Housing preferences are changing. According to (Koebel, Lang, & Danielsen, 2004; Kolson, 2016; Myers & Ryu, 2008; Shaver, 2017; Woo, 2016), the largest demographic, the millennials, prefer low- to mid-rise housing units that are in the walkable urban core areas. These areas have access to cultural activities, entertainment, restaurants, shopping and other amenities such as parks. The retiring baby boomers who are downsizing from their single-family suburban homes are also seeking the same. As suggested by Parolek (CNU. 2015).), the Missing Middle Housing types (MMH) are one possible solution to help meet the demand. However, the demand is greater than the supply (Koebel et al., 2004; Kolson, 2016; Myers & Ryu, 2008; Shaver, 2017). This review discusses the factors that affect the supply of the MMH types. It reveals that although these housing types once existed in the urban core, attempts to reintroduce them in the area meet with opposition from several stakeholders. Additional factors which hurt the supply of MMH types include land use and zoning regulations, a lack of developer interest to develop these units, and a lack of developer financing (Glaeser et al, 2009, Doherty, 2017).