Spokane ADU Cost 2026: Two ADUs Per Lot Under HB 1337
Washington's HB 1337 is the strongest ADU law in the country — Spokane must now allow up to two ADUs per lot, cannot require owner-occupancy, and caps ADU impact fees at half the single-family rate. Pair that with Spokane's alley-lined pre-1950 housing stock and it is one of the most builder-friendly ADU markets in the state.
Last updated: •Reviewed by the aduglossary editorial team
By the BVLLC Editorial Team · Cost data verified against City of Spokane ADU allowances, Washington HB 1337, and local Spokane County builder quotes. Last updated July 2026.
Quick Answer
Spokane ADUs: $55K–$350K
Permits: $2K–$3.5K · Updated April 2026
Garage conversion
$55K–$120K
Most affordable path
Daylight-basement conversion
$60K–$140K
Common in pre-1950 homes
Detached ADU (up to 975 sq ft)
$150K–$280K
New backyard build
Permit + plan review
$2K–$3.5K
+$600–$1,200 impact fees
Second ADU on same lot
Allowed
HB 1337 — up to 2 per lot
Rental income potential
$1,000–$1,600/mo
1-bed ADU, long-term
Finance Your Spokane ADU
Spokane home values climbed sharply from 2020 through 2023, and many South Hill, Perry District, and North Spokane owners who bought before the run-up now hold well more equity than a conversion or modest detached ADU costs. Local credit unions — STCU, Numerica, Canopy, and Global — write home-equity and HELOC products against that equity without disturbing a low-rate first mortgage.
Why Spokane First Quotes Don't Match Final Bills
Spokane is genuinely one of Washington's cheaper places to build an ADU, but the first number a homeowner hears is still usually short of the real all-in cost. The gap is not bad estimating — it is a predictable set of Inland Northwest items that thin quotes leave out.
The first missing line is cold-climate construction. Spokane winters demand a roof and framing engineered for the local ground snow load, footings carried below frost depth, and Washington State Energy Code insulation and air-sealing. A slab or crawlspace that would satisfy a mild-climate reviewer will not pass here, and freeze-protected supply lines add plumbing cost. These are not luxury upgrades — they are code.
The second missing line shows up on conversions. Spokane's appeal is its deep stock of pre-1950 homes with full basements and alley-loaded detached garages, but those structures often need new egress windows, foundation repair, updated wiring (knob-and-tube is common), and moisture control before they can become legal living space. A garage that looks conversion-ready frequently needs a new slab, insulation, and a heating system.
The third missing line is the second-ADU opportunity itself. Because HB 1337 lets you place two ADUs on one lot, many owners upsize their plans mid-project — and a second unit means a second kitchen, a second bath, and decisions about separate utility metering and service upgrades that a single-ADU quote never priced. Historic districts such as Browne's Addition add design review on top.
Spokane-specific budget items to price up front:
- Snow-load-rated roof/framing + frost-depth footings: $6,000–$16,000
- Older-home conversion fixes (egress, wiring, foundation, moisture): $8,000–$25,000
- Separate utility metering / electrical service upgrade: $4,000–$12,000
- Long utility runs to a rear-yard detached unit: $4,000–$11,000
- Historic district design review (e.g. Browne's Addition): $1,500–$6,000
Spokane ADU Cost Breakdown by Type and Size
Figures below reflect mid-finish construction on a typical Spokane lot with a snow-load-rated shell, frost-depth footings, and a permitted City of Spokane pathway. Historic district design review and major service upgrades are billed separately. Verify with a local builder.
| ADU Type | 400 sq ft | 600 sq ft | 800 sq ft | Notes |
|---|---|---|---|---|
| Detached ADU (new build) | $150K–$190K | $185K–$245K | $230K–$310K | Capped at 975 sq ft detached |
| Garage conversion | $55K–$90K | $80K–$125K | $110K–$160K | Existing slab + alley access saves cost |
| Daylight-basement conversion | $60K–$100K | $90K–$140K | $120K–$175K | Common in pre-1950 South Hill / Perry homes |
| Above-garage ADU | $150K–$200K | $190K–$250K | $230K–$300K | Strong long-term rental premium |
| Attached (internal) ADU | $110K–$150K | $150K–$200K | $185K–$250K | 800 sq ft cap; shared utilities lower cost |
Includes design, permits, snow-load-rated foundation and shell, and standard mid-grade finishes. Building two ADUs on one lot lowers per-unit soft costs but adds a second kitchen, bath, and utility set. Last updated July 2026.
The HB 1337 Two-ADU Advantage — and Spokane's Cold-Climate Reality
Two forces shape almost every Spokane ADU budget, and they pull in opposite directions. The first is Washington's HB 1337, passed in 2023 and the most aggressive statewide ADU reform in the country. It requires cities including Spokane to allow up to two ADUs per lot in most residential zones, forbids owner-occupancy conditions, caps ADU impact fees at 50% of the single-family rate, and bars off-street parking requirements within a half-mile of major transit. For a Spokane homeowner, that unlocks build strategies that are impossible in most other states: a basement conversion for aging parents plus a detached backyard cottage rented long-term, or two detached rentals on a single deep lot — with no requirement that you live there.
