Everything homeowners ask about building an ADU in Folsom — answered with 2026 data.
Yes, ADUs are allowed in Any parcel zoned for single-unit, two-unit or multi-unit residential use with an existing or proposed primary dwelling (FMC 17.105.010(D)); JADUs only on single-unit parcels. Historic District parcels add objective architectural-style rules (17.105.200) unless the unit is a 66323 unit. in Folsom, CA. The maximum size for a detached ADU is 1,000 sq ft with 2 or more bedrooms, 850 sq ft with 1 bedroom or fewer, measured as interior living space, minimum 150 sq ft (FMC 17.105.150(B)(2)). A new detached unit of 800 sq ft or less with 4-ft setbacks is a '66323 unit' exempt from design review, and no standard may preclude an 800 sq ft ADU (17.105.150(C)(2)). Detached ADUs on two-unit and multi-unit lots are capped at 800 sq ft (17.105.170(B)(2)). Attached ADUs can be up to 1,000 sq ft with 2 or more bedrooms or 850 sq ft with 1 bedroom or fewer, and no more than 50% of the existing primary dwelling's floor area. Folsom has an ADU-friendliness score of 7/10. Check our full Folsom ADU rules page for complete zoning details.
ADU construction in Folsom typically costs $170,000-$400,000. The cost per square foot averages $250-$500. Garage conversions are more affordable at $90,000-$200,000, while prefab ADUs cost $170,000-$340,000. Permit fees add Flat building permit fee of $4,852 per detached or attached ADU and $2,329 per JADU, covering plan review and inspections (Res. 11481, effective July 1, 2026), plus an $81 processing fee and state surcharges; design review for a non-66323 unit adds $400 (Director level). to your budget. See our detailed Folsom ADU cost guide.
Detached ADUs in Folsom can be up to 1,000 sq ft with 2 or more bedrooms, 850 sq ft with 1 bedroom or fewer, measured as interior living space, minimum 150 sq ft. Attached ADUs are limited to 1,000 sq ft with 2 or more bedrooms or 850 sq ft with 1 bedroom or fewer, and no more than 50% of the existing primary dwelling's floor area. Junior ADUs (JADUs) have a cap of 500 sq ft, entirely within the single-unit home, with its own exterior entrance and an efficiency kitchen; minimum 150 sq ft (FMC 17.105.160). A deed restriction on the city's template (Form CD-P515) is recorded before the building permit issues. Maximum height is Detached: 25 ft or the height of the main house, whichever is less, measured finished grade to peak (FMC 17.105.150(B)(5)); within 50 ft of a listed historic structure only the state heights apply. 66323 units: 16 ft, 18 ft within 1/2 mile of a major transit stop or where the roof pitch matches the house, 20 ft with both (city checklist CD-P214; Gov. Code 66321). Attached: the zone's height, but 25 ft for any part inside the primary dwelling's setback area (17.105.140(B)(5)). You are allowed up to Single-unit lot: 1 attached or detached ADU + 1 conversion ADU on a qualifying lot.
Rear setback: 4 ft minimum (FMC 17.105.150(B)(3)); the city recommends 5 ft to avoid extra Fire Code construction requirements. No setback change for a conversion of an existing house or accessory building, or for an ADU built on top of an existing accessory building that already sits closer than 4 ft. Side setback: 4 ft minimum, 5 ft recommended by the Building Division to avoid Fire Code upgrades; none for conversions of existing space. A detached ADU must be at least 6 ft from the house and from any other accessory building other than a fence or wall (FMC 17.105.150(B)(3)). Front setback: Attached units use the zone's front setback (FMC 17.105.140(B)(3)); a detached ADU may not break the front plane of the primary dwelling (17.105.150(B)(3)). The ADU may cover no more than 40% of the rear yard and the zone's lot-coverage limit applies, except for 66323 units of 800 sq ft or less, which are also exempt from required open space (17.105.150(B)(4), 17.105.130(D)). Note that garage conversions may be exempt from setback requirements if the existing structure is already within the setback area.
1 space per ADU or per bedroom, whichever is less; a driveway or tandem space counts (FMC 17.105.180(A)). Not required within 1/2 mile walking distance of public transit, in the Historic District, for a unit inside the house or an existing accessory building, near a car-share vehicle, or when permitted with a new home; no replacement parking when a garage is converted (17.105.180(B)-(C)). The city publishes an ADU Parking Requirement Map. Many cities and states have been relaxing ADU parking requirements in recent years. In California, no additional parking is required for ADUs near transit.
Not required for ADUs. JADUs: required only when the JADU shares a bathroom with the main house (FMC 17.105.160(B)(4), matching Gov. Code 66333 as amended by AB 1154, effective Jan 1, 2026); a JADU with its own bathroom has no owner-occupancy rule. Note that owner-occupancy rules vary by state law — California and several other states have eliminated this requirement for standard ADUs.
The ADU permit review timeline in Folsom is typically Completeness notice within 15 business days; decision within 60 days of a complete application, deemed approved if the city misses it (FMC 17.105.050; Gov. Code 66317). The city says merged design review saves an estimated 3-4 weeks and a pre-reviewed Master Plan about 30 days more. Plan check: Plan check is bundled into the flat ADU permit fee and covers up to three review cycles; a fourth cycle, revisions and deferred submittals bill at $194 per hour and expedited review costs 1.5x (Res. 11481, Building Fees Sec. D). Side or rear setbacks under 5 ft trigger extra Fire Code construction requirements; retaining walls over 4 ft or grading over 50 cubic yards need separate permits; work on a protected tree needs a Tree Permit. Pre-approved plans availability: Not available yet — Folsom launched an ADU Master Plan program under Gov. Code 65852.27 (designers submit detached ADU plans for pre-review; a homeowner using one, with no deviations, cuts permit time by an estimated 30 days), but the city's approved-plan gallery listed no plans as of Sept 20, 2026.
Average ADU rental income in Folsom: 1-bedroom units rent for $1,650-$2,400/month, 2-bedroom units for $2,200-$2,950/month. Short-term rentals: No — an ADU or JADU may be rented only for 30 days or longer (FMC 17.105.070(A)), and the city's ADU page states ADUs cannot be used as vacation rentals. Expected ROI: 7-14% gross rental yield on a $250,000-$400,000 detached build renting at $2,200-$2,950/mo; a $90,000-$200,000 garage conversion at $1,650-$2,400/mo can gross 10-25% (estimates before taxes, vacancy and financing).
The minimum lot size for an ADU in Folsom is No minimum lot size anywhere in FMC 17.105; the zone's lot-coverage and pervious-surface standards still apply to units that are not 66323 units (17.105.150(B)(4)). This applies to detached ADUs — garage conversions and JADUs may have different or no lot size requirements.
Garage conversions are one of the most popular and affordable ADU options. In Folsom, a garage conversion typically costs $90,000-$200,000. The key advantages are lower cost (no foundation work), potential setback exemptions, and faster permitting. Read our complete garage conversion guide.
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