Yes it's tacked on, but I wouldn't assume it's necessarily an extension to the pub; it may be a separate house. The reason it looks strange is that the shared wall that adjoins the pub isn't parallel to the gable end next to the road giving the extension a trapezoid floor plan. If you look at the shadow cast by the gutter on the extension it has the same depth on both sides of the down pipe, meaning the two walls old and new lie on the same plane.
Interesting that it has a bricked up window. It's possible it was built before 1851 when the window tax was abolished, but since the tax had been in existence from the start of the eighteenth century, why build the window in the first place?