Key takeaways
- The 2021 Washington State Energy Code, Residential took effect statewide on March 15, 2024. It is the code your new house will be permitted under unless your application lands after the next code takes over.
- A new house needs 5, 8 or 9 energy credits depending on its size. Many custom homes fall in the 8 credit band; above 5,000 square feet of conditioned floor area it is 9.
- Your heating system is the biggest single decision. An all heat pump system earns 3.0 credits before anything else is counted. A gas furnace earns none, so the house has to find all 8 or 9 credits elsewhere.
- Every new house gets a blower door test, capped at 4.0 air changes per hour at 50 pascals, and the credit options reward going much tighter with heat recovery ventilation.
- The credit options you choose have to be shown on the permit drawings, so they are design decisions, made early, not a builder’s afterthought.
If you are building a new custom home in Seattle or on the Eastside, the 2021 Washington State Energy Code decides more of your design than most people expect. It sets how thick your walls are, how good your windows have to be, how tight the house must test, how fresh air gets in, and, through a credit system, it pushes hard toward heat pumps. None of it is optional, and almost all of it has to be settled before you apply for a permit.
The code is written for builders and plan reviewers. This is my translation of it for the person paying for the house.
What the code is and when it took effect
The residential energy code in Washington is chapter 51-11R of the Washington Administrative Code, usually shortened to WSEC-R. It covers single family homes, duplexes and townhouses, along with some low-rise multifamily buildings. The current edition is the 2021 code, which became effective in all counties and cities of the state on March 15, 2024.
King County is in climate zone 4C, the marine zone, in the code’s county table. The state uses one set of prescriptive insulation and window values for its marine 4 and zone 5 counties, so the numbers below apply across Seattle, Bellevue, Kirkland, Medina and Mercer Island alike.
The code offers three ways to comply: the prescriptive path, where you meet the tables and then earn energy credits from a menu; a whole-house energy model that has to beat a reference design by a set margin; or certification to a recognized passive house standard. This page describes the prescriptive route.
Seattle and the Eastside: same residential requirements
Seattle does publish a Seattle Residential Energy Code, but the state code says it “shall be the maximum and minimum energy code for residential construction in each town, city and county.” Seattle’s own residential chapters repeat that sentence, adopt the 2021 state code with the same March 15, 2024 date, and carry the same credit targets by house size. The key changes summary the city publishes for its 2021 energy code lists changes to the commercial provisions.
A new house in Seattle is held to the same residential energy requirements as one in Kirkland or Bellevue. Process still varies: each city runs its own plan review and forms. As of September 2026, confirm the current forms with the Seattle Department of Construction and Inspections or your Eastside city before you submit. The wider permit sequence is in my Seattle building permit guide.
The credit system by house size
On the prescriptive path, meeting the insulation and window tables is only the floor. Every new house also has to earn a minimum number of energy credits from two tables in section R406. The first table, R406.2, gives credits for the type of heating system. The second, R406.3, is a menu of upgrades: better envelope, tighter air sealing with heat recovery ventilation, more efficient heating and hot water equipment, solar, and efficient appliances. You add them up until you reach the target for your size of house. Within the envelope, air leakage and heating equipment groups you can take only one option each (a connected thermostat is the one add-on allowed), so two envelope upgrades cannot be stacked.
| House or project | How the code defines it | Credits required |
|---|---|---|
| Small dwelling unit | Less than 1,500 sq ft of conditioned floor area and less than 300 sq ft of windows, glazed doors and skylights. Also additions over 500 sq ft but under 1,500 sq ft | 5.0 |
| Medium dwelling unit | Every house that is not small or large. Many custom homes land here | 8.0 |
| Large dwelling unit | More than 5,000 sq ft of conditioned floor area | 9.0 |
| Addition of 150 to 500 sq ft | New conditioned area added to an existing house | 2.0 |
Source: 2021 WSEC-R section R406.3, second edition. Additions of 150 sq ft or less are exempt from the credit requirement. Conditioned floor area means the heated or cooled floor area.
