What unique characteristic identifies a semi-detached house?

A semi-detached house is specifically characterized by sharing one wall with another house. This design allows for two homes to be built side by side, each maintaining its own separate entrance and living space while still having that common wall. This arrangement offers a compromise between the privacy of single-family homes and the economic efficiency of townhomes or duplexes.

The other options do not accurately describe a semi-detached house. For instance, having no shared walls is characteristic of a detached home. Multiple units within one building describes multi-family dwellings like apartments, rather than single-family homes. Additionally, while semi-detached homes can have separate foundations, the defining feature is primarily the shared wall, making the option focused on that aspect the most accurate representation of what makes a house semidetached.