Spokane's own code has leaned into this. The City permits detached ADUs up to 975 sq ft (or 75% of the primary building, whichever is greater) and internal ADUs up to 800 sq ft, allows a 0 ft rear setback when a lot has an alley, and runs a free Middle Housing Support Team to walk residents through feasibility before they hire anyone. Few cities in the country make it this easy on paper — which is exactly why competing builder pages already rank for “Spokane ADU.” The homeowners who win are the ones who plan for the second force.
That second force is the climate. Spokane sits in a cold, snowy zone, and the building code responds accordingly. Roofs and framing must be engineered for the local design ground snow load (commonly cited around 39 psf — confirm your site value with the City); footings must sit below frost depth (roughly 24 inches, verify); and the Washington State Energy Code drives up insulation, window, and air-sealing spend. Plumbing has to be routed and protected against freezing. None of this is optional, and all of it is why a Spokane detached ADU still lands in the $150,000–$280,000 range despite the region's low labor costs.
The practical takeaway: Spokane rewards owners who convert existing basements and alley garages (reusing a foundation is the single biggest cost avoidance) and owners who take advantage of the two-ADU rule to spread fixed soft costs across two rentable units. Price the cold-climate shell honestly, verify every fee with the City of Spokane, and the math in Spokane is among the friendliest in Washington.
City of Spokane ADU Permit Process
The City of Spokane issues ADU building permits, with Planning handling zoning verification under SMC Title 17C and the Middle Housing Support Team offering free pre-application guidance. Note that Spokane Valley is a separate city with its own ADU code — confirm which jurisdiction your address falls in before applying.
50–75 days
City permit review (complete submittal)
$2K–$3.5K
Permit + plan review fees
975 sq ft
Detached ADU max (or 75% of home)
- Up to two ADUs per lot allowed under HB 1337 — any combination of attached and detached
- No owner-occupancy required (unless the property is a short-term rental) — verify with the City
- Impact fees capped at 50% of the single-family rate; no parking required within 1/2 mile of transit
- 0 ft rear setback with an alley, 5 ft without — ideal for alley garage-to-DADU conversions
- Free Middle Housing Support Team (MiddleHousing@spokanecity.org) helps confirm feasibility before you design
Financing a Spokane ADU
Spokane's run-up in home values from 2020 to 2023 left many longtime owners with substantial tappable equity, and Washington has no state income tax on wages — which improves the after-tax return on an ADU rental compared with high-tax states at similar build cost. Because HB 1337 removes owner-occupancy limits, lenders can underwrite ADU rental income more readily than in states that still require the owner to live on-site.
Most Spokane ADU projects use one of four financing structures. Rates below are illustrative as of mid-2026; verify current rates with your lender before budgeting.
| Loan Type | Typical Rate | Best For | Notes |
|---|---|---|---|
| HELOC (STCU / Numerica) | From ~6.7% APR | Conversions under $130K | Local credit union; fixed-advance option |
| Cash-out refinance | ~7% | Owners with a higher legacy first-mortgage rate | Resets first mortgage; closing costs apply |
| Construction-to-perm (Washington Trust) | ~7.5% | New detached ADUs > $180K | Single close, converts to mortgage at C/O |
| RenoFi (after-renovation value) | ~7.5%+ | Borrowing beyond current equity | Underwrites against post-ADU appraisal |
Typical lender requirements for Spokane ADU financing: CLTV at or below 85%, DTI under 43%, and a FICO of roughly 660–680 minimum for a HELOC. Construction-to-perm loans require a Washington-licensed general contractor and a detailed budget that includes the snow-load shell and any service upgrade. Because HB 1337 allows two non-owner-occupied ADUs, rental-income underwriting on DSCR and after-renovation-value products is often stronger in Spokane than in owner-occupancy states.
10 Questions to Ask a Spokane ADU Builder
Hiring a builder fluent in City of Spokane permitting, HB 1337, cold-climate construction, and older-home conversions prevents the most common project disasters. Ask these before signing:
- How many Spokane ADUs have you completed in the last 24 months? Can I see two finished addresses?
- Have you built two ADUs on one lot under HB 1337? How did you handle separate metering and utility service?
- How do you engineer footings for frost depth and the roof for Spokane's ground snow load?
- Have you done alley-loaded detached DADUs at the 0 ft rear setback? Show me one.
- Are you registered, bonded, and insured with the Washington Department of Labor & Industries (L&I)? What is your registration number?
- Do you submit and manage the City of Spokane permit, and do you coordinate with the Middle Housing Support Team?
- What is your experience converting pre-1950 basements or garages (egress, knob-and-tube, foundation, moisture)?
- What is your typical timeline from permit submittal to certificate of occupancy on a 600 sq ft detached ADU?
- If my lot is in a historic district like Browne's Addition, have you cleared design review before?
- Will you carry a fixed-price contract with a 5–7% contingency, or only a cost-plus structure?
