Complete 2026 cost breakdown: construction, permits, financing, and expected rental ROI.
Calculate your Folsom ADU cost →Building an ADU in Folsom costs between $170,000-$400,000 for new construction, with per-square-foot costs averaging $250-$500. These figures include design, permitting, site preparation, and construction. The most affordable option is a garage conversion at $90,000-$200,000, while prefab/modular ADUs fall in between at $170,000-$340,000.
| Cost Category | Estimated Range |
|---|---|
| Architectural Plans | $3,000 – $15,000 |
| Permits & Fees | 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). |
| Impact Fees | No city impact fees for an ADU of 750 sq ft or less or a JADU of 500 sq ft or less (FMC 17.105.060(C)); above 750 sq ft each city fee is prorated by ADU floor area divided by primary-dwelling floor area. Water and sewer connection or capacity charges are not charged unless the ADU is built with a new house (17.105.060(B)). The July 1, 2026 North of 50 schedule totals about $26,200 in city impact fees for a new single-family home (roads $9,903, park equipment $8,532, general $1,936, fire $1,316, drainage $1,257, water $1,194, light rail $878, police $728, smaller items), so a 1,000 sq ft ADU behind a 2,000 sq ft house would owe roughly half; the Folsom Plan Area south of Hwy 50 has its own higher schedule. School district fees are collected separately. |
| Site Prep & Foundation | $10,000 – $35,000 |
| Framing & Structure | $15,000 – $45,000 |
| Plumbing | $8,000 – $20,000 |
| Electrical | $5,000 – $15,000 |
| HVAC | $3,000 – $8,000 |
| Interior Finishes | $15,000 – $40,000 |
Quotes for the same ADU commonly vary 20–40% between builders. We pass your request to up to 5 local builders serving Folsom, where we have them — free, no obligation.
Get Folsom builder quotes →Permit fees in Folsom are 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).. The review timeline is 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., with 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.. Impact fees are No city impact fees for an ADU of 750 sq ft or less or a JADU of 500 sq ft or less (FMC 17.105.060(C)); above 750 sq ft each city fee is prorated by ADU floor area divided by primary-dwelling floor area. Water and sewer connection or capacity charges are not charged unless the ADU is built with a new house (17.105.060(B)). The July 1, 2026 North of 50 schedule totals about $26,200 in city impact fees for a new single-family home (roads $9,903, park equipment $8,532, general $1,936, fire $1,316, drainage $1,257, water $1,194, light rail $878, police $728, smaller items), so a 1,000 sq ft ADU behind a 2,000 sq ft house would owe roughly half; the Folsom Plan Area south of Hwy 50 has its own higher schedule. School district fees are collected separately.. Pre-approved plans: 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..
| 1-Bedroom Rent | $1,650-$2,400/month |
| 2-Bedroom Rent | $2,200-$2,950/month |
| Annual Rental Income (est.) | $24,300/year |
| Payback Period (est.) | 11.7 years |
| Short-Term Rental | 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. |
| Overall ROI Estimate | 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) |
A typical ADU in Folsom generates approximately $24,300 per year in rental income based on average 1-bedroom rents. At an average construction cost of $285,000, the estimated payback period is 11.7 years — before accounting for the resale value the ADU adds, which an appraiser determines from comparable sales in Folsom and varies by market (often less than build cost).
1. Garage conversions save 40-60% vs new construction since the shell already exists. Average cost in Folsom: $90,000-$200,000.
2. Prefab/modular ADUs offer predictable pricing and faster timelines. Average: $170,000-$340,000.
3. Pre-approved plans — Folsom does not offer a pre-approved/standard ADU plan program yet, so budget for custom architectural plans (typically $3,000-$15,000). See which cities offer pre-approved ADU plans.
4. State grants — California's CalHFA ADU grant (up to $40K) is currently paused; check calhfa.ca.gov/adu for status and your state for active programs.
5. Get multiple bids — ADU contractor pricing varies 20-40% for the same project. We recommend getting at least 3 quotes.
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