Washington is the state where the code finds everybody. No county here quietly skipped adoption, and no unincorporated corner goes uninspected — the legislature adopted one building code for the whole state, then told any county without a building department to contract with somebody who has one. The county-shopping that works in Alabama or Colorado has nowhere to go. What Washington gives back is dry-side land that's still cheap, water rules that work once you know your watershed, and some of the cheapest electricity in the country to compete against. December sun is the real design constraint on both sides of the Cascades.
This page covers what decides a Washington build: what the sun does across the state's four regions, the rule that gates every residential building permit, what a stand-alone system costs installed in 2026, and the storm record behind all those batteries. At the bottom, tell us your county and rough load and Size it with the free calculators, then take the numbers to the gear lists — makers linked direct.
Why Washington Rewards Homework
Three things stack here, the same three that decide every state in this series. Land: USDA NASS put Washington farm real estate at $3,800 an acre in its Land Values 2026 Summary, up 2.4% on the year, and that average hides the spread that matters off-grid. Pasture runs $980 an acre and non-irrigated cropland $1,750, against $10,000 for irrigated cropland. The ten-to-one gap between watered and dry ground has little to do with soil — it's the price of a water right, and it summarizes the state neatly. Law: RCW 19.27.031 adopts the international building, residential, mechanical and fire codes plus the Uniform Plumbing Code as the state building code, and RCW 19.27.050 makes every county and city enforce it. Water: a household well is normally permit-exempt at up to 5,000 gallons a day under RCW 90.44.050, fifteen watersheds have had that cut to 3,000 or 950 gallons since 2018, and every building permit needs evidence of an adequate supply.
Land figures: USDA NASS, Land Values 2026 Summary (July 2026). Code adoption and enforcement: RCW 19.27.031, RCW 19.27.050 and RCW 19.27.060(4). Current editions: the 2021 codes, effective March 15, 2024, Washington State Building Code Council. Groundwater: RCW 90.44.050, with caps at RCW 90.94.020 and RCW 90.94.030.
The Sun You Actually Get — and Why December Runs the Design
“Peak sun hours” (PSH) is the number that sizes an array — the equivalent hours per day of full 1,000 W/m² sun once cloud, season and tilt are baked in. Washington's annual figures look respectable, from about 3.5 on the outer coast to roughly 5.0 in the Columbia Basin. December is another matter. A steeply tilted array in the Puget Sound lowlands sees around 1.5 PSH a day then, against about 6.0 in July — a four-to-one swing that extra panels never close, and the reason serious builds here pair a summer-rational array with generator hours through the darkest weeks. Rule of thumb: array kW ≈ daily load (kWh) ÷ (peak sun hours × 0.78) — the 0.78 covers inverter loss, wiring, temperature derate and soiling.
| Region | Example | Annual PSH/day | December PSH/day | Winter-sized array, 30 kWh/day* |
|---|---|---|---|---|
| Puget Sound lowlands | Seattle, Olympia | 3.8–4.0 | ≈1.5–1.6 | ≈24–26 kW |
| Pacific coast / Olympic Peninsula | Forks, Willapa Hills | ≈3.5 | ≈1.6 | ≈24 kW |
| Inland Northeast | Spokane, Colville | 4.2–4.4 | ≈1.4–1.5 | ≈26–28 kW |
| Columbia Basin | Yakima, Wenatchee, Tri-Cities | 4.7–5.0 | ≈1.9–2.1 | ≈18–20 kW |
*In-plane irradiation for a fixed south-facing array at 45°, from PVGIS v5.2 running the NREL NSRDB dataset (European Commission Joint Research Center). The winter-sized column is arithmetic on the December row, shown to make the scale visible; almost nobody builds it, and the practical design runs three to five battery days plus a generator. Note the swap — Spokane beats Seattle across the year and loses to it in December.
What's Actually Regulated Here (A Longer List Than Most States)
Washington runs a real statewide floor, so county-shopping doesn't work here:
- The building code applies everywhere. RCW 19.27.031 adopts the model codes as the state building code, currently the 2021 editions, and RCW 19.27.050 requires counties and cities to enforce it; a county without a building department has to contract the job out. Amendments under RCW 19.27.060 can't fall below the state minimum performance standards, and residences can't be exempted.
- Building it yourself. RCW 18.27.090 exempts an owner working on their own property from contractor registration, and the exemption is lost if you improve property you've owned under twelve months in order to sell or lease it. It waives registration and nothing else.
