A careless act with a cigarette combined with a hot day caused extensive damage to a house in Edgeware Rd.
Fire service investigators said a person was smoking on an upstairs deck of the rented property last Wednesday but didn't put the cigarette out properly and threw it into a pot on the deck.
The tenants went out not realising the cigarette was quietly smouldering in the pot.
High temperatures and warm, gusty winds were "perfect fire conditions" which had helped the cigarette butt to smoulder all day, before the deck caught fire risk management officer Steve O'Brien said.
The fire was well alight when a neighbour spotted it just after 7pm.
The house was nearly destroyed and is now unliveable.
And with no smoke alarms in the house, it was lucky no-one was home, Fire Service risk management officer Shane O'Brien said.
"The weather conditions on Wednesday were a perfect mix. It was lucky no-one was home at the time of the fire, which was our initial concern. If someone was asleep it could've been a lot worse than what it was because there wouldn't have been anything to alert them to the fire. Smoke alarms do save lives. A number of homes don't have smoke alarms because people don't think their house will ever catch fire, but a fire can strike at anytime, anywhere. Fires spread so quickly. A whole house can go up in flames in a matter of minutes.
"It's a good idea if people are going to use jars or pots to put their cigarette butts in, to make sure they are topped up with a bit of water to prevent something like this happening again. Some of the tenant's belongings are salvageable, but there is extensive heat and smoke damage throughout the house," he said.
Fire fighters from St Albans extinguished the blaze.