Seattle is very good on these issues, and really tries to be accommodating, but it does depend on their workload. As long as you have an appointment notice (even if for a different day), you can attempt to do it as a walk-in, but you should arrive extremely early, and be prepared to wait. If they can get you in, they will. An option is to reschedule the appointment.
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In Los Angeles, this is generally permissible and I do not see why it would be different in Seattle. You can find out by actually walking in to ask to be entertained and explaining why you cannot make the scheduled date.
It all depends on the work load of the ASC.
J Charles Ferrari Eng & Nishimura 213.622.2255 The statement above is general in nature and does not constitute legal advice, as not all the facts are known. You should retain an attorney to review all the facts specific to your case in order to receive advise specific to your case. The statement above does not create an attorney/client relationship. Answers on Avvo can only be general ones, as specific answers would require knowledge of all the facts. As such, they may or may not apply to the question.
I have had Clients go to their biometrics appointments at an earlier date and or time. While the facilities have thus far always been accommodating, I still tell my Clients that they do not have to accommodate you and you can reschedule by mail. I suggest going with some proof of why you can't make it at the date and time of the scheduled appointment.
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The times I went to the Tukwila USCIS office, some times the wait to go through the security checkpoint is 30 minutes, especially when there is a citizenship oath ceremony. After the security checkpoint, there is a wait to get a number. Depending on how many other persons are in front that wait can be over 15 minutes. For biometrics, about ten persons are called up at a time. The biometrics office seems to be always busy. Plus, there is a fee to park.
So, for some parking fee and about an hour, you can find out whether your biometrics can be done outside of your appointment.
You if are going, you likely should leave yourself plenty of time to wait.