Washington courts have traditionally been hostile to split custody or 50 / 50 residential schedules. However, in 2007, the Washington Legislature took a small but significant step towards split custody.
Under the revised statute the court may order the child to spend substantially equal intervals of time in each household where it is in the best interest of the child. This change has probably made courts more willing to order split custody over time.
In practice, at least in the foreseeable future, courts are probably not going to award 50/50 custody when the child is under school age and if there is a history of conflict. On the other hand, they probably will award 50/50 custody if the child is in 1st grade or older and there is a history of shared parenting. The courts will almost certainly award 50/50 custody if both parents are in agreement.