Key takeaways
- The 2021 Washington State Energy Code, Residential has applied statewide since March 15, 2024. For a remodel, the work you alter meets new-construction rules and the unaltered rest of the house is left alone (Chapter 5).
- An addition meets new-construction rules and, above 150 square feet, needs energy credits: 2.0 up to 500 square feet, 5.0 up to 1,500. A blower door test starts at 500 square feet.
- Unheated space you begin to heat, such as a garage or basement, counts as an addition.
- Replacement windows must average U-0.30 or better. A glass-only swap in an existing sash and frame is a repair.
- Seattle publishes its own residential energy code, but the requirements I compared match the state’s. Your application date decides the edition; the 2024 code is scheduled for May 3, 2027.
Short answer: a remodel has to meet the energy code only for the parts you alter, and an addition has to meet it as new construction. Open a wall and you may fill the cavity; replace windows and they average U-0.30. The heavy requirements come with three things: new floor area, space that was not heated before, and new heating or duct systems.
Piper Cole designs and administers construction; I do not build. This page reads chapter 51-11R WAC, the State Building Code Council’s printed second edition (second printing, September 2024), and the Seattle Energy Code residential chapters, all in October 2026. Where the text leaves a question open, I say so. Confirm with your city before relying on any figure.
What triggers the code, and what “unaltered” means
Chapter 5 sorts existing-building work into additions (R502), alterations (R503), repairs (R504) and change of occupancy (R505). The definitions decide which you are in. An addition is “an extension or increase in the conditioned space floor area, number of stories or height of a building or structure.” An alteration is any construction, retrofit or renovation that is not a repair or an addition.
Height is in that definition, so a roof raised to vault a ceiling can count as an addition even with no new floor area. The credit thresholds are in square feet, and the code does not say how they apply to a height-only change. Ask your reviewer.
The “unaltered portions” sentence (R501.1.1 and R503.1) is the reason a remodel does not drag the whole house up to current code. It has two limits that owners miss:
- The altered house may be no less conforming than it was before.
- It may use no more energy than it did before (R503.1). Swap a solid wall for a wall of glass and the house can use more energy than before even if every window meets U-0.30. The Total UA path (R402.1.5) is the tool for that case: it trades weak areas against strong ones.
There is also a safety valve: the code official may approve an alteration that does not fully conform when full compliance is physically impossible or economically impractical and the work improves efficiency (R503.1). It is the reviewer’s call, so I do not design around it.
What happens to each part you touch
| Work in your project | What the code asks | Section |
|---|---|---|
| Existing wall, ceiling or floor cavity exposed | May be filled instead of meeting new-construction values: at least R-15 in a 2×4 wall and R-21 in a 2×6 wall | R503.1.1, exception 2 |
| Wall, roof or floor rebuilt as a new assembly | New-construction table: walls R-20 plus R-5 continuous or R-13 plus R-10 continuous, ceilings R-60, floors R-30 | R503.1.1, R402.1.3 |
| Reroofing | Roof recover is exempt. A roof with no cavity insulation, where sheathing or insulation is exposed during reroofing, is insulated above or below the sheathing | R503.1.1, exceptions 4 and 5 |
| Windows and doors replaced | Area-weighted U-0.30 (skylights 0.50) | R503.1.1.1, R402.1.3 |
| New heating, cooling or ducts | Meet the systems section, with duct leak testing | R503.1.2, R403 |
The line to watch is between exposing a cavity and rebuilding one. Stripping drywall off a 2×4 wall and filling it to R-15 satisfies the exception, and a cavity left closed needs no upgrade (exception 3). Reframing that wall is a new assembly, and my reading is that it takes the new-construction values. The reviewer confirms which side each wall falls on, so I mark it on the drawings wall by wall.
Replacing windows and doors
If you replace a window unit, including just the sash and glazing, the replacement meets the U-factor in Table R402.1.3: 0.30 for windows and 0.50 for skylights (R503.1.1.1). You can use an area-weighted average across all the units you replace, so a few units above 0.30 are fine if others sit below it. Seattle’s forms page says the same and supplies a glazing schedule. It also links a one-page remodel worksheet from the Washington State University (WSU) Energy Program, a useful checklist that carries a few older numbers (see duct testing below). Table R402.1.3 lists no solar heat gain limit for our climate zone, so U-factor is what binds.
