I suspect it would have taken several years to build so perhaps foundations were laid out well before the completion date.
However it’s more than likely ‘something else’ occupied the site before.
Some of the trees close by are enormous and were almost certainly already impressive (protected in big house grounds?) when the place was built.
1870 is quoted in many places, but theres already a Hall marked on the 1857 OS map.
http://www.hidden-teesside.co.uk/2013/07/23/westerdale-hall/
I suspect it would have taken several years to build so perhaps foundations were laid out well before the completion date.
However it’s more than likely ‘something else’ occupied the site before.
Some of the trees close by are enormous and were almost certainly already impressive (protected in big house grounds?) when the place was built.