Why you can trust the numbers

The receipts. Specific discrepancies I verified against the law, and how the engine handles the edge cases that trip people up.

The certified Washington Administrative Code (WAC) is the only canonical source behind this engine. Published code books, the WSU C3 spreadsheet, and draft documents are reference material — useful, but not the law. When I found that the book I'd started from disagreed with the WAC, I deleted the book-based source from the data pipeline entirely so it couldn't contaminate a single value.

None of this is a knock on the state. Documents drift as they're republished and marked up; the law doesn't. Everything below is a discrepancy I verified against the WAC myself. On the point where I cross-checked the WSU C3 spreadsheet against the same WAC text, it agreed with the WAC — not with the published book.

Headline findings

Option 1.3 undervalued by a full credit for R-2 buildings

What the published document says
In the SBCC-published 2021 WSEC-R (Sept 2024) document, Table R406.3, the Option 1.3 credit for Group R-2 reads 0.5.
What the certified WAC says
WAC 51-11R-40621, Table R406.3, row 1.3 lists 1.5 credits in both columns — All Other and Group R-2.
Why it matters
A builder pricing an R-2 project from the published document would undervalue Option 1.3 by a full credit — and could end up buying a more expensive option than the law actually requires. The discrepancy persists in the state's corrected September 2024 republication.

Verified June 2026. Sources: SBCC Sept-2024 PDF (archived) · WAC 51-11R-40621

Option 3.2a lists equipment the WAC doesn't allow

What the published document says
The 2021 WSEC-R, 2nd Edition book (January 2024) lists an air-source centrally ducted heat pump (HSPF 9.5) as qualifying equipment under Option 3.2a.
What the certified WAC says
WAC 51-11R-40621, Option 3.2a allows exactly two paths — an ENERGY STAR (U.S. North) gas or propane furnace at a minimum 95% AFUE, or a gas or propane boiler at a minimum 90% AFUE. There is no heat-pump path under 3.2a in the WAC. The book doesn't just add equipment — it lists a heat pump where the WAC lists a furnace.
Independent corroboration
The state's own September 2024 markup of the document struck the heat-pump line and inserted the furnace language — matching the WAC.

Verified June 2026. Sources: 2021 WSEC-R 2nd Edition (Jan 2024) (archived) · SBCC Sept-2024 redline (archived) · WAC 51-11R-40621

How carefully I work

Closing a credit-eligibility loophole

WAC 51-11R-40621, footnote d, reads verbatim:

This option may only be claimed if serving System Type 4 or 5 from Table R406.2.

Early versions of the engine enforced none of this. A System Type 1 project — a gas furnace — could claim an option reserved for System Types 4 or 5, earn credits it wasn't entitled to, and be reported compliant. Caught in audit, fixed, and locked down with regression tests so it can't come back.

The 150-square-foot addition exemption

WAC 51-11R-50200, Section R502.1.1, reads verbatim:

Additions not greater than 150 square feet (13.9 m2) shall not be required to comply with Section R406.

The engine's tier classifier was routing additions under 150 square feet into the small-dwelling tier — demanding 5.0 credits — instead of treating them as exempt at 0 credits. Caught in audit and fixed forward with a dedicated exempt tier and 12 boundary tests.

At exactly 150 square feet, the code says two opposite things

An addition of exactly 150 square feet is, by the letter of the code, both exempt from Section R406 and required to earn 2.0 credits under it. Two provisions cover the same square footage and reach opposite conclusions.

WAC 51-11R-50200, Section R502.1.1:

Additions not greater than 150 square feet (13.9 m2) shall not be required to comply with Section R406.

“Not greater than 150” includes 150 — so a 150 sq ft addition is exempt. But Table R406.3, item 5:

Additions 150 square feet to 500 square feet: 2.0 credits

“150 to 500” also includes 150 — so the same addition owes 2.0 credits. Both readings are literal. Both are defensible from the text alone. The overlap is in the code, not in anyone's interpretation of it.

wsec.ai resolves it as exempt. R502 is the scoping section for additions — it decides which additions enter Section R406 at all, and Table R406.3 only prices projects that R502 sends there. An addition sitting exactly on the exemption line is de minimis: 150 square feet is about a 12′ × 12′ room, and the code drew its floor there deliberately. Charging the full 2.0-credit package at the exact threshold chosen as the cutoff would read the exemption out of the code.

This is an ambiguity in the WAC, not a defect in the engine, so we disclose it instead of quietly picking a side. Projects at exactly 150 square feet carry a caveat on both the calculator and the wizard showing both citations. If your building official reads it the other way, their reading governs — and the caveat gives you the citation to have that conversation before you submit.

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