What does not trigger it: storm windows over existing windows, surface-applied film on single-pane glass, and glass-only replacement in an existing sash and frame, which the code treats as a repair (R503.1.1, R504.2). Windows also carry an air leakage limit of 0.3 cfm per square foot, swinging doors 0.5 (R402.4.2). The WSU worksheet groups doors with windows at 0.30.
New openings follow the “no more energy” rule above, and a big new window wall is a candidate for the Total UA path. For credits, option 1.1 in Table R406.3 gives 0.5 for vertical windows at U-0.22.
Roof, attic and ceiling work
The table value for ceilings is R-60. Two reductions apply: R-49 over the whole ceiling counts as R-60 where the full height of uncompressed R-49 reaches over the wall top plate at the eaves (R402.2.1), and a single-rafter vaulted ceiling can drop to R-38 where the full insulation depth runs over the top plate (Table R402.1.3, footnote e).
Two rules reach beyond the roof you are working on:
- When an addition over 150 square feet adjoins an existing ceiling with an attic, the existing attic must meet the envelope section too (R502.3.1.1). The attic you meant to leave alone becomes part of the job.
- Access hatches and doors from conditioned space to attics and crawl spaces are insulated to the value of the wall or ceiling they sit in (R402.2.4), and air-permeable insulation in vented attics needs baffles at the eaves (R402.2.3).
A new vault is a new assembly, so in my reading it carries the R-38 or R-60 values; whether the rafters are deep enough to hold them is a framing question to settle first. Structure is covered in my rambler guide and split-level guide.
Additions: credits, blower door and duct tests
New envelope assemblies, ducts, hot water and lighting in an addition meet the code as new construction (R502.3). An addition also complies if it alone complies, if the existing house and addition comply as one building, or if the combined building uses no more energy than the existing one (R502.1). Above 150 square feet, credits apply:
| Addition size | Credits needed | Source |
|---|---|---|
| 150 square feet or less | None | R502.1.1 |
| Over 150 to 500 square feet | 2.0 | R406.3, item 5 |
| Over 500 and under 1,500 square feet of heated floor area | 5.0 | R406.3, item 1 |
| 1,500 square feet or more | The text does not name this case; ask the reviewer | R406.3 |
Credits come from two tables. The heating system itself earns credits first (R406.2): a heat pump 3.0, a heat pump with electric resistance or gas furnace backup 1.5, electric resistance alone 0.5, a gas furnace 0. Then you add options from Table R406.3. Three examples, with the arithmetic:
- A 350 square foot addition (2.0 needed) heated by a heat pump: 3.0 by the table, which clears it.
- The same addition on the existing gas furnace: 0 from heating, so it needs a heat pump water heater to Tier III of the Northwest Energy Efficiency Alliance specification (option 5.6), worth 2.0 by itself.
- An 800 square foot second story (5.0 needed): heat pump 3.0 plus that water heater 2.0 makes 5.0.
Ask early whether the reviewer lets an addition earn the heating credit when it shares the existing system. The permit drawings must list the options chosen and their points (R406.3). The full menu is in my new home energy code guide.
Blower door. Additions under 500 square feet of conditioned floor area are exempt (R402.4.1.2). From 500 up, the limit is 4.0 air changes per hour at 50 pascals (R402.4.1.3.1), or the addition may be tested together with the existing house at a combined 7.0 if the house predates the 2009 energy code, as most 1950s to 1970s houses do. A signed report goes to you and the code official. My reading is that an alteration alone does not trigger the test, because R503.1.1 does not list the testing section.
Duct testing. New ducts are leak tested to WSU RS-33 (R403.3.5). Rough-in leakage may not exceed 4.0 cfm per 100 square feet of conditioned floor area; ducts entirely inside the thermal envelope get 8.0, with limits on runs outside and no crawl space ducts (R403.3.6). Exemptions: additions under 150 square feet, ducts documented as previously sealed, and, for alterations, extensions of an existing system with under 40 linear feet in unconditioned space (R502.3.2, R503.1.2).
The WSU worksheet shows an exemption for additions under 750 square feet instead, wording that matches the 2018 edition of the code. The state’s text and the Council’s printed second edition both say 150. I follow the code text and ask which number your reviewer applies.