Two details catch owners out. The small band depends on glass as well as floor area, so a compact house with a lot of glazing is treated as medium. And the jump to 9 credits happens above 5,000 square feet of conditioned space, so whether a lower level or bonus room is heated can move the whole house into the next band.
How your heating choice scores
Table R406.2 gives credits for the primary heating source before you pick anything from the menu:
| Primary heating system | Credits from Table R406.2 | What it means for the rest of the house |
|---|---|---|
| Heat pump, including air to water heat pumps that heat and cool | 3.0 | A medium house needs 5.0 more from the menu |
| Electric resistance with a ductless heat pump in the largest zone, or 2 kW or less of total heat | 2.0 | 6.0 more for a medium house |
| Heat pump with electric resistance or gas furnace backup | 1.5 | 6.5 more for a medium house |
| Electric resistance only | 0.5 | 7.5 more for a medium house |
| Gas or propane furnace or boiler | 0 | All 8.0 from the menu, or 9.0 for a large house |
Gas heat is not banned under this code. It is simply made expensive in credits. To show you what that means on a real design, here are two ways to reach 8 credits on a medium house, both taken straight from the code’s tables.
Three units appear in these lists. R-value measures how well insulation resists heat flow, so higher is better. U-factor measures how fast heat passes through a window or wall, so lower is better. Air leakage is given in air changes per hour at 50 pascals: how many times the full volume of air in the house leaks out in an hour while a test fan holds it at 50 pascals of pressure. Lower is tighter.
Heat pump house: 8.0 credits
- Heat pump as the primary heating system: 3.0
- Centrally ducted heat pump at the higher efficiency level in option 3.6: 1.0
- Electric heat pump water heater meeting Tier III of the Northwest Energy Efficiency Alliance (NEEA) water heating specification, option 5.6: 2.0
- Tested air leakage of 2.0 air changes per hour or less, with a heat recovery ventilator rated at least 0.65 for sensible heat recovery, meaning it recovers roughly 65 percent of the heat in the outgoing air, option 2.1: 1.0
- Envelope option 1.2, which includes U-0.25 windows, R-38 floors and R-10 insulation under the entire slab: 1.0
That is 3.0 plus 1.0 plus 2.0 plus 1.0 plus 1.0, which makes 8.0.
Gas furnace house: 8.0 credits
- Gas furnace as the primary heating system: 0
- ENERGY STAR gas furnace rated at least 95% AFUE (annual fuel utilization efficiency), option 3.1: 1.0
- Connected thermostat, option 3.11, allowed alongside 3.1: 0.5
- The top envelope option, 1.4: U-0.18 windows, R-60 ceilings, R-21 walls plus R-16 continuous insulation, R-48 floors and R-20 under the entire slab: 2.5
- Tested air leakage of 0.6 air changes per hour or less, with a heat recovery ventilator of at least 0.80, option 2.3: 2.0
- Gas-fired heat pump water heater meeting NEEA Tier 2, option 5.5: 1.5
- ENERGY STAR appliance package, option 7.1: 0.5
That is 0 plus 1.0 plus 0.5 plus 2.5 plus 2.0 plus 1.5 plus 0.5, which also makes 8.0. The gas house needs the most demanding envelope and the tightest airtightness target in the menu. There are other combinations, including on-site solar, which earns 0.5 credit for every 600 kWh a year it generates, up to 4.5 credits. But the pattern holds: every credit the heating system does not earn has to be bought somewhere else in the building.
I will not put a price on a credit. The heating decision belongs at the start of design, because it decides how hard the walls, windows and air sealing have to work.
Walls, roof and windows
The prescriptive table in section R402.1.3 sets the minimum envelope for our climate before any credits are counted:
- Windows: a maximum U-factor of 0.30, and 0.50 for skylights.
- Walls: R-20 in the cavity plus R-5 continuous insulation, or R-13 plus R-10 continuous. Continuous insulation is a layer that runs unbroken over the framing, usually outside the sheathing.
- Ceilings: R-60, which can drop to R-38 in a single rafter vaulted ceiling where the full depth runs over the top of the wall.
