Property speculators have made a game out of tearing down historically protected homes, then retroactively applying for demolition permits, and using the now-empty lots to build massive mansions that sell for millions.

Ross Johnstone claimed that the reason he knocked down modernist architect Richard Neutra’s 1936 Twin Peaks Largent House at 49 Hopkins Ave. had nothing to do with the extra money he’d get from increasing the floor plan from 1,300 square feet to about 4,000.

Mr Johnston, who bought the property last year for $1.7m (£1.3m), had permission to renovate it but not to knock it down.

He argued instead that he wanted to live in a larger home.

The San Francisco City Planning Commission didn’t buy this argument and ordered Johnstone to build an exact replica of the house he tore down, “executed beautifully in a way that would be consistent with the home’s original expression,” and to post a plaque recounting the history of the house.

Planning commissioner Dennis Richards said he hopes the commission’s action will send a message to speculators accustomed to ignoring city planning and building laws with few or no repercussions.

“We are tired of seeing this happening in the city and are drawing a line in the sand,” said Richards. “You can have all the rules in the world, but if you don’t enforce them, the rules are worthless.”