Spokane Neighborhood ADU Snapshot
Spokane's ADU demand concentrates in established neighborhoods with alley grids and rental demand near hospitals, downtown, and the universities. Older housing stock rewards conversions; newer infill areas favor detached builds.
| Neighborhood / Zip | 600 sq ft ADU | Key local factor |
|---|---|---|
| South Hill (99203 / 99204) | $185K–$260K | Larger lots, higher home values, demand near hospitals & universities |
| South Perry District (99202) | $170K–$235K | Walkable bungalows, alley lots ideal for detached DADUs |
| Logan / Gonzaga (99202) | $165K–$230K | Steady student-adjacent rental demand near Gonzaga |
| Kendall Yards / West Central (99201) | $180K–$250K | Newer infill plus a walkable riverfront premium |
| Browne's Addition (99201 / 99204) | $190K–$265K | Historic district — design review adds time and cost |
| North Spokane / Nevada-Lidgerwood (99207 / 99208) | $155K–$215K | Affordable entry, flatter lots, lower lot competition |
| Spokane Valley (99206 / 99216) | $160K–$225K | Separate city with its own ADU code — verify local rules |
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Spokane ADU Frequently Asked Questions
How much does an ADU cost in Spokane in 2026?
In Spokane, a detached ADU typically runs $150,000–$280,000 all-in for a 500–975 sq ft unit at standard finishes — roughly $260–$400 per square foot once design, permits, and Spokane's cold-climate foundation work are priced in. Garage conversions start near $55,000–$120,000 and daylight-basement conversions around $60,000–$140,000. High-end custom detached builds can reach $450,000+. Spokane is one of Washington's more affordable ADU markets because labor and lot-prep costs sit below Seattle and the Puget Sound region — verify current pricing with a licensed Spokane builder.
Can I build two ADUs on one lot in Spokane?
Yes. Under Washington's HB 1337 (2023) — the strongest ADU law in the state — the City of Spokane must allow up to two ADUs per lot in most residential zones, in any combination of attached and detached. That means a basement conversion plus a detached backyard cottage, or two detached units. It is the single biggest change for Spokane homeowners: you can house family in one unit and rent the second long-term, or build two rentals. Lot size, lot-coverage, and setback limits still apply, so confirm your specific parcel with the City of Spokane before designing.
Does Spokane require me to live on the property (owner-occupancy)?
No. HB 1337 prohibits Washington cities, including Spokane, from imposing an owner-occupancy requirement on ADUs — you can rent out both the primary home and the ADU(s). The one exception in the City of Spokane's own guidance is that owner-occupancy may apply when the property is operated as a short-term rental. Always verify the current rule with the City of Spokane before you count on non-owner-occupied rental income.
What is the maximum ADU size in Spokane?
Per the City of Spokane's ADU allowances, an attached (internal) ADU is capped at 800 sq ft, and a detached ADU at 975 sq ft or 75% of the primary building, whichever is greater. Note that HB 1337 requires Washington cities to allow ADUs of at least 1,000 sq ft, so Spokane's exact detached cap may be updated as the City finalizes its HB 1337 code — verify the current figure with Spokane Planning (SMC Title 17C) before drawing plans.
How do snow loads and cold climate affect Spokane ADU cost?
Spokane sits in a cold, snowy climate zone, and that drives three cost items a mild-climate quote often omits: (1) a roof and framing engineered for Spokane's design ground snow load (commonly cited around 39 psf — confirm the value for your site with the City); (2) frost-protected footings set below Spokane's frost depth (roughly 24 inches, verify); and (3) higher insulation and freeze-protected plumbing to satisfy the Washington State Energy Code. Together these can add several thousand dollars versus an identical unit in a warm climate.
Do I need to add parking for a Spokane ADU?
Often not. The City of Spokane requires no off-street parking for studio and one-bedroom ADUs, and HB 1337 bars any parking requirement for units within a half-mile of a transit stop. ADUs with two or more bedrooms generally need one off-street space plus one per additional bedroom unless the transit exemption applies. Because much of Spokane's older grid — Logan, Perry, West Central, Browne's Addition — sits on or near transit, many lots owe no ADU parking at all. Verify with the City.
How long does a Spokane ADU permit take, and what does it cost?
Plan on roughly 50–75 days for City of Spokane permit review on a complete submittal, with plan check itself often 2–3 weeks. Building-permit and plan-review fees typically total $2,000–$3,500, plus impact fees of about $600–$1,200 — and under HB 1337 ADU impact fees are capped at 50% of the single-family rate. Add 4–8 months of construction for a detached unit. Verify all figures with the City of Spokane, since fee schedules change.
Are basement or garage conversions cheaper in Spokane than a new detached ADU?
Usually yes. Spokane has a large stock of pre-1950 homes with full basements, detached garages, and alley access — ideal conversion candidates. Reusing an existing foundation or slab avoids the most expensive part of a cold-climate build. Spokane also allows a 0 ft rear setback when the lot has an alley (5 ft without), which makes alley-loaded garage-to-DADU conversions especially practical. Budget for updated egress, insulation, wiring, and moisture control in older structures — verify scope with a licensed Spokane contractor.
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