A DUI is a felony in Washington state if you have prior DUI convictions within previous years or have been previously convicted of vehicular homicide or assault while DUI.
Otherwise, a DUI is not a felony in Washington, but rather a Gross Misdemeanor, which means the person is subject to a maximum punishment of a $5,000 fine and 1 year of jail. There are mandatory minimum punishments for DUI convictions, but they change depending on how many priors a person has.
You should always hire a DUI lawyer to minimize the damage this can do to your record and your future (such as getting a job), especially if you're facing a felony DUI charge, which can mean some serious jail time.
Specifically, the statute is as follows:
RCW46.61.502(6): It is a class C felony punishable under chapter 9.94A RCW, or chapter 13.40 RCW if the person is a juvenile, if: (a) The person has four or more prior offenses within ten years as defined in RCW 46.61.5055; or (b) the person has ever previously been convicted of vehicular homicide while under the influence of intoxicating liquor or any drug, RCW 46.61.520(1)(a), or vehicular assault while under the influence of intoxicating liquor or any drug, RCW 46.61.522(1)(b).
As Mr. Rands points out, the answer is "it depends." In general, DUI is a gross misdemeanor (punishable by up to 365 days in jail and/or $5,000 fine).
There are, however, certain times when a DUI may be charged as a felony -- the latest because of a change in the law allowing the State to charge a typical gross misdemeanor DUI as a felony where there are four prior offenses within a ten-year period.
The interesting thing is that the law indicates that the case shall be prosecuted as a felony, but the various state attorneys haven't necessarily been doing this, believing they have the discretion to charge as a gross misdemeanor. Recent indications, at least in Pierce County, are that they WILL be prosecuted as felonies.
A DUI can be considered a felony in Washington State, but the person charged has to have prior convictions. If they have a prior conviction for vehicular assault or vehicular homicide and get a new DUI, it can be charged as a felony. If the person has 4 prior DUI's within 10 years, then the 5th can be charged as a felony.
As a matter of practicality, the arresting officer will not know the person's criminal history and will almost always file the charge as a misdemeanor. The State (or municipality) will then have to figure out the person's criminal history. This does not take long.
If you have a situation like this, contact an attorney in your area immediately.