Seattle Transit Blog reports:
Housing units don’t directly correlate to residents, as single family houses have higher average household size, but the fact is that Seattle is not dominated by single family housing as many people think. ...
[T]he housing trends over the last ten years have seen growth in the medium/large multifamily housing (50+ units) and townhouses, which make up 4.5% and 1.4% more of the housing stock in 2010 than they did in 2000. This growth in their market share came overwhelmingly at the expense of single family housing, representing 3.4% less of the housing stock in 2010 as it did in 2000. More importantly, this trend will never reverse, because Seattle essentially has no new buildable land for single family housing. As the city continues to grow, single family housing will continue to represent a smaller and smaller percentage of the city’s housing stock.
STB also points out that, despite frequent complaints that denser housing leads to additional "noise," the reality is that most noise complaints result from bars and clubs in neighborhood commercial districts, not areas that allow low- or midrise housing. "Seattle has many examples of dense neighborhood which are peaceful and quiet, mostly located in historically dense neighborhoods such as north Capitol Hill (north of John), First Hill (between Boren, Pike, Broadway and Madison), Lower Queen Anne and areas north of Central Ballard."
Density, in short, isn't incompatible with quiet residential neighborhoods. More to the point, as Seattle grows, it's inevitable.