Complete 2026 cost breakdown: construction, permits, financing, and expected rental ROI.
Calculate your Bothell ADU cost →Building an ADU in Bothell 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 fees: a valuation-based permit fee (Table 20.02.155-A: $1,514 plus $9 per $1,000 over $100,000), plan check at 65 percent, planning review $231, site review $834, energy review $230, plumbing and mechanical 16 percent, 5 percent tech fee. A $300,000 detached ADU: about $7,700 plus impact fees |
| Impact Fees | Four impact fees at permit issuance, each capped at 50 percent of the principal unit's (BMC 12.14.135(G); RCW 36.70A.681(1)(a)). 2026 rates (form X, updated April 2026): transportation $3,311 per ADU from May 1, 2026 (BMC 17.045.070; single-family $13,387) plus a 3 percent administrative fee; fire $232.30 ($229.34 sprinklered; BMC 21.16); park "50% of Principal Unit", i.e. $779-$2,005 against the $1,557.15-$4,009.85 house brackets (BMC 21.08); school $16,550 per single-family and $5,945 per multifamily unit from Jan 1, 2026 plus a $70 administrative fee (BMC 21.12), with no ADU row on the form. Roughly $4,400-$5,650 before any school fee. Impact fees do not vest: the rate in force at issuance applies |
| 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 Bothell, where we have them — free, no obligation.
Get Bothell builder quotes →Permit fees in Bothell are 2026 fees: a valuation-based permit fee (Table 20.02.155-A: $1,514 plus $9 per $1,000 over $100,000), plan check at 65 percent, planning review $231, site review $834, energy review $230, plumbing and mechanical 16 percent, 5 percent tech fee. A $300,000 detached ADU: about $7,700 plus impact fees. The review timeline is Not published: the city's ADU checklist (form 3f, Dec 2025) gives no review time; ask Development Services at 425-806-6400. Add up to two weeks for the water and sewer availability certificate where the City is the provider, and get the address assignment first, with plan check: Plan check fee is 65 percent of the building permit fee and is due at submittal. Plans are 24 by 36 inches maximum, construction drawings at 1/8 inch to the foot or larger. The site plan must show a hard surface coverage table per BMC 12.14.140(B), distances from every structure to the property lines, footing and roof drain routing and discharge, existing and proposed parking, 2-ft contours (5-ft where slope is 15 percent or more), easements, and any environmentally sensitive area on site, adjacent, or within 200 ft (wetlands, steep slopes, streams). That last item decides whether you get two ADUs or one under BMC 12.14.135(C). A new detached ADU needs the full Washington State Energy Code worksheets (prescriptive compliance, heating system size, glazing schedule). Impact fees are Four impact fees at permit issuance, each capped at 50 percent of the principal unit's (BMC 12.14.135(G); RCW 36.70A.681(1)(a)). 2026 rates (form X, updated April 2026): transportation $3,311 per ADU from May 1, 2026 (BMC 17.045.070; single-family $13,387) plus a 3 percent administrative fee; fire $232.30 ($229.34 sprinklered; BMC 21.16); park "50% of Principal Unit", i.e. $779-$2,005 against the $1,557.15-$4,009.85 house brackets (BMC 21.08); school $16,550 per single-family and $5,945 per multifamily unit from Jan 1, 2026 plus a $70 administrative fee (BMC 21.12), with no ADU row on the form. Roughly $4,400-$5,650 before any school fee. Impact fees do not vest: the rate in force at issuance applies. Pre-approved plans: None. The city has no pre-approved ADU plan program: the ADU checklist assumes a custom plan set, and the draft Housing Action Plan presented to Council on June 9, 2026 (adoption by resolution expected December 2026) only lists "pre-designed layout plans for ADUs" as an idea to explore. Confirm with Development Services at 425-806-6400.
| 1-Bedroom Rent | $2,300-$2,700/month |
| 2-Bedroom Rent | $2,700-$3,150/month |
| Annual Rental Income (est.) | $30,000/year |
| Payback Period (est.) | 14.6 years |
| Short-Term Rental | Not regulated by an ADU-specific rule. |
| Overall ROI Estimate | 5-8% annual return |
A typical ADU in Bothell generates approximately $30,000 per year in rental income based on average 1-bedroom rents. At an average construction cost of $437,500, the estimated payback period is 14.6 years — before accounting for the resale value the ADU adds, which an appraiser determines from comparable sales in Bothell 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 Bothell: $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 Bothell, 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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