Dfyjkt

Currently planning a holiday around the flights I've booked which are:
LHR->SFO and then SEA->LHR.

We're planning on flying up to Canada (Vancouver) from San Francisco, spend a few days in Canada and fly back to Seattle. Then finally fly back to the UK.

Closest answer I could find was this:
UK Citizen flying to US, then Canada, then back to UK - How does this affect Visa Waiver Program?

However I'm concerned as I want to re-enter the US after exiting, which isn't quite what they did.

So far my TODO list is:

• Apply for an ESTA


• Apply for an eTA

What else can I do to prepare?

If your last departure from the US will be within 90 days of your first admission to the US, then you don't need to do anything else.

If your total time in North America will be longer than 90 days, then, instead of ESTA, you should apply for a B-2 visa to avoid running afoul of the Visa Waiver Program rule on short trips to Canada, Mexico, or nearby islands:

If you are admitted to the United States under the VWP, you may take a short trip to Canada, Mexico, or a nearby island and generally be readmitted to the United States under the VWP for the remainder of the original 90 days granted upon your initial arrival in the United States. Therefore, the length of time of your total stay, including the short trip, must be 90 days or less. See the CBP website. Citizens of VWP countries* who reside in Mexico, Canada, or a nearby island are generally exempted from the requirement to show onward travel to another country* when entering the United States. Learn more on the CBP website.

The "remainder of the original 90 days" bit is discretionary: the Code of Federal Regulations, at 8 CFR 217.3, says (emphasis added),

(b) Readmission after departure to contiguous territory or adjacent island.

An alien admitted to the United States under this part may be readmitted to the United States after a departure to foreign contiguous territory or adjacent island for the balance of his or her original Visa Waiver Pilot Program admission period if he or she is otherwise admissible and meets all the conditions of this part with the exception of arrival on a signatory carrier.

So you might be able to talk your way in, especially if your original 90-day period expires while you're in Canada, and you can apply for a new 90-day period. But since you'll only be in Canada for "a few days," you will probably have a hard time doing this as the border officer may decide that you are trying to abuse the VWP. In that case, you would be denied entry. With a visa, you would not have this problem unless you try to stay in the US for longer than six months.