The principle of Unity, as the name implies stresses on the fact that there should be a sense of uniformity or harmony among all the elements used. Interior design should serve as a visual guide for a person to understand a living space, and without unity, the visual guide will only end up confusing the person. All the elements used should complement one another and a smooth transition should exist from one to another. A good understanding of Alignment of objects, Similarity of color/pattern/texture, Proximity (spacing) of objects, Repetition (grouping) of elements based on similarity, Continuation and Overlapping of interior design elements are a few ways to achieve ‘Unity’ in an interior design arrangement.
Designers cheat sheet
Using elements in balance will achieve a sense of unity,
Balanced colours and tones (intentional colour-wheel based scheme)
Spare use of Motif or theme
Heights of artwork are the same