WSEC compliance and substantial-alteration analysis

Why this is the dominant compliance question on this project type

A second-story addition usually doesn't trip the substantial-alteration threshold — the new construction is the new construction, and the existing house is mostly untouched. A whole-house remodel + addition routinely does. When it does, the existing house has to be brought up to current code along with the new work — whole-building energy compliance, sometimes whole-building lateral retrofit.

This is the single biggest cost variable buried in the design phase. Settling it early is what keeps the budget honest.

What "substantial alteration" means

SDCI Tip 314 sets the threshold for when an alteration to an existing building is "substantial" enough to trigger compliance for the entire structure rather than just the altered portion. The threshold is tied to alteration value as a percentage of building replacement cost, with a sliding scale. Each piece of code (energy, seismic, accessibility) has its own threshold and its own compliance requirements when it trips.

How the analysis runs

The energy consultant or designer produces a calculation memo that:

1. Estimates building replacement cost for the existing structure.
2. Calculates alteration value for the proposed scope (addition + interior remodel + MEP).
3. Computes the percentage and compares it to each Tip 314 threshold.
4. States the conclusion for energy, structural, and accessibility separately.

A typical Seattle whole-house remodel + addition lands at 30–60% alteration value. That's well above the energy-code threshold (which triggers earlier) and frequently above the seismic threshold.

What it means when you trip the threshold

• Energy. The existing house has to meet current Seattle Energy Code as if new — wall and roof R-values, window U-factors, equipment efficiencies, mechanical ventilation. Practically, this means re-insulating the existing house's walls and roof, replacing single-pane windows, sealing the air barrier — work that wasn't necessarily in the scope and now has to be.
• Seismic. The existing house has to be retrofitted to current lateral standards — new shear walls, holdowns, anchor bolts. Significant cost and disruption; often the discovery that pushes a project from band 3 to band 4.
• Accessibility. Single-family residential alterations rarely trigger accessibility upgrades, but it's worth checking.

Compliance paths

Two main paths for the energy compliance:

• Prescriptive path. Each component (wall, roof, window, equipment) meets the table value in the code. Documented with a one-page summary.
• REScheck performance path. Trade-offs allowed across components. Use REScheck when one component is below code but offset by another.

For substantial-alteration projects, performance path often saves significant cost — you can leave one or two components below code if the rest of the package is strong enough.

Why this happens during design, not later

Discovering during permit review that you're substantially altered is expensive. The design has to absorb the additional scope (whole-building re-insulation, retrofit shear walls), and that affects every later number — bid, schedule, financing.

A good designer runs the analysis early, before construction documents are final. The bid then includes the substantial-alteration scope as a known line item, not a discovery.

When this is done

When the substantial-alteration determination is documented in writing, with calculations showing the math, and any required existing-building scope is added to the architectural and structural drawings.