It's quite faithful. The TV show mostly sticks to the essence and key plot points of the novels.
The TV series 'Granchester' is relatively true to the novels. It manages to capture the main storylines and character developments. Some changes are made to fit the TV format, but they don't stray far from the original works.
Overall, 'Granchester' on TV does a decent job of adhering to the novels by James Runcie. There might be some minor tweaks for the sake of TV adaptation, but the core elements remain intact.