- Electrical gets inspected. Labor & Industries issues electrical permits and inspects the work under RCW 19.28.101; an owner wiring their own residence or farm is exempt from licensing under RCW 19.28.261 and still needs that permit and inspection.
- Water is what to research before you buy. Domestic withdrawals up to 5,000 gallons a day are exempt from a water right permit under RCW 90.44.050, along with stock watering and up to half an acre of lawn or garden. Since 2018 the ceiling is 3,000 gallons a day per connection in WRIAs 1, 11, 22, 23, 49, 55 and 59 (RCW 90.94.020) and 950 gallons in WRIAs 7, 8, 9, 10, 12, 13, 14 and 15 (RCW 90.94.030), where drought can pull it to 350 gallons indoors. Both carry a $500 fee at building permit time.
- Wells and septic. A well has to be built by a licensed operator under chapter 18.104 RCW, with notice to Ecology at least 72 hours beforehand (RCW 18.104.048). Septic is permitted by the local health officer under WAC 246-272A-0200, and WAC 246-272A-0320 puts a lot on its own well and septic at 1 to 2 acres by soil type.
For the national county-by-county walkthrough, see our US off-grid permitting guide.
Revised Code of Washington § 19.27.097
This is the real off-grid gate in Washington. Section 19.27.097 sits inside the State Building Code chapter and opens with the line that decides more remote projects than any other in state law: “Each applicant for a building permit of a building necessitating potable water shall provide evidence of an adequate water supply for the intended use of the building.” Acceptable evidence is a water right permit from Ecology, a letter from an approved purveyor, or “another form sufficient to verify the existence of an adequate water supply” — and the loophole closes in the same breath: “An application for a water right shall not be sufficient proof of an adequate water supply.” Outside the regulated watersheds, subsection (1)(g) accepts a well report under chapter 18.104 RCW, so on raw land the well gets drilled before the house gets permitted.
What turns this from paperwork into a project killer is subsections (1)(b) through (1)(f), rewritten when the legislature passed the Streamflow Restoration Act in 2018. In twenty-nine watersheds the evidence has to match a specific rule: ten instream-flow areas regulating permit-exempt wells directly, among them Elwha-Dungeness, Wenatchee and Methow; seven routed through the 3,000-gallon cap; eight through the 950-gallon cap; three Yakima basin areas where Ecology can impose conditions for adjudicated rights; and the Skagit basin, under the state Supreme Court's 2013 decision in Swinomish Indian Tribal Community v. Department of Ecology. Two parcels either side of a watershed line can get different answers.
Two provisions cut the other way. Subsection (4) exempts buildings that don't need potable water facilities, which is how an unplumbed shop or dry cabin sidesteps the question. Subsection (5) quietly makes older land more valuable: a permit-exempt withdrawal tied to a well built under chapter 18.104 RCW before January 19, 2018 counts as evidence of adequate supply, so a parcel with a legal pre-2018 well has cleared a gate the bare parcel down the road may never clear. The sequence is what people get wrong. Buyers assume it runs land, house, water. Here water comes first.
Source: RCW 19.27.097 (Building permit application—Evidence of adequate water supply), via the Washington State Legislature's own RCW portal; amended by 2018 c 1 s 101 (Streamflow Restoration Act), enacted at 1990 1st ex.s. c 17 s 63 (Growth Management Act); caps at RCW 90.94.020 and RCW 90.94.030. Verified August 2026.
What an Off-Grid Build Costs in Washington in 2026
Hardware is priced nationally — panels and batteries cost the same delivered to Colville as to Sacramento — so Washington's pricing sits inside the national band below. What moves here is the winter design. December sun in the 1.4 to 1.6 range on the west side and in the northeast means a larger array, a larger bank, or generator hours and fuel storage, and all three cost money. Add snow-load and seismic engineering, a permitted electrical install, and the $500 watershed fee. Full national numbers, size by size, in our cost of off-grid solar report, and the battery-bank math in battery bank sizing.
| System class | Array / battery | 2026 installed price |
|---|---|---|
| Seasonal cabin / small homestead | 3–5 kW / 10–15 kWh | $28,000–$48,000 |
| Full-time off-grid home | 7–10 kW / 20–30 kWh | $54,000–$80,000 |
| Larger home + shop, well pump, winter autonomy | 12–15 kW / 35–45 kWh | $86,000–$130,000 |
Same national hardware and labor blend as our full cost report. Expect a west-side or inland-northeast build in the upper half of every band, because December forces oversized generation, extra battery days or a generator into the quote; the Columbia Basin is the one region that prices in the lower half. One offset is real money: RCW 82.08.962 allows a remittance of state and local sales tax on solar generating equipment, up to 100% for systems no larger than 100 kW AC where the seller meets its conditions, running to January 1, 2030. The federal 30% residential credit (Section 25D) ended for systems completed after December 31, 2025; what's still live (USDA REAP, business-owned 48E) is in the tax credit guide.
