How do you create a relation between a building and 3D building parts?

Most 3D buildings in the database do not use building relations. In the absence of relations, building parts simply belong to the building that geometrically contains them. Situations where that simple rule doesn't work are very rare, but they do exist: When multiple building outlines overlap, relations are needed to avoid ambiguity.

Except in those rare cases, building relations are redundant and provide no additional information. Adding them would not only be a waste of time, it would also further raise the barrier of entry for new mappers (who often consider relation editing challenging and counter-intuitive). As such, adding them unnecessarily is generally not a good idea.

To create a building relation in JOSM, follow the following steps:

• Select all the building parts and building outlines (use shift+click or draw a selection box).
• Create a new relation by using the "create new relation" button.
• The relation editor will open. Add all the selected elements to the relation.
• Add the role part to all parts, and the role outline to the outline.
• Add the tag type=building to the relation itself.
• Close the relation editor by pressing "OK".

You can also create a buildling relation in iD as follows:

• Select the building outline.
• In the "all relations" section on the bottom left, click the "+" button and select "New relation".
• There's no preset for building relation, so select the plain "Relation".
• In the "type" field, enter building.
• In the "all members" section below that, add the role outline for your building outline.
• Now perform the following steps for each building part:
• Select the building part
• In the "all relations" section on the bottom left, click the "+" button and select your recently created building relation from the drop-down list.
• Add part in the role field that appears.

"Except in those rare cases, building relations are redundant and provide no additional information. Adding them would not only be a waste of time, it would also further raise the barrier of entry for new mappers (who often consider relation editing challenging and counter-intuitive). As such, adding them unnecessarily is generally not a good idea."

Tordanik, while I appreciate the fact that relation processing is difficult, relations by itself are not "bad". To continuously stress that building relations should be avoided, is in my opinion unnecessary and counter productive. If you want to ignore them in processing, you always can, but they do also have clear merits in relation to (web)navigation of parts, and as such I object to your statement that they "provide no additional information". I have seen many complex 3D buildings that were almost incomprehensible and impossible to navigate on the main OSM website without a type=building relation to group the parts in a logical way. The simple answer as to which parts belong to the building, is often most easily answered on the main OSM website through a building relation. If setup carefully, you can than easily drill down different parts or even levels through weblinks, greatly easing insight into the structure in a plain 2D view.