Mill Valley has extended the temporary housing ordinance it passed in December, giving the Planning Commission more time to review and ensure compliance with new state laws intended to promote affordable housing.

“This is the biggest single planning change I can recall in my 10 years of public service,” Councilman John McCauley said. “It’s gigantic. And it came out in October, November, and was effective on Jan. 1.”

The council unanimously approved a 10-month extension on a temporary ordinance that adds regulations for accessory dwelling units — with the exception of keeping its 500-square-foot floor area garage bonus in place because a few residents were concerned its removal would reduce home values.

Bill Silvestri, a Mill Valley resident, calculated the potential aggregate loss to property values in the city at $3 billion. He sent a letter to the council expressing his concern.

“This proposal constitutes an illegal taking of property value and property rights without any required finding of facts or data,” Silvestri wrote. “The recommendation of this action is based purely on personal opinion and whimsical feelings of certain staff members and council members.”

The bonus allows property owners to build a garage that is 500 square feet without including it in the lot’s total floor area ratio.

The ordinance was set to eliminate the bonus, but the council decided to keep the exemption to allow the Planning Commission to review how it would affect accessory dwelling unit development and to allow for community input, said Jim McCann, city manager.

“The council allowed the interim measure by urgency,” McCann said. “Given that some residents were upset and concerned about this one component of a major ordinance, the council said OK, but let’s not forget we have to work comprehensively and quickly. So they voted to have more conversations and bring recommendations back to council in a few months’ time.”

The council was initially concerned that developers would use a loophole created by the new state law that allows for accessory dwelling units exceeding 16 feet to be built as long as they are on top of a garage, McCauley said.

Mayor Jim Wickham said it is unlikely that there will be a surge of developers building accessory dwelling units on top of garages over the next six months, and suggested the commission take the lead on this discussion.

McCauley said community concerns that the council is trying to secretly pass an ordinance to remove the 500-square-foot garage bonus are untrue, and said it is simply trying to keep up with state legislation.

“An interim ordinance is not a policy,” he said. “Either way we flip the coin and say we are going to take out the garage bonus for the next six months or we don’t, it has nothing to do with our long-term policy. I have no view on what our long-term policy should be yet, it’s way too hard to figure out. I think the Planning Commission should have multiple meetings on this topic and engage with architects and towns beyond.”

The temporary ordinance was originally passed on Dec. 2 to update local rules and regulations to be in compliance with state law, said Patrick Kelly, planning director.

Kelly said the staff was directed by the council to toy with the idea of removing the 500-square-foot garage bonus and came up with the ordinance presented to the council on Monday.

“Staff would feel better if this goes to the Planning Commission,” he said. “That’s where these ordinances should start.”