Allowing for two units on just 10% of single-family lots would boost development almost 27% over current levels.

Allowing for even modest amounts of new density in the nation's overwhelmingly single-family neighborhoods could lead to millions of new homes nationwide, according to a Zillow® analysis, helping alleviate a housing affordability crisis that has been decades in the making. Building at the status quo over the next two decades is expected to produce about 10 million new homes across the 17 large metros analyzed nationwide, a more than 20% boost over the almost 50 million homes these markets currently have. Allowing for two units on just 10% of single-family lots would add an additional 3.3 million homes on top of that 10 million, a boost of almost 27% over current levels. Single-family neighborhoods account for the lion's share of land in metropolitan America, and over the years have generally become insulated from denser redevelopment by a thickening tangle of regulations, reflecting entrenched local interests that benefit from keeping a neighborhood as it is. In America's expensive coastal cities especially, natural barriers and environmental concerns have exacerbated that trend, limiting most new housing to islands of density – often near transit or in formerly non-residential areas – in an otherwise stagnant sea of no-growth. 

Source: Zillow