What are some synonyms for relationship which encompass how something fits - like how a couch fits in a room, or how a building fits on a block, or how a neighborhood fits in a city. Its "fitting-in-ness", or ability to properly fit into a place and even improve it because it fits so well
One meaning of harmony is "a pleasing combination of elements in a whole."
Harmonious means "having component elements pleasingly or appropriately combined: a harmonious blend of architectural styles."
Maybe "congruity" or "compatibility"?
I don't know if you're looking to use it in a specific context or you just have this question buzzing around your head... I'm sure there must be a great noun out there, but otherwise perhaps you could turn the phrase around and use a verb, which might sound less clunky? Something like "complement", "enhance", "blend", or more colloquially "look right at home"?
For what it's worth!
Suitability fits. For instance, one would refer to the suitability of the couch (for the living room).
Feng shui shows us how our spaces can be designed and re-arranged in order to provide us the right balance and harmony. Simple things such as the position in which we place our desk, can become determinative to the harmony we feel in that space...
Feng shui encompasses many philosophical concepts as well; however, when applied as a borrowed term in English, the term seems to focus on the harmony of objects within the context of their surroundings. Wordnik mentions:
feng-shui (noun): rules in Chinese philosophy that govern spatial arrangement and orientation in relation to patterns of yin and yang and the flow of energy; the favorable or unfavorable effects are taken into consideration in designing and siting buildings and graves and furniture the favorable or unfavorable effects are taken into consideration in designing and siting buildings and graves and furniture
It might not always be a suitable term, but, at the right time and place in a conversation, it could fit rather nicely.