How Much Does an ADU Cost in Woodinville, WA?

Construction Cost Breakdown

Building an ADU in Woodinville costs between $250,000-$450,000 for new construction, with per-square-foot costs averaging $300-$500. These figures include design, permitting, site preparation, and construction. The most affordable option is a garage conversion at $120,000-$250,000, while prefab/modular ADUs fall in between at $180,000-$350,000.

Permit Costs & Timeline

Permit fees in Woodinville are Confirm current amounts with Woodinville Development Services at (425) 489-2700. Development fees are set by City Council resolution rather than in the code (WMC 21.13.040), the city publishes a 2026 Development Services fee schedule, and the Director may require an advance deposit for outside consultant review — which is common where a critical areas report has to be peer-reviewed (WMC 21.13.050 and 21.13.060). A disputed fee estimate is appealable to the Hearing Examiner (WMC 21.13.070). The review timeline is Confirm with Woodinville Development Services at (425) 489-2700 — no published ADU review timeline was found in a city source, and the city's website could not be reached from this research session. Budget additional weeks if a critical areas report or a SEPA determination is required, with plan check: Standard building plan review under Chapter 21.62 WMC. Prefabricated units are allowed provided they sit on a permanent foundation (WMC 21.41.020(4)(c)). Conversions of existing enclosed structures, detached garages included, are allowed even where the structure does not comply with current building coverage or setback standards, provided the structure and the nonconformance were legally established (WMC 21.41.020(4)(a)). If the lot has a wetland, stream, geologically hazardous area, frequently flooded area or critical aquifer recharge area, expect a critical areas report under WMC 21.51.060, mitigation sequencing under WMC 21.51.050, and a minimum 10-foot building setback from the edge of any riparian management zone or wetland buffer under WMC 21.51.050(3). Impact fees are Better than most Washington cities. School impact fees: an accessory dwelling unit approved under WMC 21.41.020 is fully exempt (WMC 3.38.040(3), Ord. 800 § 2, 2025). Park impact fees: an ADU pays only 50 percent of the park impact fee calculated for the largest primary dwelling unit it is associated with (WMC 3.36.080(2), Ord. 811, 2026). The park fee itself runs $2,054 for a unit of 1,500 sq ft or less, $4,886 from 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 attached to a 2,000 sq ft house owes about $2,443. Transportation impact fees are assessed at $3,760 per p.m. peak-hour person trip (WMC 3.39.085, Ord. 772, 2024) with no ADU exemption in Chapter 3.39. RCW 36.70A.681(1)(a) caps ADU impact fees at 50 percent of the principal unit's — but that cap is one more thing RCW 36.70A.681(4) switches off on a critical-area lot, so cite Woodinville's own WMC 3.36.080(2) and 3.38.040(3) instead, which carry no such carve-out. Payment may be deferred to final inspection or certificate of occupancy against a recorded lien (WMC 3.36.060(3)). Pre-approved plans: No city pre-approved ADU plan library was found in a Woodinville source — unlike neighbouring Kirkland and Redmond. Confirm with Development Services at (425) 489-2700, and note that Washington State Department of Commerce maintains a statewide pre-approved plan effort you may be able to draw on.

Rental Income & ROI

A typical ADU in Woodinville generates approximately $25,800 per year in rental income based on average 1-bedroom rents. At an average construction cost of $350,000, the estimated payback period is 13.6 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).

Ways to Reduce ADU Costs in Woodinville