SB 9 Lot Splits: Objective Standards In Atherton

Thinking about an SB 9 lot split in Atherton but not sure where to start? You are not alone. SB 9 opened doors for two-unit projects and ministerial lot splits on single-family parcels, but the path runs through local objective standards. In Atherton, details like access, trees, and utility service often decide feasibility. This guide walks you through the key checkpoints so you can assess your options with confidence. Let’s dive in.

What SB 9 does in Atherton

California’s SB 9 established ministerial approvals for two key actions on eligible single-family parcels:

• Building up to two housing units on one lot.
• Splitting one lot into two legal parcels through a ministerial map process.

Local agencies must apply clear, objective standards. In practice, Atherton can set objective design and development rules and require a complete submittal, but cannot require discretionary review for eligible SB 9 projects. The primary statutes guiding this are California Government Code sections 65852.21 for two-unit developments and 66411.7 for ministerial lot splits.

Check eligibility first

Before you sketch plans or hire a surveyor, confirm the parcel is eligible. SB 9 applies only to parcels zoned for single-family residential use that meet state and local criteria.

Common disqualifiers

• Historic resources or districts where SB 9 is limited or prohibited.
• Protected or rent-restricted housing, or projects that would remove protected units.
• Prior SB 9 lot splits on the same original parcel or prohibited condo conversions.
• Parcels in protected land types such as open-space preserves or certain habitat areas.
• Recorded CC&Rs or easements that prohibit subdivision or restrict added units.
• Conflicts with local coastal programs where applicable.

In Atherton, conduct practical checks early: confirm zoning with Planning, review title for CC&Rs and easements, and verify any overlays such as floodplain or conservation areas. Because private covenants still apply, do not assume SB 9 overrides your CC&Rs.

Owner-occupancy affidavit

SB 9 includes an anti-speculation requirement. You should expect to sign an owner-occupancy affidavit stating you intend to occupy one of the resulting parcels or one of the units as your principal residence for a period commonly three years after approval. Atherton may require the affidavit to be recorded against title. Ask Planning about the required form, recording process, and penalties for noncompliance.

Lot split math and map

The 40/60 rule

SB 9’s parcel split provisions include the “40/60 rule.” A ministerial split cannot create a lot smaller than 40 percent of the original parcel’s area. In practice, the two lots created must each fall within about 40 to 60 percent of the original lot size, subject to any objective local minimums.

Minimum sizes and frontage

Atherton can apply objective standards such as minimum lot size, minimum frontage, and parcel shape rules. These cannot be exclusionary, but they do matter. Many Atherton lots are large, yet you still need to test your concept against any applicable minimums and frontage requirements.

Survey and parcel map

You will need a preliminary survey and a parcel map prepared by a licensed surveyor. The map should show lot lines, existing improvements, topography, trees, and easements so reviewers can verify the split proportions and compliance with local parcel configuration standards.

Objective standards that shape design

SB 9 projects must comply with objective development standards. These rules determine your buildable envelope and the practical size of your units.

Setbacks, height, and coverage

• Confirm front, side, and rear setback requirements measured from the new lot lines after a split.
• Verify maximum building height and any roofline allowances.
• Check lot coverage and floor area limits for two-unit projects.

These standards are typically listed in Atherton’s SB 9 handout or development standards table. Objective standards cannot be applied in a way that effectively blocks SB 9 where state law allows it, but they will define your project’s form and size.

Parking and ADU interactions

Local, objective parking standards can apply to SB 9. Some jurisdictions reduce parking in certain contexts, but you should not assume a waiver. Also remember that accessory dwelling unit law is separate from SB 9. Adding an ADU versus pursuing an SB 9 two-unit or lot split involves different approvals, utility setups, and parking rules. Map out which path better aligns with your goals and the parcel’s constraints.

Access and fire safety

Emergency access is often decisive in Atherton, especially for flag lots.

Driveways and flag-lot access

Expect objective minimum access widths for private driveways or access strips serving a rear lot. Many agencies look for at least 20 feet of clear width, though exact Atherton and fire district standards must be confirmed. Narrow, long flag lots that cannot provide adequate access are likely to be rejected or require significant redesign.

Turnarounds, slope, and hydrants

The local fire authority may require emergency vehicle turnarounds, limits on driveway slopes, and specific surface materials. Hydrant spacing and water flow can also drive design changes or off-site improvements.

Fire safety features

If your site is in a high fire severity zone, you may face additional requirements like defensible space and residential fire sprinklers. Coordinate with the fire marshal early to understand what will apply.

Utilities and meters

SB 9 does not exempt you from utility requirements. Plan ahead and contact providers during feasibility.

Water service

Ask the water provider about service availability, new meters, meter relocations, and backflow requirements. Many providers prefer or require separate meters per parcel or per dwelling unit.

