Which restriction is likely to render an architectural review covenant unenforceable?

An architectural review covenant that is focused on aesthetic preferences is likely to be deemed unenforceable due to issues surrounding vagueness and subjectivity. Courts often seek to enforce covenants that provide clear, objective standards to guide compliance. When a covenant's requirements are based solely on personal taste or subjective opinions about aesthetics, it can lead to disputes and arbitrary enforcement decisions.

For example, if a homeowner must receive approval based on a vague description of what is considered "aesthetically pleasing," it can be difficult to determine whether a proposed modification meets the standard, making the covenant harder to enforce. The absence of concrete standards can lead to claims of discrimination or favoritism in the decision-making process, further complicating enforcement. Therefore, covenants should have measurable and observable criteria to guide review processes, which is often not the case when the requirements are merely aesthetic in nature.

The other options tend to rely on more concrete and objective factors, such as physical distancing or historical styles, which can typically be assessed more clearly and consistently.