The 66ft (20m) diameter hole spread across a front garden and driveway on Fontmell Close and is 33ft (10m) deep.
Hertfordshire County Council said five homes were evacuated and 20 people were taken to a centre set up at the nearby Batchwood Sports Centre.
Residents are said to have heard a crash before the huge crater appeared.
Firefighters received a call at 01:30 BST to say the hole had "opened up substantially" overnight.