Garage and basement conversions
This is the easiest trap to fall into. Space that was not heated and becomes heated is an addition under the code (R501.2, R502.2), however it looks on the plans. Seattle’s code note says so: a converted garage or unheated basement counts as an addition, and over 150 square feet the credit requirement applies. The same note says an accessory dwelling unit built entirely inside already heated space needs no credits.
“Conditioned” is also wider than the thermostat. The definition includes space that communicates through openings with conditioned space or contains uninsulated ducts or piping. A basement you meant to leave unfinished can be conditioned by that definition.
A conversion has three routes to compliance:
- Full prescriptive. Ceiling R-60, walls R-20 plus R-5 continuous or R-13 plus R-10, floor R-30, windows U-0.30, slab edge R-10 for 4 feet (a footnote deems R-7.5 continuous insulation over an existing slab equivalent).
- The UA exception. The conversion complies where the combined UA of the existing house, the converted space and any alterations is no greater than the existing house had on its own (R502.3.1). You can pay for a garage conversion with better windows or attic insulation elsewhere.
- A modeled path (R502.4, R405), which allows 110 percent in some cases (R502.2).
A new accessory dwelling unit also needs its own programmable thermostat (R501.1.2). Bellevue’s page says garage-to-living conversions fall outside its no-plan-review remodel permit. More in garage conversions and basement dig outs.
Heat pumps, ducts, ventilation and electrical readiness
Heat pumps. New heating and cooling equipment is sized with ACCA Manual J loads and Manual S selection, and the output may not exceed the smallest available size above the calculated load (R403.7).
The credit tables above are the adopted 2021 tables. The Council’s 2024 update is still in rulemaking, so check the edition before you rely on any credit value.
Ventilation. The code requires whole-house mechanical ventilation in buildings that comply with the air leakage section, and tests the flow (R403.6, R403.6.2). Chapter 5 sends new systems to R403 as a whole and does not say how the ventilation rule applies to an alteration, so ask your reviewer. Bellevue states that remodeled bathrooms, laundry rooms and kitchens need exhaust fans to M1505.4 (50, 50 and 100 cfm). Equipment choices are in my HRV and ERV guide.
Electrical readiness. I found no electrical-readiness credit in the energy code tables. The electric vehicle circuit (one 40 amp, 208/240 volt dedicated circuit) sits in the state residential code amendments and applies to “construction of new dwelling units” with attached garages or carports (R309.6). A remodel of an existing house is not a new dwelling unit; a new attached accessory dwelling unit might be, so ask. Heat pumps and induction ranges add load, so have the electrician check service capacity early.
Where Seattle differs from the Eastside
State law makes the residential energy code preempt each city’s (RCW 19.27A.020), with a narrow exception for stricter city codes adopted before March 1, 1990, and the code calls itself the maximum and minimum in every city. The remodel provisions I compared match; the packaging differs.
| Topic | Eastside cities | Seattle |
|---|---|---|
| Code name | State code, chapter 51-11R WAC; Redmond’s adopting ordinance and Bellevue’s permit page both refer to the state code | Seattle Residential Energy Code, which adopts the 2021 state code with the March 15, 2024 date |
| Remodel chapter | Chapter 5, R501 to R505 | Same sections, with city notes on ADUs and conversions |
| Historic buildings | R501.6 | R501.6, plus the R101.6 landmark provision |
| Forms | Each city’s own; Bellevue sends you to its building desk | SDCI lists duct leakage, heating sizing, glazing schedule and a remodel worksheet |
I read both Chapter 5 texts side by side and the substantive provisions match. My Seattle permit guide covers the sequence.
Who does what, and which code applies
On the drawings. Permit documents show the compliance path, insulation values, window U-factors with the area-weighted calculation, mechanical design criteria, duct sealing and air sealing details, and the credit options with their points (R103.2, R406.3). The heating sizing calculation comes from whoever designs the system.
In the field. Inspectors check framing and rough-in, wall insulation before it is covered, mechanical rough-in and the final (R105.2). A tester runs the blower door and duct tests and signs the reports. A consultant becomes useful on the Total UA or modeled path, because someone has to run the calculation. I have no sourced figure for what that adds to a schedule. The schedule I can stand behind is design to permit submission typically 2 to 4 months, then a Seattle remodel permit in about 2 to 3 months.
Which edition. A complete application is considered under the code in effect on the day you apply (RCW 19.27.095). The Council’s schedule, revised August 14, 2026, lists final adoption of the 2024 energy codes for November 20, 2026 and implementation on May 3, 2027. Schedules move, so confirm with the Council and your city.
