What is another name for an up and down duplex?

The term "mother-in-law or legal suites" accurately describes what is commonly referred to as an up and down duplex. This type of residential structure typically consists of two separate living units, one located above the other, allowing for shared occupancy while maintaining individual living spaces. This setup is often used to accommodate extended family members, such as in-laws, hence the name "mother-in-law suite."

The other choices do not appropriately reflect this configuration. A shared unit generally refers to a dwelling where multiple individuals share a single living space but doesn't denote the two-level aspect. Split-level refers to a style of home that features staggered floor levels and doesn’t imply separate units. A row house, on the other hand, describes a type of housing that shares walls with adjacent homes but is distinct in its architectural design and layout, lacking the duplex arrangement. Thus, "mother-in-law or legal suites" is the most fitting alternative for an up and down duplex.