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

Pasco ADU permits — the key details

Washington state law (RCW 36.70A.697, amended 2023 and reinforced by HB 1337 in 2024) mandates that cities allow one ADU per residential lot, regardless of local zoning. Pasco, like all Washington jurisdictions, must permit ADUs on single-family parcels without reducing parking requirements, reducing lot size, or imposing owner-occupancy restrictions. However, this does NOT mean the permit is automatic or fast. The City of Pasco Building Department still requires a full building permit application (Form PO-1), site plan, floor plans, electrical and mechanical schedules, and proof of separate utility connections (or sub-metering). The IRC R310 egress requirement applies: every habitable room in the ADU, including bedrooms, must have a window or door meeting minimum size and sill-height specifications (typically 36 inches wide, 43 inches tall, opening directly to grade or a 36-inch-wide egress window well). If the ADU is detached or new construction over a crawl space, IRC R401-R408 foundation design applies, and soil and frost-depth engineering is required. Pasco's municipal code defers to IBC 2021 (with amendments), which means sprinkler requirements can be triggered if the total square footage of primary dwelling plus ADU exceeds certain thresholds or if the lot layout triggers occupancy classification changes.

The single largest surprise in Pasco ADU permits is the utility separation and impact-fee requirement. You cannot simply tap into the main house water, sewer, or electrical panel. A separate water meter and sewer-service line (or on-lot septic approval if outside city limits) are mandatory. If you use sub-metering to avoid dual service, the sub-meter still requires a permit and compliance with WAC 296-128 (electrical safety). Sewer impact fees in Pasco run $1,200–$2,000 per new fixture; water impact fees are $500–$1,500. Additionally, if the ADU is detached or is a garage conversion, you must address stormwater runoff (Pasco's stormwater manual requires flow-control swales or bioretention for 95th percentile rainfall, roughly 1.5 inches per 24 hours). The city will not issue a final permit unless drainage plans are stamped by an engineer or approved as exemption-eligible (typically small roof areas under 750 sq ft with permeable downslope surfaces). Many homeowners do not budget for the $2,000–$4,000 engineering and civil work required to show compliant stormwater management.

Exemptions do not exist for ADUs in Pasco. Even a 300-square-foot detached 'junior ADU' (sleeping room plus kitchenette, no full bathroom) requires a permit. A garage conversion that adds a bedroom and bathroom to an existing structure triggers a remodel permit, foundation review (to confirm the structure can handle added live and dead loads), and egress verification. The reason: life-safety code requires proof that every occupant can exit to safety within 35 feet, and the IRC does not grant exemptions based on unit size. However, there are gray areas in how Pasco interprets 'habitable.' A detached structure with no kitchen (sleeping room only, bathroom only) might be classified as a 'guest house' or 'dwelling unit' depending on fixture count and intent. If you declare it as a non-habitable accessory structure (e.g., art studio with a daybed but no kitchen and no full bathroom), you may avoid some review steps — but this is a risky classification that courts have challenged. The safest approach is to design the ADU as a full dwelling unit and let Pasco classify it; the city will ask you to clarify in the application.

Pasco's climate and soils create specific design pressures. The valley floor (elevation 300-500 feet) is glacial till with seasonal high groundwater in spring. Frost depth is 12 inches, so typical frost-protected shallow foundation (FPSF) designs work. However, if the ADU is on a slope or in the east Pasco foothills (elevation 800+ feet), frost depth increases to 30+ inches, and basements or full crawl spaces are more common. Volcanic soils (pumice, tephra, basalt) in the Finley and Waitsburg areas drain rapidly but are prone to dust infiltration and earthquake shaking (seismic zone 2, per USGS). If your lot is in a designated flood zone (Pasco straddles Sacajawea Flats and Snake River inundation zones), FEMA compliance and elevation certificates add $1,000–$2,500 and 2-4 weeks to permitting. The city's online permit portal (https://eservices.co.franklin.wa.us or check current city portal) allows you to e-file applications, but plan review is not truly over-the-counter; expect two to three rounds of markups for structural, mechanical, and planning sign-offs.

Next steps: Gather your site survey, preliminary floor plan, and lot dimensions. Call the City of Pasco Building Department (Franklin County Building Services manages Pasco permits; verify the phone number locally — (509) 545-3522 or similar) to confirm whether your lot is in-city or unincorporated (which changes jurisdictional rules and septic vs. sewer). Request the ADU-specific checklist (Pasco published one in 2024 to comply with HB 1337). If detached or a garage conversion, hire a civil engineer to produce a site plan with setbacks, drainage, and utility routing (cost $800–$1,500). Submit the permit application with all plans and impact-fee calculations. Expect plan review to take 8-10 weeks minimum; most projects are deemed complete within 2-3 weeks but require one or two revision cycles. Once approved, construction can begin, and inspections happen in sequence: foundation (72 hours after concrete pour), framing (before sheathing/insulation), rough trades (electrical/mechanical/plumbing before drywall), insulation, drywall, final (includes utility sign-off and occupancy sign-off). Total permitting and construction timeline is typically 16-20 weeks from application to certificate of occupancy.

