Expansion date is 25 October and it's to everything within the north and south circular routes.
It'll be a much bigger area than the congestion charge zone (which is a separate charge) and has stricter emissions criteria than the existing LEZ.
Warning signs are already up, and it should be pretty clear. But if you weren't expecting it, it might be an unwelcome surprise, especially if you didn't notice and end up with the penalty charge
It's £12.50 per day if your vehicle is not compliant, and the only day it will not apply is Christmas Day. There is a temporary exemption until 2025 for vehicles which are in the disabled/disabled passenger tax class.
Map and postcode checker
tfl.gov.uk/modes/driving/ultra-low-emission-zone/ulez-where-and-when
More info, including a car checker, here
tfl.gov.uk/modes/driving/