Four fire engines and around 25 firefighters tackled a fire at a workshop on Mount Pleasant in Wembley.
Half a tonne of refuse was destroyed by the fire and a small part of the walls of two workshop units were damaged by the fire.
The fire is believed to have been accidental and caused by an outdoor heater left unattended.
A spokesperson for London Fire Brigade said: “Many fires involving fire pits and heaters can easily be prevented. Carefully consider whether you have enough space in your garden for an outdoor heater or fire pit.
“Most people know to clear a fireproof space around their heaters, but sometimes they don’t realise that this can mean above and below the heater too. You must be able to place it at least three feet away from anything that could catch alight, like fences, sheds and overhanging trees, but also awnings, gazebos and certain types of wood finish that may not withstand the heat. If you can’t, don’t use them.
"It's also important that you never leave outdoor heaters or fire pits unattended. It only takes a moment for a fire to start."
The Brigade was called at 1433 and the incident was over for firefighters at 1704. Crews from Harrow, Northolt, Acton and Wembley fire stations attended the scene.