Arsonists struck at an empty property in Hyde Park, Doncaster, last night.
Fire crews from Doncaster and Edlington stations were sent to Milton Walk just after 11pm to reports of an outhouse on fire.
Smoke from the burning structure had entered the house but it was not occupied.
A fire service spokesman said the fire was started deliberately.
A skip fire in Cooke Street, Bentley, at 8pm had started accidentally.