Additions get evaluated against the energy code differently than a whole new house. Here's what that means for your compliance path.
If you're adding square footage to an existing house, the good news is that REScheck doesn't require you to bring your entire 1970s house up to current code — it only evaluates the new envelope area you're building. The addition is treated essentially as its own small building for compliance purposes, using the same UA trade-off methodology as new construction, but scoped to just the new walls, windows, ceiling, and floor you're adding. This page covers when an addition triggers REScheck, exactly what gets evaluated, and how the trade-off calculation works.
Yes, in most jurisdictions — DOE's own guidance states that REScheck determines compliance for additions in the same manner as new construction, and building departments generally require it whenever an addition increases conditioned floor area. Per the Building Energy Codes Program (energycodes.gov), when entering an addition into REScheck, only the new portions of the building need to be shown in the software — you are not required to model or upgrade the existing house's envelope to current code just because you're building an addition onto it. This is a common point of confusion: homeowners sometimes assume a REScheck for an addition means the whole house needs new insulation or windows to pass. It doesn't. The scope is the new construction only.
Only the walls, windows, doors, ceiling/roof, and floor assemblies that are part of the addition itself — the existing house's envelope is excluded from the calculation entirely. If you're adding a 400-square-foot family room, REScheck evaluates the new exterior walls of that room, its windows, its ceiling or roof assembly, and its floor (if it's not over existing conditioned space), and compares the total UA (U-factor × area) of those new assemblies against what code-minimum construction would produce for the same area in your climate zone. The existing house's walls, windows, and insulation levels — which may well be well below current code, especially in an older home — are not part of the compliance calculation for the addition permit. That said, if the project also alters existing envelope components (replacing windows in the existing house as part of the same project, for example), those alterations may trigger separate exemption rules or get folded into the trade-off, depending on what's being changed.
REScheck calculates the total heat-loss rate (UA — U-factor multiplied by assembly area, summed across every wall, window, ceiling, and floor in the new construction) for your proposed addition design, then compares it against the UA that a code-minimum version of the same addition would produce. If your proposed UA is equal to or lower than the code-minimum UA, you pass — even if one individual component falls short of the prescriptive table, as long as another component overperforms enough to compensate. This is the "trade-off" part: oversized windows with a strong U-factor can offset by adding extra ceiling insulation, for instance, rather than every single assembly having to independently hit the prescriptive minimum. This Total UA Alternative trade-off path is specifically an envelope-only trade-off — it does not include any credit for mechanical equipment efficiency, so a high-efficiency furnace or heat pump can't be used to offset a weak wall assembly in this calculation. The exact section number for this provision varies by IECC edition, so confirm the citation against whichever edition your jurisdiction has adopted.
Generally yes, if the addition adds conditioned (heated/cooled) floor area, regardless of size — the requirement is triggered by adding to the conditioned envelope, not by a minimum square footage threshold, though some jurisdictions set local minimums or exemptions. A small conditioned addition still needs its new envelope assemblies to meet or trade off to code, following the same UA methodology as a large addition. An unconditioned space, like an unheated three-season porch that doesn't touch the home's HVAC system, typically falls outside REScheck's scope since it's not adding to the conditioned envelope — but check with your local building department, since local amendments vary on how strictly this is enforced.
It depends on whether the addition adds or extends HVAC equipment — if it does, you likely need both a REScheck for the new envelope and a Manual J for the mechanical sizing, since these are two separate code requirements evaluated on different criteria. See our REScheck vs. Manual J vs. COMcheck guide for the full breakdown of how these two documents differ, and our Do I need a Manual J? guide for how addition projects specifically are treated under IRC M1401.3. In practice, most conditioned additions trigger both documents: the REScheck proves the new envelope meets code, and the Manual J proves whatever HVAC equipment serves the addition (whether that's extended existing ductwork or new dedicated equipment) is properly sized.
Alterations are evaluated differently than additions: REScheck applies exemption rules to specific alteration types, and only non-exempt assemblies get pulled into the trade-off calculation. Per DOE guidance, common exemptions include storm windows installed over existing fenestration, glass-only replacements within an existing sash and frame, and existing wall/ceiling/floor cavities that are exposed during construction and filled with insulation. If no exemption applies to a given assembly, its proposed and required U-factors get included in the trade-off calculation, similar to how additions are handled. This is a separate use case from a straightforward addition and follows its own rule set — the specific exemption list has shifted somewhat between IECC cycles, so check the current REScheck user guidance for your jurisdiction's adopted code edition.
No — this is the most persistent myth around addition permits. REScheck for an addition evaluates only the new construction, not the existing house. Your 1960s ranch's original single-pane windows and R-11 walls are not part of the compliance calculation for a new family room addition, as long as the new room's envelope meets or trade-offs to current code on its own. The confusion usually comes from conflating "addition" with "whole-house renovation," which is a different scope entirely and may trigger different requirements depending on the extent of work.
Travis Smith, ICC-Certified Residential Energy Inspector / Plans Examiner
Travis reviews addition permits regularly and built The Plans Examiner's REScheck service to scope correctly to just the new construction — no unnecessary whole-house modeling, no inflated report. See our REScheck page — $149, opening soon.
ICC-certified plans examiner — private review for your permit package. Travis Smith — ICC-Certified Residential Energy Inspector / Plans Examiner · E&O insured · general contracting background in Washington. The person who reviews your plans at the building department is a plans examiner. I hold the same ICC certification — except I work for you.