Energy code compliance plan (WSEC alterations)

How alterations differ from new construction

Good news: the Seattle Energy Code treats additions and alterations more pragmatically than new construction. The new portion has to meet current code; existing portions you're not touching mostly stay grandfathered. You don't have to retrofit the whole house to current standards just because you're adding a second story.

What applies to the new portion

• Wall, ceiling, and floor insulation to current R-values.
• Window U-factor at current code.
• Air sealing at the new envelope, with a blower-door test of the new + existing combined building at completion (step 5.6).
• Mechanical equipment for the new space — usually heat-pump-based given current Seattle requirements for new heating and water heating systems.

What does NOT apply to existing portions

• You don't need to upgrade existing wall insulation that you're leaving in place.
• You don't need to replace existing windows you're not touching.
• The 2021 Seattle Energy Code does not require Table 406.2 efficiency credits for alterations, only for new buildings.

When existing portions DO get pulled in

If the project hits the substantial-alteration threshold in SDCI Tip 314 — generally when alteration value exceeds 60% of the building replacement value — additional whole-building requirements kick in. This is the single most consequential threshold in the project; the designer should know exactly where you sit relative to it before submittal.

What the documentation looks like

A 2–4 page compliance narrative attached to the permit set: which prescriptive path you're taking, which assemblies meet which R-values, where the air-barrier transition happens between new and existing. Most designers handle this in-house; some outsource to a third-party energy consultant for $500–$1,500.