I don't think there is necessarily a discrepency in the distance between the Mansion house and the Cottage.

The way it is worded, it sounds like you can see the Great House if you travel a quarter of a mile past the Cottage--but not that they are necessarily that distance apart. The aspect (or view) of the Great House is a quarter of a mile past the Cottage. Or, in other words, you might be able to see the Great House a quarter of a mile before you actually arrived there, and that makes up the half mile.

Uppercross Cottage, with its viranda, French windows, and other prettinesses, was quite as likely to catch the traveller's eye, as the more consistent and considerable aspect and premises of the Great House, about a quarter of a mile farther on.