The short answer
The new work complies. The rest of the house is not made worse. That is the whole rule.
Under the Building Act 2004, all building work must comply with the Building Code (section 17), so the new parts of an alteration, an addition or a conversion meet H1 at today’s levels. For the existing building, section 112(1)(b) asks only that after the work it continues to comply with the Code at least to the same extent as it did then comply
. Nothing in H1 or the Act asks you to upgrade what you are not touching.
MBIE’s guidance page, Building work repairs, replacements and alterations – H1 compliance, turns that into a method in two sentences. For an addition, only the new building elements go into the heat-loss equations, on both the proposed side and the reference side. For work within the existing envelope, make sure the building’s overall thermal resistance is not diminished, adding insulation elsewhere to offset any loss.
Thermly follows that method line by line. This page walks through what it means for the jobs designers, builders and homeowners actually do, and says as clearly what is not required as what is.
01 · The method
Two tests, and nothing else.
Every alteration job breaks into two questions. A pure addition asks only the first. A straight window swap asks only the second. A knock-through-and-extend job with a few replaced windows asks both. Neither question ever reaches fabric the work does not touch.
Test 1
The new building work complies
Building Act s 17
Run the H1/AS1 Calculation Method over the new thermal envelope only: the addition’s roof, walls, floor, windows and doors. The reference building is built from those same new elements, so the existing house appears on neither side. The usual Sixth Edition rules still bind the new construction: the direct minimums for roofs, walls and floors other than slabs (H1/AS1 2.1.2.11), the heated-element minimums (2.1.2.12) and the 40% wall-glazing limit (2.1.2.5), which MBIE lets you check over the whole altered building or over the new work alone.
Test 2
Existing fabric you touch is not diminished
Building Act s 112(1)(b) · s 42A(2)(b) for consent-exempt work
For every existing envelope element the work alters and that stays part of the envelope (a replaced window, an opening cut or enlarged, a re-roofed section), compare heat loss after the work with heat loss before, across the set you touched. Voluntary upgrades elsewhere count as offsets. Untouched fabric is never in this test, and there is no upgrade duty: the target is the building’s own prior performance, not today’s levels.
Why leaving elements out is exact
Taking an unchanged roof out of both sides of the comparison changes nothing about the result for the new work. It is the arithmetic form of what section 112(1)(b) says in words: that element’s compliance is a settled fact, and the new work is judged on its own. Nothing is being waived.
What “thermal envelope” means
The boundary between heated (conditioned) space and the outside or an unheated space such as a garage or roof space (H1/AS1 Appendix B). Walls and floors between two heated spaces are not envelope and are left out of the calculation (2.1.2.4). That one definition does a lot of the work in the examples below.
Heat loss is measured in W/K, watts per kelvin: the heat an element lets through for each degree of temperature difference between inside and out. Lower is better. It is simply area divided by construction R-value, summed over the elements in question.
02 · Worked examples
Four common jobs, and what each one has to show.
The numbers below are illustrative and rounded. They use the Sixth Edition reference values for climate zone 3 (roof 6.6, wall 1.6, wall glazing 0.46, slab-on-ground 1.5) so the two tests can be seen working; a real assessment uses the project’s own zone, areas and construction R-values.
Adding a room
A 24 m² family room built off the north wall of a 1970s house.
Only the first test applies. The room’s own roof, walls, floor and windows go into the Calculation Method, and the reference building is built from those same elements. The existing house appears on neither side.
In the comparison · new work
- New roof, 24 m²
- Three new external walls, 33.6 m² gross
- New windows, 8 m²
- New slab, 24 m²
Not in the comparison
- The existing roof, walls, windows and floor, whatever their insulation
- The old external wall the room is built against
The wall that becomes internal is out because of a definition, not a concession.
Once the room is built, that wall separates two heated spaces. It is no longer part of the thermal envelope, so it leaves the assessment entirely (Appendix B definitions; 2.1.2.4). MBIE’s guidance says the same in its own extension example: parts of the house that formed the thermal envelope but are now enclosed by the alteration are no longer part of it. Cutting the doorway through it alters no envelope fabric, so it does not enter the second test either.
| New work only | Heat loss |
|---|---|
| Proposed: roof R3.3, walls R1.8, windows R0.46, slab R1.6 | 53.9 W/K |
| Reference: the same elements at the Table 2.1.2.7 values | 56.3 W/K |
| Proposed does not exceed reference | Passes |
If the addition fails on its own, it is the addition that changes: better windows, more roof insulation, an insulated slab edge. The existing house neither rescues it nor sinks it.
Replacing windows
Three tired timber windows on a 1980s house swapped for new double-glazed units.
