Appearance
Alterations and additions
Sources and verification
This page describes how Thermly handles building work on an existing building. The approach follows MBIE's guidance Building work repairs, replacements and alterations - H1 compliance and Building Act 2004 sections 112(1)(b) and 42A(2)(b). H1/AS1 6th edition and NZS 4214:2006 are both silent on alterations, so that guidance is the controlling text. Verify against current MBIE guidance, and confirm with the building consent authority how it wants an alteration presented.
On a new building, every element is new building work and all of it goes into the heat-loss comparison. On an alteration or addition that is no longer true: some fabric is new, some is existing and untouched, and some is existing but altered by the work. Thermly lets you say which is which, row by row, and then shows two separate results.
For the rules themselves - when section 112 applies, what "not diminished" means, how replacement windows are treated - read the public guide, H1 for alterations and additions. This page covers doing the job in Thermly.
The two tests
- The new building work complies. This is the ordinary Calculation Method comparison, run over the new elements only: the addition's roof, walls, floor and glazing. MBIE's guidance is explicit that only the new building elements go into the heat-loss equations of the proposed and the reference building. Unchanged existing fabric appears in neither equation. The minimum construction R-values (§2.1.2.11) and the heated-element minimums (§2.1.2.12) bind the new work exactly as they do on a new building.
- The existing fabric is not diminished. For existing fabric the work alters - a replaced window, a new opening cut into an existing wall, a re-roofed section - heat loss after the work must not exceed heat loss before it. Thermly sums area ÷ R over the altered elements, before and after, and compares the two. An upgrade elsewhere in the altered set counts as an offset, which is how a new opening in an old wall can be made good.
A pure addition engages only the first test. A like-for-like window replacement engages only the second. "Extend out the back and replace three windows" engages both.
Untouched fabric never enters either test: there is no duty to upgrade it, and Thermly claims no R-value for it.
Switch the project to an alteration
In Project Information, set Building work to Alteration or addition (the default is New building work). A hint appears under the switch, and every roof, wall and floor row gains an Existing fabric choice. A new-build project is unaffected by any of this: leave the switch alone and nothing on the page changes.
Mark each row
Each element row's Existing fabric choice has three settings:
| Setting | What it means | What Thermly asks for |
|---|---|---|
| New building work | Built as part of this job. | The usual area and assembly. It enters the heat-loss comparison. |
| Existing, unchanged | Existing fabric the work does not touch. | Only its extent in m², for the record. The row collapses to a summary marked as not assessed under s 112(1)(b): no assembly and no R-value are asked for or claimed, and it is left out of both heat-loss equations and the minimums. |
| Existing, altered | Existing fabric the work changes. | The after-state as normal (area and assembly), plus its before-state in a Before the work panel. |
If a row already held an assembly when you mark it unchanged, the assembly stays stored but plays no part.
The before-state of altered fabric
The not-diminished check needs to know what the fabric was before the work. The Before the work panel asks you to choose a basis, and it is deliberately not defaulted:
- Evidenced construction - the R-value the existing construction had, from drawings, a survey or the original consent. Enter the R-value and a note naming the evidence.
- Conservative bound - take the before-state at the Table 2.1.2.7 reference value for the climate zone. This over-protects: where the old fabric was really worse than the reference value, it can call for offsets the law does not strictly require. It is the fallback when no evidence exists.
Area before the work covers fabric whose extent changes; leave it blank when the area is unchanged. Until a basis is chosen the panel says so, and the comparison stays incomplete with its sums withheld, so a missing input never reads as a pass.
Windows, doors and skylights in existing fabric
An opening under an existing wall or roof starts as unchanged, along with its parent. Give it its own setting where the work touches it: Existing, replaced or altered for a replaced unit, or New opening in existing fabric for one cut into the old wall or roof, with the tick box that records nothing was there before. A touched opening stands on its own state; it does not inherit its wall's.
Read the result
The report bar's verdict is the ordinary one - PASS or FAIL on the heat-loss comparison and the hard rules - computed over the new building work.
The results panel adds a card, Existing building fabric (Building Act s 112), with three parts:
- Excluded - the existing, unchanged elements by type and area, recorded but not assessed.
- The glazing cap on both bases - MBIE allows the 40% cap (§2.1.2.5) to be checked over the entire altered building or over the new building work only. Thermly shows both. Where only one is met, the report states that one as the basis relied on. The cap fails only where neither is met, and that is the one blocking finding this route can raise.
- The not-diminished comparison - heat loss over the altered fabric after the work against before it, with the margin, shown as met, not met, or incomplete. Show per-element working lists each altered element's area and R-value before and after, with the basis its before-state rests on. Where it is not met, compensating insulation elsewhere in the altered set is creditable as an offset.
On the plan
Existing fabric is shaded on the marked-up plan from the row settings, so it always matches the form: hatched for existing fabric that is unchanged, cross-hatched for existing fabric the work alters, and plain colour for new building work. The legend marks each such element, and a key line under the plan explains the shading. Because the shading is drawn from the rows each time, changing a row's setting updates every plan that shows it.
On the report and the take-off
Whenever any element is existing or altered, the H1 report carries a section headed Existing building fabric - Building Act s 112. It is not optional and sits straight after the compliance result. It states the statutory basis and the MBIE guidance followed, explains the two tests, tabulates the unchanged elements that were excluded, gives the glazing cap on both bases, and sets out the altered fabric before and after with the not-diminished result. The report's introduction says that the comparison covers the new elements only, and altered rows are marked as such in the schedules.
The insulation take-off lists altered existing fabric in a group of its own, outside the main order roll-up.
AI Review also checks an alteration job: for example, an altered element with no before-state basis comes back as a finding.
What this route does not show
The card and the report both carry a scope note. This route relies on s 112(1)(b) for consented work and s 42A(2)(b) for exempt work. Where the work creates one or more household units where none existed before, s 115(a) requires compliance with the Building Code as nearly as is reasonably practicable in all respects, which this route does not demonstrate. Whether that applies to a job is a question for the territorial authority.
A space that was never part of the thermal envelope has no prior performance to preserve. Converting an unheated garage into a living space, for example, makes its walls, roof and floor thermal envelope for the first time, so they are new building work at their actual values, not existing fabric.
You can always assess the whole altered building at actual values instead, with every element marked New building work. Passing that way demonstrates compliance at current levels, which is more than section 112 asks, and with old fabric it is usually out of reach.