On 19 Mar, 16:54, Conor wrote:

Maximum height of a vehicle in the UK is 15ft 9in. Above that it's an
abnormal load. Take off the height of two adults standing up and that
leaves you under 4ft to create a third deck, have the
wheels/chassis/drivetrain etc.

Practically a height of about 15' 9" is correct although there is no
"legal" height limit in the UK.

Theoretically you could try the old lowbridge double deck bus idea of
a side corridor generating height under the seats of the upper deck.
Put one corridor on one side and the second on the other, then try to
load/unload in the rush hour and stay popular!

George