The wall of any attached garage and adjacent living space must be a fire wall, no penetrations without fireproof sealant. Both spaces need ventilation, soffit/ridge vents are best, but anything that lets air pass works.
'Sealed for air leaks' are the key words. The possibility of carbon monoxide passage is the problem. If you put in a door or hatch into the new attic space make sure it's weatherstripped and stays closed.