The introduction date of the very first commercial double decker bus (just like the first bus itself) seems to be open to some debate, as different versions of models and claims appear all over the internet. Generally what is agreed upon is that commercial buses were first widely introduced in the 1820′s, (with a few previous unsuccessful attempts briefly recorded in France as early as 1662) and as this is some time before the introduction of the combustion engine, they were horse drawn as you would expect. These early buses were called ‘Omnibuses‘ and ran in France and England, and it wasn’t until 1847 when Adams & Co of Fairfield works, manufactured a vehicle with a clerestory roof with built in upper seats running the length of the bus. These were first operated by the Economic Conveyance Company of London, and to encourage people to use these the fare was half of the cost of sitting inside.
Source: Double Decker Bus