Hi Rob, 
welcome to OSM! Good way to start the year with a contribution to open source projects. :)

I see you requested a review for the edit you made, so I had a look into it.
You adjusted the shape of the building and there are three things that could be done a bit better here:
1. You deleted the existing buildings and added new ones. Unless it's a completely newly built building that replaces one that was torn down, it's better to just change the existing building to keep the history of that object.
2. The creation of the new buildings made what OSM calls a "multipolygon". This is only needed for complex building shapes (e.g. a building with a hole in the middle). 
3. The building you created is not "squared", meaning the walls are not at 90 degree angles.

I'll do a quick revert on the changes and see if I can walk you through how to do the change easily while keeping the history of the object and making the angles straight, I'll write again in a bit.

All the best,
Paul