If your mid-century lot has room for a second, smaller home, California law now requires cities to allow one, and often more than one. The harder question is design. A mid-century modern ADU that ignores the low roofline, the glass walls and the private garden of the main house can undo much of what made the property special.
This guide covers what state law allows on a single-family lot today, where an accessory dwelling unit fits with least disruption, how to design one that belongs with an Eichler or other post-and-beam house, and the fees, taxes and paperwork to plan for. It draws on the California Department of Housing and Community Development's March 2026 ADU Handbook and the City of Palo Alto's Eichler design guidelines.
What California ADU law allows on a single-family lot
State ADU law sets a floor that every city and county has to meet. Your city's ordinance can be more generous.
Three kinds of units. On a single-family lot, local agencies must allow at least one ADU built from existing space, one junior ADU (JADU) and one newly constructed detached ADU.
A JADU is small and inside the house. It is no more than 500 square feet of interior living space, created within the existing or proposed home, and it can be carved out of an attached garage.
Room for at least 800 square feet. Lot coverage, floor area ratio, open space and front setback rules cannot block an ADU of at least 800 square feet with four-foot side and rear setbacks.
Setbacks of no more than four feet from the side and rear lot lines for a new attached or detached ADU. No setback applies to an ADU created within existing space, or rebuilt in the same location and to the same dimensions as an existing structure.
Height. A city cannot set a detached ADU height limit lower than 16 feet. That rises to 18 feet within a half-mile walk of a major transit stop or high-quality transit corridor, plus another two feet for a roof pitch that matches the main house. An attached ADU can go to 25 feet or the main house's height limit, whichever is lower.
Size caps. A city's maximum size must allow at least 850 square feet, or 1,000 square feet for an ADU with more than one bedroom. Without a compliant local ordinance, the defaults are 1,200 square feet for a new detached ADU and up to 50 percent of the main house's floor area for an attached one.
A firm clock. The permitting agency must make a written completeness determination within 15 business days, and approve or deny a complete application within 60 days when there is an existing home on the lot.
HCD's handbook also addresses single-story zoning, which matters in many Eichler neighborhoods. If a detached two-story ADU fits within the state height allowances and complies with the building code, HCD says a local agency cannot deny it even where zoning restricts the main house to one story. Whether a two-story unit suits the house is a separate question.
Start with the space you already have
Before you plan a new building in the backyard, look at what the house already contains.
Garage conversions. Many original houses have an attached garage or carport. State law lets a JADU be created within an attached garage, and an ADU converted from an existing accessory structure can expand up to 150 square feet for entry and exit. A city may not require you to replace the parking spaces lost when a garage or carport is converted to an ADU.
Space inside the house. A converted ADU needs exterior access. A JADU is created inside the house itself.
Palo Alto's Eichler design guidelines, a final draft from February 2018, say that a detached ADU behind an Eichler is likely to reduce valued yard space and to be highly visible from the main house and its neighbors. They recommend exploring a garage conversion or an attached ADU first because those options may present fewer privacy challenges. Our Eichler renovation guide covers the systems you will touch when you open up an original house.
Design a detached ADU that belongs with the house
If a detached unit is the right answer, the Palo Alto guidelines give a useful checklist that applies to post-and-beam houses well beyond that city:
Relate the structure to the house. Because original Eichlers were designed by notable architects, the guidelines suggest considering post-and-beam construction for the ADU. They discourage prefabricated detached units with no aesthetic relationship to the main house.
Keep the roof low. Design with as low a height and roof slope as possible. Flat, low-slope gable and low-slope shed roofs, all used on original Eichlers, are encouraged. High-pitched gable, hipped and gambrel roofs are discouraged.
Place it with care. Set a detached ADU at a distance from the main house, keep the footprint as small as the living space allows, and orient it so only one facade faces the main house.
Use clerestories for privacy. Do not place eye-level windows facing the main house's primary outdoor space. Clerestory windows above eye level work better. Keep in mind that a loft can defeat the purpose of clerestories.
Turn the entry away. Where possible, put an attached ADU's entrance on a facade that does not face the street. The same approach applies to garage conversions.
