Some mapping hints

    Buildings can be very close to each other, but in most cases do not actually touch each other - Try to map buildings close to each other without letting them connect or share nodes. Zooming some more can help you. If you made a mistake, you can also disconnect features:
        In iD: select the feature and press 'D'. This "disconnects" nodes. Press now 'M' and move your selected feature.
        In JOSM: selecting the feature and press ‘G’. This "un-glues" connected nodes, and you are now able to move the feature you want.

    Most buildings are rectangular, so make buildings rectangular even if a shadow, tree or roof pitch make it appear otherwise.
Best regards and keep mapping