A hobby room is considered a “living space” and a “habitable room” on this side of the pond and probably on your side of the pond too, because the code is written by the International Code Council (ICC). (See ICC R202 Definitions)
Likewise, Garages are to be separated from single family residences by 1/2” gypsum board on garage side ONLY, provided the garage does not open into a sleeping room. (See 2017 edition ICC Table R302.6.) It DOES NOT matter if it’s a bearing wall or not.
Also the door between the garage and house shall be a solid wood door a minimum of 1 3/8” thick and is not permitted into a sleeping room. (See ICC R302.5.1.1.)
In order for the 1/2” gypsum board to be allowed and effective, it must be nailed into a minimum of 1 1/2” thick x 3 1/2” material at 16” on center. (5/8” thick gypsum board can be nailed at 24” on centers.)
Side note: You are creating a “fire wall” between the garage and living spaces. Usually gypsum board is required on both sides of the wall, but in this case, the “hazard” is located in the garage.
Oh, BTW, it’s not required on the ceiling of the garage unless there is a habitable room above.
AND no step is required at the doorway. In fact, the inside cannot be more than 1 1/2” difference than the outside, unless the door does not swing out over the landing then it can be 8”, or if it’s a handicap accessible route...in which case the maximum difference is 1/2”. (See ICC R311.3.1)