The search for a missing toddler in Perth will continue through the night if necessary, police say.

Two-year-old Sam Trott walked out of the family home on Walbrook Mews in Landsdale about 10.30am today when tradesmen left the front door open.

Police have extended the search area for the toddler, combing surrounding streets, parks and bushland on foot and on horseback, assisted by police dogs, the air wing unit, local rangers, State Emergency Service volunteers and neighbours.

Sam is known to like water so lakes surrounding the nearby primary school are also being searched by police divers.

Sam is missing. (Supplied)

Inspector Dom Wood said another 40 officers and 40 SES volunteers were being brought in to boost search efforts before nightfall.

Insp Wood said the search would continue until Sam was found.

People arriving home in the area are urged to check their backyards as concerns for Sam's welfare increase as darkness falls.

"Check your backyards, check your front yards, check in your bushes," father Matt Trott said.

"If you've got little drains, you know, he's autistic, so he may not play with normal toys, so he could be hiding in the garden."

He said his son could have run a fair way since he disappeared.

"We're asking anyone (to look for him), maybe within sort of a 5km radius.

Grandfather Geoff Trott said Sam doesn't talk much so he may not respond to his name being called.

"We're just hopeful at this moment he's in somebody's backyard, maybe sifting through a bit of sand or something."

Mother Lyndal Trott said it took only 30 seconds for their son to disappear from home.

"He doesn't know how to open doors but the door was wide open," she said.

Sam is blond-haired and was last seen wearing a blue polo shirt and shorts with a grey stripe.

© AAP 2014