Sewer connections

Confirm sewer lateral locations, capacity, and permitting with the sewer district. Some projects need new laterals or upgrades.

Electricity and gas

Contact the electricity and gas utility for meter plans and service extensions. In the Atherton area, Pacific Gas and Electric is typically the provider. Requirements for separate meters or panel upgrades can affect your budget and layout.

Stormwater and drainage

Atherton’s public works standards may require on-site stormwater treatment and post-construction runoff controls. These needs can influence grading, driveway design, and landscape plans.

Trees, rights-of-way, and other constraints

Atherton’s mature tree canopy is a defining feature, and tree rules can shape your project. Heritage or protected trees, whether on private property or in the public right-of-way, can trigger permits, arborist reports, and mitigation planting. Tree preservation zones may reduce the buildable area or influence driveway location. You should also expect careful review of driveway connections and any frontage improvements to protect the town’s streetscape and utilities.

Private recorded restrictions still matter. If your property is within an HOA or has CC&Rs limiting subdivision, those restrictions are enforceable and can prevent a split even if the town would otherwise approve it.

The process, timing, and fees

SB 9 approvals are ministerial, so you avoid discretionary hearings. That said, you will work through several agencies and submittals.

• Application and plans. Submit a complete SB 9 application with site plan, elevations, and any required objective design checklists.
• Survey and parcel map. Provide a map prepared by a licensed surveyor that confirms the 40/60 split and shows existing conditions.
• Owner-occupancy affidavit. Sign the affidavit and follow Atherton’s recording procedures.
• Utility confirmations. Obtain letters or confirmations from water, sewer, and electric/gas providers regarding service and metering.
• Public works and fire review. Address driveway widths, access, stormwater controls, and fire safety requirements.

Timelines vary by jurisdiction. Ministerial reviews are faster than discretionary processes, but coordination with utilities and fire can extend schedules. Fees can include application fees, parcel map and recording fees, plan check and building permit fees, utility connection fees, and potential frontage or off-site improvement costs. Ask Planning for current processing times and fee schedules.

A practical checklist for Atherton owners

• Confirm zoning and SB 9 eligibility with Atherton Planning.
• Pull a recent title report to review CC&Rs, easements, and prior maps.
• Order a survey and site plan showing structures, trees, utilities, and topography.
• Check historic status and any conservation or hazard overlays.
• Test the 40/60 lot split and any minimum lot size or frontage rules.
• Contact water, sewer, and electric/gas providers about service and meters.
• Meet with the local fire authority to confirm driveway width and turnaround needs.
• Review setbacks, height, floor area, and lot coverage objective standards.
• Prepare and record the owner-occupancy affidavit as required.
• Budget for application, map, utility, and frontage improvement fees.
• Submit a complete SB 9 application and coordinate follow-ups with public works and fire.

How we help you plan with confidence

If you are weighing an SB 9 lot split or two-unit plan in Atherton, a clear feasibility path can save you time and money. The Pam Tyson Sales Team brings local insight on parcel patterns, access challenges, and buyer demand on the Peninsula. We help you frame options, connect with the right local contacts, and understand whether an SB 9 path supports your broader goals, including holding, building, or selling.

If selling makes more sense than splitting, our Compass-enabled marketing helps you maximize value. With Concierge, we can coordinate and front the cost of select pre-sale improvements that may boost your net proceeds. Private Exclusives can provide discreet market exposure, and bridge financing can help you move on your timeline.

Ready to explore your options with a local advisor? Schedule a personalized market consultation with the Pam Tyson Sales Team at [Unknown Company].

FAQs

Does Atherton allow SB 9 lot splits?

• Yes, SB 9 is state law, and Atherton must process eligible applications ministerially using objective standards; confirm parcel-specific eligibility with Atherton Planning.

What is the owner-occupancy requirement for SB 9 in Atherton?

• Expect to sign an affidavit to live in one unit or on one parcel as your principal residence for a period commonly three years; ask Planning about the required form and recording.

How does the 40/60 rule affect my split?

• A resulting lot cannot be smaller than 40 percent of the original parcel area, so your two lots typically fall between about 40 and 60 percent of the original; also verify any local minimums.

Can I create a narrow flag lot under SB 9?

• Only if it meets objective access and fire requirements; many proposals fail due to insufficient driveway width, lack of turnaround, or utility access constraints.

Will I need separate utility meters for two units or two lots?

• Often yes; utility providers commonly require separate meters or service arrangements for each parcel or dwelling unit, so contact providers early.

Do CC&Rs or HOAs override SB 9 rights?

• Private recorded restrictions can prohibit subdivision or additional units even if the town would otherwise approve, so review your title report and CC&Rs.

How do tree rules impact SB 9 projects in Atherton?

• Heritage and protected trees can limit siting, trigger permits, and require mitigation; early arborist review helps avoid redesigns and delays.