According to The Plan Collection, a 1 1/2-story house is a house design that allows for expansion of the roof to provide more living space. The top level typically contains enough space for a bedroom, bathroom and recreation room or second bedroom.
The Plan Collection states that there are several reasons why a 1 1/2-story house might be more advantageous than a two-story house. For example, the former often helps with budget considerations. A potential homeowner might not have the budget to afford a two-story house, so buying a 1 1/2-story frees up funds for other necessary expenses. Furthermore, these houses are suitable for home buyers who are unsure of whether or not they need the additional space for the future.