Tiny houses on wheels, the cheapest and easiest small units to build, were initially left out of San Diego's granny flat ordinance.

That changed in July 2020, when the city, seizing on their potential as low-cost housing, decided to allow movable tiny houses on residential lots. Even in the pricey coastal zone.

Last Friday, the Coastal Commission approved the changes, which regulate the number, size, location, and appearance of the homes, but with several modifications - none likely to appease critics.

The prefab look, some have argued, could undercut community character, and planning groups claimed there was a dearth of local review. That left it for the Commission to weigh the potential effects of tiny houses on coastal communities.

The units, classified as recreation vehicles, are defined by the city as being 150 - 430 square feet, not more than one story, unable to move on their own power, and lacking a separate address from the main house. Their wheels must be hidden.

Only one movable tiny house and one junior unit will be permitted on a premise, in addition to guest quarters, and they can't be placed in front yards. They must be at least five feet from an adjacent lot line.

Like accessory dwelling units, they're free from development impact fees, but most will still require a coastal development permit.

As complete living quarters that just have to be hooked up to utilities, their advantages as affordable housing are clear.

Cost estimates to build an accessory dwelling, based on a pilot program by the San Diego Housing Commission, range from $116,803 for a 224-square-foot studio to $342,078 for a 1,199-square-foot three-bedroom unit.

Tiny movable units, in contrast, generally cost between $40,000 and $85,000. And while an accessory dwelling may take 10 - 26 months to build, mobile units can be installed quickly.

Another boon to property owners: the mobile homes are only subject to personal property tax. They would be licensed with the Department of Motor Vehicles.

The commission found the amendment protects coastal views and complies with San Diego's environmentally sensitive land-use regulations but disagreed with the proposal to remove parking for all granny flats.

To avoid spillover parking interfering with coastal access, one space will be required for each movable tiny house in areas with high parking demand or which lie outside the transit priority area, and already have one or more accessory dwellings.

Sea rise was another problem for the units. The city's Local Coastal Program lacks development regulations, like flood resistant design, for homes found at risk from future flooding. That could lead to more homeowner attempts to build seawalls.

Vulnerable areas not yet within the mapped Special Flood Hazard Area include portions of La Jolla Shores, Mission Beach, Pacific Beach, Ocean Beach, and Peninsula. Property owners in those areas will have to enter into an agreement with the city, acknowledging the risks.

In addition, the amendment will ban shoreline armoring for movable tiny houses.