Only the second test applies, and it is a modest one. Replacement joinery has to perform at least as well as the joinery it replaces. It does not have to meet today’s H1 levels. That is the position in LBP Codewords 113, resting on sections 42A and 112 of the Act.
MBIE’s guidance describes four routes that all satisfy it:
- Like-for-like: the same material and a comparable unit.
- A frame of similar performance, such as uPVC in place of timber.
- A lower-performing frame, such as aluminium in place of timber, with insulation added elsewhere to offset the loss.
- A better-performing unit outright.
Most of these jobs need no building consent: Schedule 1 of the Act exempts like-for-like replacement (clause 1) and work on windows and exterior doorways in homes of up to two storeys (clause 8). Exempt is not the same as unregulated. The work must still comply, and the building must still comply at least to the same extent afterwards (sections 17 and 42A(2)).
MBIE also accepts a slightly lower performance where an identical replacement is no longer available, for instance because standard sizes have changed, provided the effect is minimal.
Nothing else changes. The walls the windows sit in, the roof and the floor stay untouched and out of scope.
Enlarging an opening
A 1.2 m kitchen window in an uninsulated 1950s wall is widened to 2.4 m to open the room to the garden.
More glass in place of wall means more heat loss, even with double glazing, so MBIE says to add enough insulation elsewhere in the thermal envelope to offset the loss. That is the second test with an offset. The panel of wall around the window and the window itself are the altered fabric; the ceiling over the same wing, topped up at the same time, is the offset.
Swipe the table sideways to see the “after” column.
| Altered fabric | Before | After |
|---|---|---|
| Wall panel with the window, 9.6 m² gross: window grows from 1.44 m² to 2.88 m² | 8.2 W/K | 10.5 W/K |
| Offset: ceiling over the same wing, 40 m², topped up from R1.9 to R4.0 | 21.1 W/K | 10.0 W/K |
| Altered set: after does not exceed before | 29.3 W/K | 20.5 W/K |
Before-state here taken at the stated conservative bound (wall R1.6, glazing R0.46). The wider window alone would have diminished the building by 2.2 W/K; the ceiling top-up more than covers it.
The before-state needs a basis, and evidence usually helps you.
The heat loss “before” has to come from somewhere: evidence of what is actually in the wall, or a stated conservative assumption at the reference value for that kind of element. For most older walls the conservative bound over-states the loss (an uninsulated wall near R0.5 loses less by being cut than an R1.6 wall does), so evidence of the real build-up usually shrinks the offset you need. Thermly always asks which basis you are using and prints it in the report; it never assumes one silently.
A new or enlarged opening in a home of up to two storeys is often consent-exempt under Schedule 1 clause 8 as well. As with replacement windows, that changes who checks, not whether H1 applies.
Converting a garage
An attached single garage becomes a bedroom and study for the same household.
Change of use? No. The 2005 change-the-use regulations put a household’s garage in the same use category as the house (SH), so converting it for the same household is not a change of use. Creating a separate household unit, such as a self-contained flat with its own kitchen and bathroom, is, and section 115 then requires the building in its new use to comply as nearly as is reasonably practicable with the whole Code. Which category a proposal falls into is the council’s call.
H1: everything at actual values. The garage was an unheated space, so its walls, ceiling and floor were never part of the thermal envelope. There is no “same extent” to preserve and nothing to leave out: the conversion is new work, assessed with the first test at the actual values of what is built. The wall and door between the garage and the house stop being envelope. The garage’s external walls, its ceiling or roof, and whatever fills the garage-door opening become the new envelope and are checked like any new room. That matches MBIE’s guidance on attached garages: while a garage is unheated and outside the envelope it has no H1 duty of its own, and the walls, windows and doors between it and the house are the envelope.
The slab reality.
The floor is almost always an uninsulated slab, and it counts at its actual value under H1/AS1 Appendix E. That is usually below the reference slab value, so the new walls, ceiling and windows have to make up the difference. H1/AS1 itself acknowledges that slab insulation cannot realistically be retrofitted (Appendix E, E.1.2.1 comment) and recommends insulating garage slabs at the outset for exactly this reason. Thermly counts the slab at its actual value and shows where the margin goes rather than hiding it.
03 · What is not required
Things you do not have to do.
Most of the anxiety around H1 for alterations comes from things the rules never ask for. Each of these has a clause behind it.
- Upgrade the rest of the house.
- Section 112(1)(b) asks only that the building keeps complying at least to the same extent as it did before the work. MBIE’s guidance says plainly that the rest of the building does not have to be upgraded when an extension is added.
- Insulate walls you are not opening.
- There is no upgrade duty on untouched fabric. A wall the work never touches is in neither test, whatever its insulation.
- Invent R-values for the existing building.
- MBIE’s method leaves unchanged existing elements out of both heat-loss equations. You do not need to work out what a 1960s wall achieves, and you should not give it a notional value: only the new work is compared with the reference building.
