2.10.5. Map items: split.
You can only split polygonal items. For example, you can use this operation to split a single building polygon into sections of various heights if necessary.
To split a building:
Click the item on the map.
In the attribute panel that opens, click Edit .
Click Splitting mode .
Draw the line that you want to split the item along:
In the attribute panel, click Save.
The building will be split along the lines you set. The values of the new buildings' attributes will remain the same as they were before you split the original building: