Sacha writes:

One of the family is replacing a rotten fence but wants to make sure that
the hedgehogs which visit frequently can get in and out. She needs to make
sure also that her v. small Jack Russell can't. Does anyone happen to know
the desired height under which a hedgehog can get into its favoured habitat?
We reckon something between four and six inches?

A lot less than that. I rebuilt the side door of my garage a few years
ago and had to build up a new shaped bit for the bottom. Before I
fitted it I was sitting out in the garden one evening when one of the
local hedgehogs wandered along, squeezed under the door, spent some
time investigating the inside of the garage and came out again. I've
just measured the screws that now fix the bottom bit at two inches up
from ground level.

Anthony