Multifamily and Duplex Properties for Sale in Atherton: Zoning and Rent Rules

What multifamily in Atherton looks like

Atherton has almost no existing apartment buildings or duplexes. It is a town of single-family estates, and until 2024 its code had no multifamily zoning. Today, rental income in Atherton comes mainly from ADUs and junior ADUs on large lots, from leasing whole estates, and, for developers, from the multifamily overlays the town adopted to meet its Housing Element. Anyone looking for a conventional two-to-four unit building will usually find it in neighboring Menlo Park or Redwood City; see our Menlo Park multifamily guide. For financing, see financing 2-4 unit properties and apartment building loans.

Where the zoning allows it

Chapter 17.30 lists four base districts and, since Ordinance 669 in 2024, three overlays, two of them for multifamily housing.

RM-10. Chapter 17.35 sets objective standards so multifamily projects on designated parcels are by right. Eligible parcels are those named in the chapter and the Housing Element, or designated later by the Council.

• Setbacks. Fronts start at 30 feet. Sides and rears start at 20 feet and grow to 40 and 50 feet next to single-family lots, plus more for taller walls.
• Exception. The larger R-1 setbacks do not apply where the neighbor is also an RM-10 site with multifamily housing.
• Still discretionary. Lot redesignations, variances and special structure permits.

RM-20/40. On PFS overlay sites, objective standards require 40-foot setbacks, or 50 feet next to R-1 land, 40% building coverage, and rules on parking, facades, windows and lighting. The minimum densities of 12 and 24 units an acre mean a low-density project will not qualify.

ADUs. Chapter 17.52, amended by Ordinance 675 in 2025, is the main income tool on estate lots.

• Per lot. A single-family lot can have one ADU and one JADU, or a detached ADU up to 1,200 square feet.
• Bonus ADU. On lots of two acres or more, an extra detached ADU is allowed if deed-restricted to low-income households for 15 years or very low-income households for 10. Bonus units must be rented for at least nine months at a time.
• Rental terms. All ADUs must be rented for more than 30 days, and ADUs generally cannot be sold separately.
• No owner occupancy. None applies to units permitted on or after January 1, 2020.
• Multifamily lots. Up to 25% of existing units can be added in non-living space, plus up to eight detached ADUs capped at the existing unit count.
• Fees and review. No impact fees under 750 square feet, and ministerial review within 60 days.

SB 9. Chapter 17.53 sets objective standards for urban lot splits and two-unit projects. See our SB 9 lot split guide and ADU investment guide. Inclusionary rules are in Chapter 17.57; we did not review them.

Rent rules: state law only

The Attorney General’s rent cap page, updated July 16, 2026, places San Mateo County in the San Francisco Area, allowing covered units up to 8.8% for increases from August 1, 2026 through July 31, 2027, and lists East Palo Alto as the only San Mateo County city with local rent stabilization. In Atherton, most rentals are single-family homes and ADUs, which may be exempt from AB 1482 when owned by individuals who give the required notice; units owned by corporations or REITs, and any new multifamily buildings once they are more than 15 years old, are covered. See our AB 1482 guide and just cause eviction guide.

Atherton rental property rules

• No transient lodging. Section 17.30.050 bars new hotels, motels, boardinghouses and transient guest quarters, including bed and breakfasts, anywhere in town. ADUs must be rented for more than 30 days.
• Boardinghouses. Because boardinghouses are on the prohibited list, renting rooms to many unrelated tenants as a business is not a path in Atherton; SRO units are allowed only on RM-20/40 overlay sites.
• Transfer tax. Only the County’s documentary transfer tax of $0.55 per $500 appears to apply; we found no town transfer tax chapter.
• Small lots. On the Parker Avenue overlay, small R-1A lots get a floor area allowance of 0.165 times lot area plus 1,500 square feet, with second stories capped at 1,132 square feet.

Large lots, older estates and building issues

Atherton’s estate lots and older homes raise their own issues, chiefly the cost of bringing an older main house and its utilities up to current standards when adding a unit. The ADU chapter helps by not requiring fire sprinklers in the existing home when an ADU is added and not requiring nonconformities to be corrected. Older homes merit a seismic retrofit review. New buildings of three or more units on RM sites will eventually need SB 721 balcony inspections if they have elevated decks.

Underwriting an Atherton multifamily purchase

1. Confirm whether the parcel is listed for RM-10 or RM-20/40, and run the minimum density.
2. For an estate, count legal ADUs and JADUs and check whether a bonus ADU is possible on two acres or more.
3. Underwrite rentals of more than 30 days only; lodging uses are barred.
4. Classify units under AB 1482 and model increases at or below 8.8% for covered units.
5. Collect leases and estoppel certificates, including for any ADU tenants.
6. Rebuild expenses with reassessed taxes, estate maintenance and insurance in our rental property analysis.

Questions people ask

Sources & notes

1. Atherton Chapter 17.30, Use Districts (GoCodebook)
2. Atherton Chapter 17.35, RM-10
3. Atherton Chapter 17.36, PFS and RM-20/40
4. Atherton Chapter 17.37, Parker Avenue Overlay
5. Atherton Chapter 17.52, ADUs
6. San Mateo County Clerk-Recorder, documentary transfer tax
7. California Attorney General, rent caps

Note: We did not review Chapters 17.53 (SB 9 standards), 17.57 (inclusionary) or 17.59, or the list of parcels carrying each overlay.

General information, not legal or tax advice. Last reviewed October 2026 by Matthew Martinez.

Related: Atherton real estate guide · Commercial real estate in Atherton · Multifamily investing guide · Investor services