Where I would start
- List what the project touches: walls opened, roofs reframed, windows, systems, and any unheated space that will be heated.
- Total the new heated floor area against 150, 500 and 1,500 square feet.
- Choose heating and water heating early, because they set your credits.
- Ask the reviewer about the gray areas: height-only changes, ventilation in a remodel, additions of 1,500 square feet or more, and the duct test threshold.
Before design starts
Get the energy strategy settled first
Tell me what you want to change and where the house stands today. The basic feasibility check is free and usually takes about a day; the in-depth analysis is $1,500 and takes 2 to 3 weeks.
Questions
Does the Washington energy code apply to my remodel?
Yes, to the work you alter. Under the 2021 Washington State Energy Code, Residential (chapter 51-11R WAC, effective March 15, 2024), alterations must meet the requirements for new construction without requiring the unaltered portions of the house to comply (R503.1). The altered house also may not use more energy than before the alteration. Additions follow the new-construction rules in R502.
What insulation is required when I open walls in a remodel?
If an existing wall, ceiling or floor cavity is exposed during construction, you may fill it instead of meeting new-construction values: at least R-15 in a 2×4 wall and R-21 in a 2×6 wall (R503.1.1, exception 2). A cavity that is not exposed needs no upgrade. A wall rebuilt as a new assembly is held to the new-construction table, such as R-20 plus R-5 continuous insulation.
What are the energy code rules for replacing windows in Washington?
A replacement window unit, including just sash and glazing, must meet the U-factor in Table R402.1.3, which is 0.30 for windows and 0.50 for skylights, and you may use an area-weighted average across the units you replace (R503.1.1.1). Glass-only replacement in an existing sash and frame is a repair (R504.2), and storm windows over existing windows are exempt.
Do additions need energy credits, and how many?
Additions of 150 square feet or less need none. Additions over 150 and up to 500 square feet need 2.0 credits, and additions over 500 and under 1,500 square feet of heated floor area need 5.0 (R502.1.1, R406.3). Credits come from the heating system table (a heat pump earns 3.0) and the options in Table R406.3. The text does not name additions of 1,500 square feet or more, so ask your reviewer.
Does an addition need a blower door test?
Additions under 500 square feet of conditioned floor area are exempt. From 500 square feet up, the limit is 4.0 air changes per hour at 50 pascals, or the addition may be tested with the existing house at a combined 7.0 if the house was built before the 2009 Washington State Energy Code (R402.4.1.2, R402.4.1.3.1). The tester provides a signed report to the owner and the code official.
Is converting a garage or basement into living space covered by the energy code?
Yes. Space that is not heated and becomes heated is treated as an addition and must meet R502, including full compliance for the converted envelope unless you use the UA tradeoff or a modeled path (R502.2, R502.3.1). Seattle’s Energy Code notes that converting a garage or unheated basement counts as an addition, and above 150 square feet the credit requirement applies.
Does Seattle have a different residential energy code from the Eastside?
Seattle publishes the Seattle Residential Energy Code, but it adopts the 2021 state code, and state law (RCW 19.27A.020) makes the state residential energy code preempt each city’s, with a narrow exception for a stricter city code adopted before March 1, 1990. The Chapter 5 provisions I compared match the state’s. Differences are administrative: landmark wording, references to Seattle codes, and the forms the Seattle Department of Construction and Inspections asks for.
Do new ducts have to be tested in a remodel?
Yes, new ducts are leak tested to WSU RS-33 (R403.3.5). For alterations, extending an existing system with under 40 linear feet of ducts in unconditioned space is exempt, as are existing ducts built, insulated or sealed with asbestos (R503.1.2). For additions, exemptions are additions under 150 square feet, ducts documented as previously sealed, and existing asbestos ducts (R502.3.2). A WSU worksheet shows 750 square feet instead; confirm with your reviewer.
Which energy code will apply to my permit, 2021 or 2024?
A complete building permit application is considered under the code in effect on the date you apply (RCW 19.27.095). The State Building Code Council’s schedule, revised August 14, 2026, shows final adoption of the 2024 energy codes on November 20, 2026 and implementation on May 3, 2027. Dates can change, so confirm with the Council and your city.
Authoritative sources
Regulatory requirements referenced in this guide are set by the official agencies below. Rules change, always confirm current requirements with the source:
