Based on per capita income, one of the more reliable measures of affluence, Sudden Valley ranks 97th of 522 areas in the state of Washington to be ranked.
According to the United States Census Bureau, the CDP has a total area of 8.1 square miles (21.0 km²), of which, 6.2 square miles (16.1 km²) of it is land and 1.9 square miles (4.8 km²) of it (22.99%) is water.
As of the census of 2000, there were 4,165 people, 1,675 households, and 1,185 families residing in the CDP. The population density was 668.8 people per square mile (258.1/km²). There were 1,984 housing units at an average density of 318.6/sq mi (123.0/km²). The racial makeup of the CDP was 92.24% White, 0.79% African American, 1.61% Native American, 1.97% Asian, 0.12% Pacific Islander, 0.91% from other races, and 2.35% from two or more races. Hispanic or Latino of any race were 3.03% of the population.