The Real Washington Angle: Cheap Hydro Power, on Lines Under Trees
Washington's grid is neither expensive nor weak. Hydro held the average retail price at 10.13 cents per kilowatt-hour across all sectors in 2024, 39th in the country and among the cheapest power any off-grid system competes with. The pressure here is mechanical: big conifers over long rural distribution lines through a wet winter, and the National Weather Service outage record reads like a maintenance schedule. The Hanukkah Eve windstorm of December 14–15, 2006 cut power to close to 1.5 million customers in western Washington, the strongest since the Inauguration Day windstorm of January 1993; four people died in the storm and eleven more afterward from carbon monoxide poisoning, downed lines and a candle fire. The January 19, 2012 ice storm produced roughly half a million outages, and on November 17, 2015 one system took out roughly 370,000 customers in the west plus about 180,000 Avista customers in the east — the worst natural-disaster damage in that utility's 126-year history.
It hasn't slowed. The June 2021 heat dome pushed Avista into two deliberate blackouts affecting about 8,300 customers, in a month with eight known heat-related deaths in Spokane County. A dry cold front on September 25, 2024 cut power to 12,394 customers in Spokane County and 7,471 in Stevens, and on November 19, 2024 a deepening low drove unusual easterly winds across Puget Sound, causing extensive tree damage, prolonged outages and two deaths from falling trees. That's the real case for going off-grid here: cheap power while it flows, parcels at the end of the longest and most tree-shaded spans, and restoration that reaches them last. The eleven indirect deaths after 2006 are worth reading twice, because most came from how people generate and heat once the power stops.
Do I need a building permit for an off-grid home in Washington?
Yes, everywhere in the state. RCW 19.27.031 adopts the model building, residential, mechanical, fire and plumbing codes as the state building code, and RCW 19.27.050 requires every county and city to enforce it — a jurisdiction without a building department has to contract the job out. Local governments can amend the code under RCW 19.27.060 and exclude some classes of building by use, though never single-family or multifamily residences. The 2021 editions took effect March 15, 2024, and the 2024 editions arrive May 3, 2027.
Can I drill a well on my own land without a permit in Washington?
A domestic withdrawal of up to 5,000 gallons a day is exempt from the water right permit process under RCW 90.44.050, which covers any normal household. The well still has to be built by a licensed operator under chapter 18.104 RCW, with notice to Ecology at least 72 hours before work starts. The catch is location: in fifteen watersheds the Streamflow Restoration Act cut the ceiling for new domestic connections to 3,000 or 950 gallons a day, with a $500 fee at building permit time.
Is there enough sun in Washington for off-grid solar?
Across the year, yes — annual peak sun hours run about 3.5 on the outer coast, 3.8 to 4.0 around Puget Sound, 4.2 to 4.4 in the inland northeast and 4.7 to 5.0 in the Columbia Basin. December is what people underestimate: a steep south-facing array sees roughly 1.5 PSH a day around Puget Sound and about 1.4 near Spokane, against 6 to 7 in July. That gap is why systems here get designed around battery days and a generator, and why the Columbia Basin, at 1.9 to 2.1 December PSH, is the cheapest place in the state to run off-grid year-round.
Can I collect rainwater in Washington?
Yes. Ecology's interpretive policy POL-1017, effective October 9, 2009, puts on-site storage and beneficial use of rooftop or guzzler collected rainwater outside the water right permit process, provided the roof belongs to a fixed structure whose primary purpose is something other than collecting rainwater. Ecology reserves the right to set local restrictions if cumulative harvesting affects instream values or existing rights. That answers the water-right question only: your building permit still has to satisfy RCW 19.27.097, whose named evidence is a water right permit, a purveyor letter or a well report.
None of this changes whether off-grid makes sense on your parcel — it changes the order you check things in. Find the water resource inventory area before you make an offer, ask whether a well report was filed before January 19, 2018, budget a generator instead of buying past December with panels, and get a 2026 quote from someone who has filed a water-availability form in that county.