house car endorsement as described in this subdivision.[...]
Keeping in mind that I am not a lawyer and this is therefore not legal advice, the short English translation is that you can drive anything titled as an RV (in California, the registration designation is "MH" for "motor home") on a class C license, unless it is over 40 feet long.
Under Forty Feet
Obviously, (a) is your salvation, so long as your vehicle is under forty feet long. Then you still don't need an upgraded license, regardless of GVWR. This measurement is taken from bumper to bumper, and does not include mirrors, which may extend past the forty foot limit. No additional testing is required, regardless of equipment.
Over Forty Feet or Towing Over 10,000 lb
If your bus is over forty feet, you will need a Class B license. If you are towing over 10,000 pounds, same. If your house car has air brakes, you need to do only the in-cab portion of the air brake test - you do not need to inspect or adjust the air brakes.