- Floors: R-30. Slabs: R-10 to a depth of 4 feet, and R-10 under the whole slab where the slab is heated.
Continuous insulation has the most design consequences. It thickens the wall, which changes how windows are set and flashed and how the siding attaches, and on a tight lot a thicker wall is floor area you do not get back. Those details belong in the drawings, not with the framing crew.
Glass is the other one. Every square foot of window is a weaker part of the envelope than the wall it replaces. A waterfront house with a wall of glass facing the lake often ends up proving compliance with the total UA alternative, a calculation that trades a weaker area against stronger ones elsewhere, or with a full energy model. It simply has to be planned for.
Air leakage testing and ventilation
Every new house is tested with a blower door, a calibrated fan set into an exterior doorway that measures how much air the house leaks. For single family homes the test follows RESNET/ICC 380, and the maximum is 4.0 air changes per hour at 50 pascals under any compliance path. The code requires a signed written report of the result, provided to the owner and the code official. Keep it.
The credit menu rewards going much further. Option 2.1 is 2.0 air changes per hour, 2.2 is 1.5, and 2.3 is 0.6, the tightest level in the menu. Each one is paired with a heat recovery ventilator of rising efficiency, because a tight house needs a controlled supply of fresh air. The code requires mechanical ventilation in every new house regardless, and the system has to be tested to show it delivers the required airflow.
If you are choosing between a heat recovery ventilator and an energy recovery ventilator, I have set out the differences for our climate in HRV vs ERV for Pacific Northwest homes. If you are thinking of going to 0.6 or beyond, read the passive house architect page first, because at that point certification may be worth the extra step.
The EV circuit
The electric vehicle requirement is not in the energy code. It sits in Washington’s amendments to the residential building code, section R309.6, effective March 15, 2024. For a new house with an attached private garage or attached carport, at least one 40 amp, 208/240 volt dedicated branch circuit has to be installed in the electrical panel for each dwelling unit. It has to end at a junction box, a receptacle outlet or the charger itself.
One circuit is the minimum. If you expect two electric cars alongside heat pumps and an induction range, size the electrical service for all of it on the drawings rather than upgrading later. If your garage is detached, ask your city how it applies the rule.
How the code shapes design and budget
The code is less a separate budget line than the standard every line is priced against. Current construction for a new custom home runs $400 to $900 per square foot, and a 2,500 sq ft custom home lands between $1.2M and $2.5M all in, as planning ranges. Those ranges already describe houses built to this code. The breakdown is in what it costs to build a house in Seattle.
Where the code moves your number is in the choices it forces early:
- Heating system first. Heat pump or gas decides if you start with 3 credits or none, and therefore how much of the envelope and air sealing menu you need.
- Size band second. Landing under or over 5,000 square feet of conditioned space changes the target from 8 to 9.
- Glass third. Large window walls push you toward the UA calculation or a model, and toward better windows.
- Then the credit package. The code requires the permit drawings to identify which options were chosen and the points for each. Once the permit is issued, swapping an option means revising the drawings.
So I settle the energy strategy during schematic design, alongside the plan and the structure. Permitting typically takes 6 to 10 months for a new house in Seattle and about 2 to 3 months for a remodel, and changing the credit package mid-review wastes some of that. If you want to go further than code, the same decisions move up a level; my net zero home guide covers where that line sits.
Remodels and additions trigger it too
The energy code is not only for new houses. An addition has to meet the code as new construction, without requiring the unaltered part of the house to comply. Additions over 150 square feet also need energy credits: 2.0 for 150 to 500 square feet, and 5.0 above that up to 1,500. Where an addition over 150 square feet adjoins an existing attic, that attic has to be brought up to the code’s insulation requirements too.
Alterations work the same way. The work you touch is held to the code for new construction, and the parts you leave alone are not. Turning unconditioned space, such as a garage or an unheated basement, into living space brings that space into full compliance. On a deep remodel that can reach further than planned, which I cover in remodel or tear down.
