Complete 2026 cost breakdown: construction, permits, financing, and expected rental ROI.
Calculate your Woodinville ADU cost →Building an ADU in Woodinville costs between $325,000-$550,000 for new construction, with per-square-foot costs averaging $400-$650. These figures include design, permitting, site preparation, and construction. The most affordable option is a garage conversion at $100,000-$225,000, while prefab/modular ADUs fall in between at $300,000-$450,000.
| Cost Category | Estimated Range |
|---|---|
| Architectural Plans | $3,000 – $15,000 |
| Permits & Fees | 2026 schedule (Resolution 668): permit fee on valuation ($148.28/sq ft for wood-frame R-3, WMC 21.62.060), plan review 65% of it, fire plan review 13.2% of that, $6.50 state surcharge. A 1,000 sq ft detached ADU comes to about $3,480, plus $119 plus $25 per fixture for mechanical and plumbing |
| Impact Fees | School impact fee: ADUs approved under WMC 21.41.020 are fully exempt (WMC 3.38.040(3), Ord. 800, 2025); a new three-bedroom house pays $17,550 (WMC 3.38.060), so the exemption is real money. Park impact fee: an ADU pays 50% of the fee for the largest primary unit on the lot (WMC 3.36.080(2), Ord. 811, 4 Aug 2026); the schedule is $2,054 for a unit of 1,500 sq ft or less, $4,886 from over 1,500 to under 2,300 sq ft and $7,718 at 2,300 sq ft and over (WMC 3.36.060(2)), so an ADU behind a 2,000 sq ft house owes $2,443 and behind a 2,400 sq ft house $3,859. Transportation impact fee: $3,760 per p.m. peak-hour person trip (WMC 3.39.085, Ord. 772, 2024), trips predicted from the ITE Trip Generation manual (WMC 3.39.090(4)); Chapter 3.39 has no ADU exemption, but RCW 36.70A.681(1)(a) caps any ADU impact fee at 50% of the principal unit's, a cap RCW 36.70A.681(4) switches off on a critical-area lot, where only the city's own 3.36.080(2) and 3.38.040(3) apply. You may ask for a non-binding advance calculation (WMC 3.39.090(3)). Fees are fixed at building permit issuance; deferral to final inspection against a recorded lien is written for one-unit dwellings and single-family construction (WMC 3.36.060(3), 3.39.090(2)), so ask whether an ADU permit qualifies |
| 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 Woodinville, where we have them — free, no obligation.
Get Woodinville builder quotes →Permit fees in Woodinville are 2026 schedule (Resolution 668): permit fee on valuation ($148.28/sq ft for wood-frame R-3, WMC 21.62.060), plan review 65% of it, fire plan review 13.2% of that, $6.50 state surcharge. A 1,000 sq ft detached ADU comes to about $3,480, plus $119 plus $25 per fixture for mechanical and plumbing. The review timeline is WMC 21.80.210 (Ord. 782, 2025): the city must say whether the application is complete within 28 days and decide a construction permit within 120 days of the completeness date; the clock pauses while it waits for your corrections. Plan review covers two resubmittals; a critical areas report adds time, with plan check: Standard building plan review under Chapter 21.62 WMC. Prefabricated units are allowed on a permanent foundation (WMC 21.41.020(4)(c)). Conversions of existing enclosed structures, detached garages included, are allowed even where the structure breaks current coverage or setback rules, provided it and the nonconformance were legally established (WMC 21.41.020(4)(a)). The Director may approve deviations from zoning standards for accessibility features (WMC 21.41.020(3)(e)). If the lot has a wetland, stream, steep slope, flood area or critical aquifer recharge area, expect a critical areas report (WMC 21.51.060), mitigation sequencing (WMC 21.51.050) and the 10-ft building setback from any riparian zone or wetland buffer (WMC 21.51.050(3)). Structural review sent to an outside consultant is billed at cost, with an advance deposit if it is expected to exceed $5,000 (2026 fee schedule, note 4). Impact fees are School impact fee: ADUs approved under WMC 21.41.020 are fully exempt (WMC 3.38.040(3), Ord. 800, 2025); a new three-bedroom house pays $17,550 (WMC 3.38.060), so the exemption is real money. Park impact fee: an ADU pays 50% of the fee for the largest primary unit on the lot (WMC 3.36.080(2), Ord. 811, 4 Aug 2026); the schedule is $2,054 for a unit of 1,500 sq ft or less, $4,886 from over 1,500 to under 2,300 sq ft and $7,718 at 2,300 sq ft and over (WMC 3.36.060(2)), so an ADU behind a 2,000 sq ft house owes $2,443 and behind a 2,400 sq ft house $3,859. Transportation impact fee: $3,760 per p.m. peak-hour person trip (WMC 3.39.085, Ord. 772, 2024), trips predicted from the ITE Trip Generation manual (WMC 3.39.090(4)); Chapter 3.39 has no ADU exemption, but RCW 36.70A.681(1)(a) caps any ADU impact fee at 50% of the principal unit's, a cap RCW 36.70A.681(4) switches off on a critical-area lot, where only the city's own 3.36.080(2) and 3.38.040(3) apply. You may ask for a non-binding advance calculation (WMC 3.39.090(3)). Fees are fixed at building permit issuance; deferral to final inspection against a recorded lien is written for one-unit dwellings and single-family construction (WMC 3.36.060(3), 3.39.090(2)), so ask whether an ADU permit qualifies. Pre-approved plans: None. Woodinville publishes no pre-approved ADU plan set: the city's Permitting and Applications & Forms pages (archived 6 Jan 2026) list none, and the code contains no expedited ADU track, unlike Kirkland's four registered plans. Confirm with the Permit Center at 425-489-2754 whether plans approved elsewhere would shorten review.
| 1-Bedroom Rent | $2,400-$2,700/month |
| 2-Bedroom Rent | $2,800-$3,150/month |
| Annual Rental Income (est.) | $30,600/year |
| Payback Period (est.) | 14.3 years |
| Short-Term Rental | No. "Accessory dwelling units are prohibited from being rented for periods of fewer than 30 consecutive nights" (WMC 21.41.020(5), re-read 20 Sept 2026, pointing to the short-term rental definition in RCW 36.70A.696). |
| Overall ROI Estimate | 5-9% gross annual yield on a $325,000-$550,000 detached build renting at $2,400-$3,150/mo, before vacancy, taxes, impact fees and financing; a garage conversion at $100,000-$225,000 yields roughly double |
A typical ADU in Woodinville generates approximately $30,600 per year in rental income based on average 1-bedroom rents. At an average construction cost of $437,500, the estimated payback period is 14.3 years — before accounting for the resale value the ADU adds, which an appraiser determines from comparable sales in Woodinville 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 Woodinville: $100,000-$225,000.
2. Prefab/modular ADUs offer predictable pricing and faster timelines. Average: $300,000-$450,000.
3. Pre-approved plans — available in Woodinville, which can save $3,000-$10,000 in design costs and 2-3 months in review time. 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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