Hi Joseph-97,
You're doing some good work adding to the map! Here's a few tips for adding buildings:
- First, when aligning aerial imagery, use the "OSMUK Cadastral Parcels" overlay (scroll down the Background Settings list). This is the best way of ensuring consistency across the UK.
- Second, after drawing the building outline at roof level, it should be moved to match the location at ground level.
- If you delete what's there and start again from scratch (as you have done here), be sure to add back relevant tags that were on the original building.

Happy mapping!
Jez

https://wiki.openstreetmap.org/wiki/Armchair_mapping