A brick chimney teetering on a pile of rubble. Blackened palm trees, stripped of their crowns. Ash-covered clay pots and warped steel beams.

The infernos that whipped across Los Angeles left little behind of the homes they incinerated.

Residents are eager to rebuild their homes and move back into their neighborhoods. Many are starting to hire contractors and figure out what it will take to return to the lives they knew.

But before construction can begin, lots must be cleared of toxic materials and debris. Last week, crews donned protective suits and began clearing mountains of hazardous remains from the roughly 12,000 homes destroyed by one of the worst disasters in the history of the Los Angeles region.

While the clearing process could take mere days on any given lot, it could be as long as 18 months before the very last property is ready for rebuilding, according to the U.S. Army Corps of Engineers. That estimate, which has been mentioned in community meetings, has made homeowners afraid that their own houses could be among the last ones to be cleared.

The timeline became a political flashpoint when Los Angeles residents told President Trump that they could not bear to wait 18 months to begin rebuilding their homes. Mr. Trump told Karen Bass, the Los Angeles mayor, “I’m sure you can get it down to, I would say, not even 18 days. They are ready, mayor, they are ready to start.”

“They should be able to start tonight,” he said.

Ms. Bass said that she had signed orders to expedite work but explained that there were necessary precautions to take to ensure the safety of residents.