After spending several years tightening up restrictions on basement construction, the Atherton City Council is loosening the town's rules to allow larger basements.

The new rules allow homeowners to build basements that extend beyond the footprint of a house and to put basements under accessory structures such as guest cottages.

Currently, basements are allowed only underneath the main house, and cannot extend beyond the house's perimeter. Basements are not allowed underneath accessory structures.

Under new rules slated to take effect in September, basements can be larger than the house — up to 130 percent of the first floor's area — if the builder gets a use permit from the Planning Commission. The vote was 5-0 at the City Council's July 18 meeting.

Basements, once virtually unheard of in California, have become a popular amenity in new homes on the Peninsula. In Atherton, they're used for everything from home theaters and wine cellars to his-and-hers gyms and playrooms for kids.

Atherton resident Carol Flaherty successfully pushed to get the basement size limit bumped up from a proposed 120 percent to 130 percent by telling the council that a 6,000-square-foot L-shaped house otherwise wouldn't be able to fit a home theater and a hallway in its basement.

"At 130 percent, you can accommodate almost anything," she told the council at its June meeting.

The September start date coincides with the council's expected adoption of new drainage rules following a town-wide study of storm run-off and groundwater.

At the July meeting, the council also approved stricter standards for installing fire suppression sprinklers. Under the new rules, all new basements, no matter the size, will have to include sprinklers.

Putting a new basement of more than 250 square feet under an existing house would trigger the need to install sprinklers in the entire house, including the basement.

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