I have worked with a colour system for the viewport to organize the room in a better way.
I have found it hard to work with the fillers, so I have tried to build an edgewall (green).
To close the gap, I have extruded this vertical filler. Maybe a good idea to work with a filler height about 3,25.
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I have organized the different floor levels of the house in own collections, so I can hide one with one click.
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Thoughts: maybe it is better not to work with fillers, but with groundfloors instead.
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This is only a test. It is not so neat as it should be, but I understand the modeling concept and it´s advantages.
Modular building structures is a kind of funny thing. I love it really and enjoy experimenting with architecture.