Product designers can use a wide range of design principles to influence the appearance of new product designs. One of these principles, ‘unity-in-variety’, states that product designs combining a maximum of unity or order with as much variety as possible are the most aesthetically pleasing. Even though designers are thought to intuitively use the principle of unity-in-variety during the design process, it has never been empirically assessed whether they, similar to lay-men, also positively appreciate product designs that adhere to this principle. We show that for automotive design students both unity and variety can simultaneously and independently positively influence aesthetic appreciation of car interior designs. Furthermore, automotive design students attribute much more of the aesthetic appreciation of car interiors to unity, than they do to variety. We suggest that this difference arises because they are trained to attach more weight to the unifying aspects of a car interior design.