A city with immaculate manners, and one collection at the end of an avenue that quietly declines to have any.
April, if you can choose. The avenue is bare and the light is flat, which suits the stone.
Walk up to it rather than arriving at the door. The approach is half the building's argument, and a taxi throws it away for the sake of four minutes.
The modern extension at the back was the right decision, and people will tell you otherwise for as long as you let them. Go through into it and look back at the old house from inside the glass; that view settles the matter.
April is my preference — flat light, bare trees, and a city not yet full. Bath in high summer is a lovely place to queue, which is not the same as a lovely place to look at things.