What changes next
The State Building Code Council is working on the 2024 edition. Its adoption schedule, revised August 14, 2026, shows the proposed energy code rules filed on July 1, 2026, public comment closing August 7, 2026, a final adoption meeting scheduled for November 20, 2026, and an implementation date of Monday, May 3, 2027. Schedules can move, so treat those dates as current as of September 2026 and confirm them with the council.
If your permit application will land around that date, ask your city early which code will govern it, and design to that one.
Before design starts
Get the energy strategy settled first
Tell me roughly how big the house is, where it is, and how you would like to heat it. I will tell you which credit band you are in and what that means for the design.
Questions
When did the 2021 Washington State Energy Code take effect for houses?
The 2021 Washington State Energy Code, Residential (chapter 51-11R WAC) became effective in all counties and cities of Washington on March 15, 2024. It applies to new single family homes, duplexes, townhouses and some low-rise multifamily buildings, and to additions and alterations to existing houses.
How many energy credits does a new house need under the 2021 WSEC?
Section R406.3 of the 2021 Washington State Energy Code requires 5.0 credits for a small dwelling unit (under 1,500 sq ft of conditioned floor area with under 300 sq ft of windows, glazed doors and skylights), 8.0 credits for a medium dwelling unit, and 9.0 credits for a large dwelling unit over 5,000 sq ft. Additions of 150 to 500 sq ft need 2.0 credits.
Does Seattle have its own residential energy code?
Seattle publishes its residential provisions as the Seattle Residential Energy Code, but it adopts the 2021 Washington State Energy Code, which states that it is the maximum and minimum energy code for residential construction in every city and county. Seattle’s residential chapters carry the same credit targets as the state code, so a new house in Seattle meets the same requirements as one on the Eastside. Confirm current forms with Seattle’s Department of Construction and Inspections.
Can I still heat a new house with gas in Washington?
Yes. The 2021 residential energy code does not ban gas heat, but a gas furnace or boiler earns 0 credits under Table R406.2, while a heat pump earns 3.0. A medium house heated with gas has to find all 8.0 required credits from the upgrade menu, which in practice means a much better envelope, very tight air sealing with heat recovery ventilation, and efficient water heating.
What is the air leakage limit for a new house in Washington?
Under the 2021 Washington State Energy Code, a new single family home may not exceed 4.0 air changes per hour when tested with a blower door at 50 pascals. The test follows RESNET/ICC 380 and a signed report goes to the owner and the code official. Energy credits are available for testing at 2.0, 1.5 or 0.6 air changes per hour when paired with a heat recovery ventilator.
Does a new house in Washington need an EV charger?
It needs a circuit, not a charger. Washington’s residential code amendment R309.6, effective March 15, 2024, requires new dwelling units with an attached private garage or attached carport to have at least one dedicated 40 amp, 208/240 volt branch circuit per dwelling unit, ending at a junction box, receptacle or charging equipment.
What insulation does the 2021 Washington energy code require for walls?
The prescriptive table R402.1.3 requires wood frame walls of R-20 cavity insulation plus R-5 continuous insulation, or R-13 plus R-10 continuous. It also sets R-60 ceilings, R-30 floors, R-10 slab edge insulation to 4 feet and windows with a maximum U-factor of 0.30. Higher levels earn energy credits.
Does the energy code apply to my remodel or addition?
Yes, to the new work. Additions must meet the code as new construction, and additions over 150 sq ft also need energy credits: 2.0 for 150 to 500 sq ft and 5.0 above that up to 1,500 sq ft. Alterations must meet the code for the parts you change, and converting unconditioned space to living space brings that space into full compliance.
When will the 2024 Washington State Energy Code take effect?
The State Building Code Council’s schedule, revised August 14, 2026, shows final adoption of the 2024 energy codes scheduled for November 20, 2026 and an implementation date of May 3, 2027. The dates can change, so confirm them with the council and ask your city which code will govern your permit application.
Authoritative sources
Regulatory requirements referenced in this guide are set by the official agencies below. Rules change, always confirm current requirements with the source:
