ADU & Home Addition Permit Calculations (REScheck + Manual J/S)
Building departments scrutinize detached backyard ADUs, garage conversions, sunrooms, and second-story additions closely—especially when high-glass walls or oversized ductless mini-splits are proposed. Get a passing IECC REScheck and ACCA Manual J/S report in 24 hours.
How IECC Section R502 Applies to Home Additions & Conversions
When you add conditioned square footage to an existing home—or convert an unconditioned garage, porch, or basement into habitable living space—the International Energy Conservation Code (IECC Section R502) requires the new thermal envelope to demonstrate compliance on its own.
- Addition-Only REScheck Modeling: You do not need to bring the entire existing house up to current code when filing an addition-only REScheck. We model only the new/altered walls, roof/ceiling, floor/slab, and windows.
- High Window-to-Wall Ratio Solutions (Sunrooms & Backyard Additions): Additions often feature large sliding glass doors or picture windows that fail prescriptive U-factor tables. Using the IECC Section R402.1.5 Total UA Trade-Off path in REScheck, we trade higher ceiling R-value (e.g., R-49 or spray foam) or low-U glazing (U-0.27 to U-0.30) so your glass passes cleanly.
- Garage Conversions & Uninsulated Slabs: Converting an existing uninsulated slab in Climate Zones 4–8? We balance slab perimeter F-factor penalties using high-density wall/ceiling cavity insulation or continuous exterior/interior rigid foam.
Why Inspectors Reject ADU & Addition Mini-Split Permits Without Manual S
A 600–900 sq ft detached ADU or primary-suite addition built to 2018 or 2021 IECC standards often has a total peak cooling load of only 9,000 to 15,000 BTUh (0.75 to 1.25 tons). When a contractor proposes a 2.0-ton or 2.5-ton mini-split using old "rule-of-thumb" sizing, plan reviewers reject the permit for exceeding ACCA Manual S oversizing limits.
Our ACCA Manual J 8th Edition + Manual S report calculates the exact room-by-room sensible and latent load and pairs it with a compliant variable-speed inverter heat pump (e.g., 9k, 12k, 15k, or 18k BTUh Mitsubishi, Daikin, Fujitsu, Carrier, or Gree) with AHRI certificate verification.