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.
Sudden Valley is located at (48.720311, -122.340219).
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.
Before 1969, Sudden Valley was nothing more than "a Ranch" owned by Glen and Betty Corning.
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.
There were 1,675 households out of which 31.4% had children under the age of 18 living with them, 61.5% were married couples living together, 6.1% had a female householder with no husband present, and 29.2% were non-families. 20.7% of all households were made up of individuals and 4.9% had someone living alone who was 65 years of age or older. The average household size was 2.49 and the average family size was 2.87.
In the CDP, the age distribution of the population shows 23.5% under the age of 18, 8.2% from 18 to 24, 30.5% from 25 to 44, 28.3% from 45 to 64, and 9.6% who were 65 years of age or older. The median age was 38 years. For every 100 females, there were 105.7 males. For every 100 females age 18 and over, there were 102.2 males.