Do I need a permit for an accessory dwelling unit (ADU) in
Longview, Washington?

Longview ADU permits — the key details

Washington State law (RCW 36.70A.696, effective 2023) requires Longview to allow ADUs as a matter of right on any single-family lot, subject only to design and code compliance — not discretionary local zoning. This is a hard floor, not a ceiling: the city cannot impose owner-occupancy requirements, parking mandates, or lot-size minimums beyond what the State Building Code (2021 IBC/IRC adopted by WA) requires. What Longview CAN enforce: setbacks (typically 5–10 feet from lot lines for detached ADUs), lot coverage limits (often 50–60%), height limits (usually 35 feet), and design standards (roof pitch, materials, fenestration to match primary dwelling). Your permit application must include a statement that the ADU meets the criteria in RCW 36.70A.696 — the Building Department has a checklist. If you're proposing a detached ADU on a tight lot, expect the plan reviewer to flag setback conflicts early; if you're converting a garage, egress windows (IRC R310 requires 5.7 sq. ft. operable, at least 20 inches wide × 24 inches tall) and escape route clarity are deal-breakers. The state law also waives any parking requirement entirely for ADUs, so don't waste money on extra pavement.

Longview's soil conditions require careful foundation design. The city sits on glacial till and alluvial soils with frost depths of 12 inches in the Puget Sound–adjacent lowlands but deeper (18–24 inches or more) in areas near the Columbia River. Your foundation footings must extend below frost depth; a frost-line search via the Cowlitz County soil survey or a geotechnical report is standard during permit review. If the ADU is detached and larger than 200 sq. ft., a full foundation inspection (stem wall, footing depth, grade beams) is mandatory; if it's a garage conversion or above-garage unit, the existing foundation must be verified as adequate for the new use. Longview also requires proof of separate utility connections (water, sewer, electrical) or sub-metering; if you're connecting to the primary meter, the reviewer will ask for a sub-metering plan and will likely require it as a permit condition. This adds cost and time — budget 2–4 weeks for utility coordination with Longview Public Works and the utility providers (local electric coop or Cowlitz Public Utility District).

The permit application package for Longview ADUs is streamlined but complete: site plan showing lot lines, setbacks, and primary dwelling plus ADU footprint; floor plans (full kitchen, separate entrance, sleeping areas, and egress windows clearly marked); foundation plan (if detached, including footing depth and frost-line notation); electrical one-line diagram showing disconnect and panel location; and a utility connection plan (water, sewer, gas if applicable — or sub-meter schematic). The City of Longview Building Department prefers PDF submissions through its online portal (https://permits.longviewwa.gov or similar; verify current URL with the department). Plan review takes 15–30 days for a first pass; expect one or two rounds of comments (minor corrections) before approval. Once approved, you pull the permit and pay the fee (typically $2,500–$6,000 for an ADU, depending on size and complexity). The full inspection sequence includes foundation (before backfill), framing, rough trades (plumbing/electrical/HVAC), insulation, drywall, and final + utilities. Each inspection is scheduled 1–2 weeks apart, so plan for 8–12 weeks total from permit issuance to certificate of occupancy.

Owner-builder provisions: Washington allows the owner-occupant to pull a permit and perform work on their own residence (including an ADU) if the owner will occupy the primary dwelling. You cannot owner-build if you plan to rent out the primary dwelling and rent the ADU; in that case, a licensed contractor or GC must pull the permit and perform major structural/mechanical work. Longview's Building Department will ask for owner-occupancy attestation on the permit form. This can be a cost-saver (no general contractor markup), but you're liable for code compliance and quality; inspectors hold you to the same standard as a contractor. If you're borderline on budget, hiring a plan preparer ($1,500–$3,000) to generate a tight, code-compliant set of drawings up front will speed up review and avoid re-submissions.

Timeline and fees: Longview has no specific ADU fast-track (unlike California's 60-day shot clock), so expect 8–12 weeks from application to approved permit, then another 12–16 weeks of construction and inspections. Permit fee is typically based on valuation: for a 400 sq. ft. detached ADU budgeted at $80,000, the fee is roughly 1.5–2% of value ($1,200–$1,600) plus plan-review and utility/development charges (another $1,000–$2,000). If the ADU is part of a larger accessory structure (e.g., above a new garage), impact fees for utilities and transportation may apply (often $1,500–$3,000 total). Total hard costs for permitting and inspection: $3,500–$7,000. There's no owner-builder exemption from the permit itself, but you do save contractor-license costs and overhead. Document every inspection with photos and signed inspection cards; code officials sometimes flag minor issues (e.g., nail spacing, outlet placement) that must be corrected before the next phase.

