A supervisor’s proposal to stem the growth of monster homes in San Francisco would instead kill many modest home additions, increase the time and cost of minor upgrades, and make it harder to add housing units, officials said Thursday.

City planners, building officials and an array of architects, builders and brokers said at a hearing that Supervisor Aaron Peskin’s proposed legislation would bring the planning department to a halt as it struggles to deal with a deluge of minor projects, such as removing dry rot or upgrading a home’s facade.

At a time when Mayor London Breed has made streamlining city housing approvals a priority, the legislation would have the opposite effect, planning officials said. Permits that can now be obtained in an afternoon would require a yearlong process.

Serina Calhoun of Syncopated Architecture called the proposal “one of the most terrifying pieces of legislation I have ever seen in my 18 years of practice.” She was among dozens of small architects and builders who said that the legislation would make typical expansions impossible.

Known as the Protect and Preserve Act, the legislation would create a single definition for “demolition” in the city. The idea is to penalize speculators who illegally knock down smaller, more affordable homes to replace them with multimillion-dollar mansions. Supporters of the bill — which included some neighborhood activists and tenant groups — argue that affordable units in older buildings have sometimes been replaced by much larger and more expensive units, which means fewer opportunities remain for middle-class residents who can’t afford a $2.5 million home or $5,000 apartment.

The proposed law, which is co-sponsored by Supervisor Rafael Mandelman, would broaden the definition of “demolition” to include any project that removes 50% of external walls, 25% of walls facing a public street, or 75% of a building’s internal structure. It would also prohibit the demolition of any qualified historic resource, any rent-controlled or affordable housing unit, or any unit that had been occupied by a tenant within the past seven years. Demolition would also be barred if the replacement unit is more than 1,200 square feet or does not “provide density and affordability greater than the structure proposed for demolition.”

“We all agree and have agreed for many years that there is a problem relative to the definition of demolition in the code,” said Peskin.

The loss of affordable units could also be due to Ellis Act evictions and owner move-ins. Planning and Department of Building Inspection staff presented several case studies of how the legislation would impact run-of-the mill projects. In one example, a Sunset District homeowner applying to replace a faux-stone facade with a stucco front would have to obtain a conditional use authorization — a year-long process — because the project would require the removal of more than 25 percent of a street-facing surface. Under the current rules, that homeowner could get the permit in a few hours.

Another case study showed a Noe Valley property owner who recently received planning permission to replace a 1,700-square-foot nonhistoric home with a pair of 2,000-square-foot flats. The project would not have been allowed under the legislation because the value of the units would be “equal to or greater than the building being demolished,” and both exceed the 1,200-square-foot limit, the officials said.

Commissioner Rich Hillis asked Peskin’s aide, Lee Hepner, who has been working on the legislation for more than a year, whether the intent of the legislation is to prevent single-family homes from being knocked down to make way for multifamily buildings.

If you have a non-historic home in an area zoned to allow three units per lot, Hillis asked, “should you be able to demolish it and build three units?”

Hepner declined to answer, saying that he was unfamiliar with the specific project that was presented.

The hearing was a joint meeting of the Planning Commission and the Department of Building Inspection Commission. Next, the legislation will go to the Board of Supervisors’ Land Use and Transportation Committee, most likely in the fall.

Several planning commissioners said that the legislation would dramatically increase the number of projects that come before them for approval. Commissioner Joel Koppel, who said that the legislation would take away the commission’s discretion, accused the sponsors of the bill of “slighting” the panel.

“I feel like we do a good job making rulings,” he said.

Commission President Myrna Melgar said that commissioners are paid “a couple of hundred dollars” for “30 or 40 hours of work” a week, and that the legislation would dramatically increase the volume of cases. She asked if the legislation included a raise for commissioners.

Architect Michael Morrison said that his office has proposals that would add 13 new units, none of which would be allowed under the legislation.

“This is not just throwing the baby out with the bathwater, it’s burning the whole house down,” said Morrison.

While taking issue with the legislation, many speakers acknowledged that illegal demolition is a problem and that the current penalties are not big enough to dissuade speculators.

George Wooding, a homeowner who often opposes development, said that he likes the idea of creating clear rules and penalties around demolition.

Hepner said that the “fines should be scary enough that you are not willing to run afoul of the code requirements. This is not the cost of doing business, which has been the case for far too long.”

After the hearing, Peskin acknowledged that he “would be the first to admit that this legislation may have tried to solve too many problems at once.” He said he would work to narrow the bill to focus on two objectives: increasing penalties for illegal demolition and establishing a single definition for what constitutes demolition, something that is defined three ways in different sections of the city building and planning codes.

“My desire to stop bad demolition actors is undiminished,” he said.

J.K. Dineen is a San Francisco Chronicle staff writer. Email: jdineen@sfchronicle.com Twitter: @sfjkdineen