After 25 years of EE I don't recall there ever being any mention of a cellar in the houses in Albert Square. However, it does explain how so many people can live in these houses, the cellars must have been converted into bedrooms.
On another point, When all the ruckus began at Patrick's house and the family escaped, Jack phoned for the police & ambulance. Two coppers arrived and one immediately said to Lucas, "I'm arresting you on suspicion of false imprisonment". How did he know Lucas had been holding Denise against her will?