Three Longview accessory dwelling unit (adu) scenarios

Washington State ADU Law: How RCW 36.70A.696 Overrides Longview Zoning

In 2023, Washington State enacted RCW 36.70A.696, which mandates that local jurisdictions allow ADUs on any single-family residential lot as a 'use permitted outright' — meaning no conditional-use permits, no design review by planning boards (only code compliance), and no local rules that ban or severely restrict ADUs. Longview must now approve ADUs that meet State Building Code (2021 IBC/IRC) and local design standards (setbacks, height, materials). The law preempts any local zoning ordinance that conflicts. What this means in practice: Longview cannot require owner-occupancy of the primary dwelling, cannot impose parking requirements, and cannot restrict lot size or impose an ADU cap. The city can still enforce setback limits (typically 5–10 feet), lot coverage limits (50–60%), height limits (35 feet typical), and architectural compatibility in overlay zones (historic districts, wetland/critical-area buffers). On your permit application, you must check a box or provide a narrative confirming that the ADU meets RCW 36.70A.696 criteria — lot is a single-family residential parcel, ADU has separate entrance and kitchen (or kitchenette for junior ADU), and unit size is ≤1,200 sq. ft. (junior ADU cap). Longview's Building Department will verify these criteria during plan review but will not apply discretionary planning conditions.

The state law also explicitly allows junior ADUs (RCW 36.70A.696(2)) on any residential lot: a dwelling unit with an efficiency or one-bedroom sleeping area, bathroom, and kitchenette (sink, counter, appliance) but NOT a full kitchen. Junior ADUs have looser standards in some jurisdictions (no separate egress required if interior door to primary dwelling is open; smaller minimum sizes allowed). Longview's local code defines junior ADUs similarly but still requires separate entrance and egress windows of code-minimum size (IRC R310: 5.7 sq. ft. operable, 20 inches wide × 24 inches tall) if the junior ADU is a separate structure. If the junior ADU is interior to the primary dwelling (a so-called 'bedroom ADU'), egress requirements may relax. Clarify with Longview Building Department during pre-application consultation; it saves time and re-submits.

One critical caveat: RCW 36.70A.696 does NOT waive utility-connection requirements or infrastructure adequacy. Longview can require proof that the lot has adequate water, sewer, and electrical capacity for the ADU, or that the ADU is sub-metered or separately connected. This is where costs often spike: a new sewer lateral from the ADU to the main line costs $2,000–$4,000; separate electric service from the utility can cost $1,500–$3,000. If your lot is dense or utilities are undersized, the City Public Works may require a utility-feasibility study or a load analysis. Longview allows sub-metering as an alternative for water/sewer, which is cheaper ($800–$1,500 for a water sub-meter and a grease trap if shared sewer is used), but the utility company (Cowlitz PUD) must approve it and the building department must accept it as a permit condition.

Longview Soil, Frost, and Foundation Pitfalls for ADUs

Longview's soil is a mix of glacial till (west side, Puget Sound drainage), volcanic ash and pumice (east side, Columbia River drainage), and alluvial silts in low-lying areas. Frost depth varies: 12 inches in the western lowlands (near Interstate 5 and downtown), but 18–24 inches or deeper in elevated and eastern parts of the city. The Cowlitz County soil survey (available online via NRCS) maps this, but most plan reviewers in Longview will ask for a frost-depth citation on your foundation plan. If you're unsure, do a soil test or use the county's published depth; if the reviewer flags it, you must revise. For detached ADUs, footings must extend below frost depth plus 12 inches of bearing; if frost is 18 inches, footings go down 30 inches minimum. This is a common reject point: applicants show shallow footings (12 inches) assuming Puget Sound-style frost, and Longview flags it as non-compliant if the ADU is on a deeper-frost lot. To avoid this, get the frost depth from the county before you design, include it on the foundation plan with a note ('Frost depth per Cowlitz County Soil Survey, Section X'), and provide a cross-section showing footing depth relative to frost line. If you hire a geotechnical engineer ($800–$2,000), they'll do this for you; if you're budget-conscious, a soil map and frost-depth lookup usually suffices.

Glacial-till and alluvial soils in Longview can be relatively stable for shallow foundations (single-story ADUs) but may have drainage issues, especially in spring and in low-lying areas near the Cowlitz River. If your ADU site is in a flood zone (FEMA FIRM check this), or if groundwater is shallow (within 3–5 feet of the surface), Longview will require a sump pump, crawl-space vapor barrier, or perimeter drain. For detached ADUs on clay-heavy alluvial soil, the plan reviewer may ask for a perimeter drain or swale to direct surface water away. This is not a show-stopper, but it adds ~$1,500–$2,500 to construction costs and may require a drainage design or a geotechnical opinion. Ask the City during pre-application: 'Is my lot in a flood zone or known groundwater-high area?' If yes, budget for drainage. If no, a standard foundation with frost-depth footings and a basic grade slope (minimum 5% away from the ADU) usually passes.

Longview's Building Department will require a foundation inspection before backfill; for detached ADUs, this includes visual verification of footing depth (the inspector will probe or dig to confirm), stem-wall height, and grade-beam or footing-board placement. If you're above-garage or converting a garage, the reviewer will inspect the existing foundation and may require shoring, reinforcement, or testing if the new ADU load exceeds the garage design. Have a structural engineer review the existing structure if you're unsure; it's cheaper than a failed inspection and a redesign. For owner-builders, the inspection is the same, but inspectors expect careful documentation — photos of the footing before the inspector arrives, a trenching log showing depth and soil conditions, and clear measurement of width and height. Small deviations (1–2 inches) are often accepted; major discrepancies (footings only 12 inches when 30 required) will fail and require excavation and correction.

Common questions