Match materials. Choose cladding, windows and doors that relate to original Eichler designs, even in a rear yard where the unit is not visible from the street.
Screen with planting. Bands of trees or other tall plantings can screen a detached ADU from neighboring properties.
One caution about the 2018 document: its statement about the maximum ADU height reflects state law at that time, and the height rules have since changed. Use the guidelines for design ideas, and use the city's current ordinance and the state handbook for numbers. Our mid-century modern design guidelines page collects the design principles these houses share.
Historic districts and local review
ADUs are allowed in historic districts and on lots where the main house is subject to historic preservation. State law lets a local agency impose standards that prevent adverse impacts on property listed in the California Register of Historical Resources, but those standards must be objective so that the ADU can still be reviewed ministerially, without a discretionary hearing.
Palo Alto's National Register districts. The 2018 guidelines say ADUs proposed in the Green Gables and Greenmeadow National Register districts are reviewed by city staff for compliance with the Secretary of the Interior's Standards for the Treatment of Historic Properties. State law now requires objective standards and ministerial review, so ask the city how it applies that review today.
Parking. A local agency cannot impose parking standards on an ADU in an architecturally and historically significant historic district, within a half-mile walk of public transit, or created within the house or an accessory structure, among other cases. Elsewhere, parking cannot exceed one space per ADU or per bedroom, whichever is less.
If you are not sure whether your tract has a historic designation, ask the city's planning department before you design. Our mid-century modern design team can help you think through options that respect the original house.
Fees, taxes and paperwork to plan for
Impact fees. No impact fees may be charged on an ADU of 750 square feet or less of interior living space, or a JADU of 500 square feet or less. Above 750 square feet, fees are charged in proportion to the ADU's size compared with the main house.
Solar. A newly constructed detached ADU is subject to the California Energy Code solar requirement, with some exceptions. The California Energy Commission says the system can go on the ADU or on the main house. ADUs built within existing space or as additions are not subject to that requirement.
Owner occupancy. A local agency cannot require owner occupancy for an ADU, except in a narrow nonprofit-sale case. A JADU that shares sanitation with the main house requires owner occupancy; one with its own sanitation does not.
Rentals. If rented, the three units state law guarantees on a single-family lot, including a JADU, must be rented for terms longer than 30 days. A city may set a similar minimum for other ADUs.
Certificate of occupancy. You must get one from the local agency before anyone lives in the ADU or JADU.
Property tax. New construction is reassessed. According to the Board of Equalization, the assessor gives the newly constructed portion its own base year value while the rest of the property keeps its existing base year value, and completed new construction triggers a supplemental assessment.
The details of state ADU law are in the HCD ADU Handbook, and Palo Alto's design advice is in its Eichler design guidelines. This is general information as of November 2026, not legal or tax advice. State ADU law changes often (the March 2026 handbook covers bills that took effect January 1, 2026), and local ordinances vary, so check with your city's planning department, a tax advisor and, for anything involving deeds, rentals or a separate sale, a real estate attorney before you build.
Frequently asked questions
Can my city stop me from building an ADU behind my Eichler?
Not if the project meets state ADU law. A city can apply objective standards, including standards for property listed in the California Register, but it must allow at least the units, sizes, heights and setbacks state law requires.
Can I build a two-story ADU in a single-story overlay?
HCD says that if a detached two-story ADU fits within the state height allowances and meets the building code, a local agency cannot deny it because zoning limits the main house to one story. Height limits for a detached ADU start at 16 feet, so a second story is not always possible.
Do I have to live on the property if I add an ADU?
Not for an ADU; state law bars cities from requiring owner occupancy except in a narrow nonprofit case. A JADU that shares sanitation facilities with the main house does require owner occupancy.
Will an ADU raise my property taxes?
The new construction is assessed and gets its own base year value, while the rest of your property keeps its existing base year value. Expect a supplemental tax bill after the ADU is complete, and ask your county assessor how the project will be valued.
Related reading
Thinking about an ADU, or buying a mid-century home with one in mind? Call or text the Boyenga Team at (408) 373-1660 or (650) 383-8606, or contact the Boyenga Team to talk through the lot, the house and the options.