Three Pasco accessory dwelling unit (adu) scenarios

Washington State ADU Law and How It Overrides Pasco Zoning (RCW 36.70A.697 and HB 1337)

Washington state law (RCW 36.70A.697, updated in 2023 and again in 2024 via HB 1337) requires all cities to allow at least one ADU per lot zoned for residential use — regardless of local zoning code restrictions. This means Pasco cannot deny an ADU permit based on lot size, zoning district, setback variance, or owner-occupancy requirements. HB 1337 (effective January 1, 2024) expanded the mandate: cities must allow both detached ADUs and ADUs within existing structures (garage conversions, upper-floor conversions, basement ADUs) without imposing parking requirements, lot-size minimums, or affordability restrictions. The city cannot charge impact fees that are disproportionate to single-family development. This is a seismic shift from historical Pasco zoning, which previously treated ADUs as conditional uses or prohibited them in many zones.

What Pasco still requires (even under state mandate): a full building permit and plan review. The state law does not create a permit exemption or a fast-track administrative process. Pasco applies IBC 2021 (with amendments) and does not waive IRC R310 (egress), IRC R401-R408 (foundation), electrical, mechanical, or plumbing codes. In practice, this means the Pasco Building Department's review timeline is 8-10 weeks for a standard ADU, because staff must review structural stamped plans, egress windows, electrical sub-panel, plumbing separation, and site drainage. Some Washington cities (Spokane, Olympia, Seattle) have published pre-approved ADU plans that can be filed with minimal review; Pasco has not adopted this yet, so applicants should expect full plan-check timelines. One exception: if the property is within the City of Pasco limits AND within the Pasco School District (not all properties are — some follow Keller or East Pasco districts), certain ADU-friendly provisions in the comprehensive plan may apply, though these do not reduce permit requirements.

The owner-occupancy myth: You do NOT have to live in either the primary dwelling or the ADU under Washington state law. HB 1337 removed owner-occupancy mandates entirely. However, some lenders (if you refinance) and some HOAs (if your neighborhood has deed restrictions) may still impose occupancy limits. Confirm with your lender upfront. Also note: if you rent the ADU and the primary home, both are now treated as rental properties for tax purposes, so your property-tax classification may change from residential to investor-class, affecting your assessed value and tax basis.

Cost and timeline context: The state law does NOT reduce permitting costs or timelines in Pasco. In fact, because Pasco did not have an ADU-friendly culture historically, staff are still adapting to the new mandate. You may encounter resistance or requests for over-engineering (e.g., sprinkler systems flagged when not strictly required). Budget 8-14 weeks for plan review and $4,000–$6,000 in permits and impact fees. This is not a fast or cheap process in Pasco, even though state law requires the city to issue the permit.

Utility Separation, Sub-Metering, and Pasco's Sewer Impact Fees: The Real Cost Driver

Pasco requires separate utility metering for ADUs in most cases. For water, you need either a second meter (hard-piped from the city main) or a sub-meter on the primary line. A second meter typically costs $1,200–$2,000 (meter installation, pit, disconnect valve, Pasco utility tap). A sub-meter (usually installed in an outdoor panel or house panel) costs $1,500–$2,500 (meter, enclosure, contactor, inspection). Sewer is trickier: the city may require a separate service line from the city sewer main to the ADU if the distance is reasonable (under 100 feet). If the distance is excessive, a sub-meter arrangement for billing purposes may be allowed, but the physical drain lines must be independently identified and sized. In unincorporated areas (Finley, Waitsburg, areas outside city limits), on-lot septic systems are the norm. A septic system originally sized for one dwelling (typically 1,500 GPD) must be evaluated for two dwellings (3,000 GPD). Most existing septic tanks are 1,500-2,000 gallons and will fail under dual occupancy; a new 2,500-3,500-gallon system must be installed ($5,000–$8,000, including drainfield expansion).

Sewer impact fees are the biggest surprise cost. Pasco's current impact fees (subject to change annually) are approximately $1,200–$1,800 per equivalent dwelling unit (EDU) for sewer and $500–$1,000 for water. An ADU is assessed as 0.5-0.75 EDU, depending on unit size and fixture count. So expect $600–$1,350 for sewer impact and $250–$750 for water impact, totaling $850–$2,100 in impact fees alone. Schools impact fees are typically an additional $500–$1,500 if the city assesses school capacity. These fees are nonnegotiable and do not vary based on design; they are tied to the number of additional dwelling units (one ADU = one additional unit).

Sub-metering and utility billing: If you choose sub-metering to avoid running a second service line, Pasco's utility billing system (managed via their online portal) will recognize the sub-metered unit as a separate account for billing purposes. However, the city still requires the physical water and sewer lines to be designed and inspected as separate systems, even if they share the primary main initially. This is a code-compliance issue: each dwelling must have independently operable and isolatable utilities, so a valve malfunction or main-line break in one unit does not affect the other. The cost is in engineering and trenching, not meter purchase. Budget $3,000–$5,000 for utility routing and separate service identification.

Stormwater is often overlooked. Because each new ADU adds impervious surface (roofing, patios, walkways), stormwater runoff increases. Pasco's stormwater manual (based on the Puget Sound Technical Manual) requires flow control (limiting discharge to pre-development rates) for most additions. A 400-sq-ft detached ADU on a 12,000-sq-ft lot may not trigger engineered stormwater design if the downslope is permeable (native soil, grass, trees). However, on a tight lot (0.25 acres with existing structures), a rain garden or permeable pavers ($1,500–$3,000) is often required. This cost is borne by the applicant and must be shown on the site plan before permit approval. Drainage violations result in permits not being deemed complete, so factor this into your timeline.

Common questions