Here’s our experience in case it’s helpful to others:
Where to park?
That’s a bit difficult. It’s easier to use public transport but if you can’t - I booked parking through the just park app. It’s a bit weird because you’ve 2 options - use the car park of the Holiday Inn nearby or someone’s driveway! We opted for the first one which was absolutely fine but to be honest the driveway would’ve been closer
Annoyingly there’s a Waitrose car park right next to the entrance if you want to take a gamble.
Which entrance?
The south pavilion entrance for the American citizen services. There’s an outbuilding where you go through security and there was a fairly big and slow moving line when we were there (9.45am appointment). It took us 40 min to go through security but there was no queue after that. We had a big stroller and 3-4 water bottles but that wasn’t a problem. They allow phones and I believe iPads but I don’t think they’ll allow laptops and larger electronic devices (however I’m not sure)
What next?
They’ll guide you to the second floor, where you go to one desk to get a number, wait to get called and give your documents, wait to get called and sent to Desk 25 to pay the fees (used a debit card, they don’t accept amex), wait a bit more, get called again where they’ll ask you a few questions and voila, your child becomes an American retrospectively from birth. The last step is to get called to Desk 27 to pay your DX currier fees which start from £9 unless you want fancy services (I believe you can bring a pre-paid envelope as an alternative). Overall, it took about an hour or so but it didn’t feel that bad as you get called periodically to do various bits.
What are the facilities?
Brand new and shiny. Seems like an improvement vs the previous embassy minus the location!
There are comfy seats, kids area, nice bathrooms with changing tables.
Hope that helps