Do I require release forms to sell footage of historic/recognisable buildings filmed from outside, or that appear in general footage of the area - like a cityscape?
That depends on the building. You will have to do some research. Historic buildings, no.
The Shard - London. Needs property release.
Colosseum - Rome. Does not need property release.
Any modern sculpture/artwork would also need a property release. But an old statue would not.
If the building is a small part of a big skyline, it generally does not need a property release. If you see The Shard in the background of a London cityscape clip, it would not need a release.