- Bring replacement windows up to current H1 levels.
- Replacement joinery has to perform at least as well as the joinery it replaces. A lower-performing frame is still allowed if insulation is added elsewhere to offset the loss.
- Treat “no consent needed” as “no H1”.
- Sections 17 and 42A(2) of the Building Act keep consent-exempt work inside the Code: the work complies, and the building afterwards complies at least to the same extent as before. Schedule 1 exemptions change who checks, not what applies.
- Apply “as nearly as is reasonably practicable” to H1.
- That upgrade duty in section 112(1)(a) covers means of escape from fire and access for people with disabilities. It never reaches energy efficiency.
- Treat a garage conversion for your own household as a change of use.
- A household’s garage sits in the same use category as the house (SH). Creating a separate household unit is different, and is the one route by which existing fabric can be pulled up for H1.
- Fail the 40% glazing limit on the extension alone.
- MBIE lets the wall-glazing limit be checked over the whole altered building or over the extension only, at the designer’s option. A glassy family room can pass on the whole-building basis, and the report should say which basis it used.
04 · In Thermly
How Thermly assesses an alteration or addition.
Thermly runs MBIE’s method as two visible tests and writes the reasoning into the consent report, so a processing officer can see which parts of the building were assessed, which were left out, and why.
- 01
Set the job type.
Under “Building work”, choose “Alteration or addition”. That switches on the existing-fabric control on every element and the section 112 narration in the report. A new build never sees them.
- 02
Mark each element.
Each roof, wall, floor, window and skylight has an “Existing fabric (s 112)” menu with three states: new building work, existing fabric the work does not touch, or existing fabric the work alters. Untouched elements need no build-up and claim no R-value; they are excluded from both sides of the comparison and from the minimums. Altered elements take a before-state, from evidence of what is there or a stated conservative assumption, never a silent default.
- 03
Read the two results.
Test 1 runs the Sixth Edition Calculation Method over the new work only. Test 2 sums heat loss before and after across the altered set, offsets included. The 40% glazing limit is checked on both bases, whole building and new work only, and the report states which basis it relied on.
- 04
Generate the consent report.
Whenever any element is existing, the report carries a section headed “Existing building fabric: Building Act s 112”: the statutory basis, the two tests, a table of the excluded elements marked “Existing: not assessed, s 112(1)(b)”, and the before-and-after table with the basis of each before-state. The Building Act and MBIE’s guidance page join the references list.
- 05
Show where the work is.
The insulation-extent diagrams drawn from the plan shade existing fabric apart from new work, so a reviewer can see at a glance which parts of the building the assessment covers.
Screenshot to follow
Screenshot to follow
Screenshot to follow
See it on a real job
A read-only sample alteration project, a family room added to a single-storey house with two windows replaced and one opening enlarged, is being prepared for every Thermly project list. A link will appear here when it is published.
Thermly’s boundary
Thermly assesses against H1/AS1 Sixth Edition only, for housing within the Calculation Method’s scope. It flags the change-of-use question when a job may create a separate household unit, but it does not assess it: use-category determinations rest with the territorial authority. It does not perform H1/VM1 modelling.
Free during open testing
Assess the new work, leave the rest alone, and show your working.
Construction R-values, plan take-off, the two tests and a consent report with the section 112 reasoning written in, in one Thermly project.
05 · Common questions
H1 for alterations and additions: FAQ.
Do I need to insulate the whole house if I add a room?
No. The new room has to meet H1 on its own, and the rest of the house only has to be no worse than it was before the work. Section 112(1)(b) of the Building Act asks for compliance to at least the same extent as before, and MBIE’s guidance says the rest of the building does not have to be upgraded when an extension is added.
Do replacement windows have to meet current H1 levels?
No. Replacement joinery must perform at least as well as the joinery it replaces, which is the position set out in LBP Codewords 113. Like-for-like, a similar-performance frame or a better unit all qualify. A lower-performing frame is allowed if insulation is added elsewhere to offset the loss.
Is converting a garage into a room a change of use?
Not when the room stays part of the same household. The 2005 change-the-use regulations place a household’s garage in the same use category as the house (SH). Creating a separate household unit, such as a self-contained flat, is a change of use and triggers section 115 of the Building Act. The use category is determined by your council, not by this page or by Thermly.
I don’t need a building consent for new windows. Does H1 still apply?
Yes. Consent status and H1 are separate questions. Section 17 of the Building Act says all building work must comply with the Building Code, and section 42A(2) attaches the same “at least to the same extent as before” condition to consent-exempt work. Schedule 1 makes many window and exterior-door jobs in homes up to two storeys exempt from consent, but not from H1.
Does the wall between the old house and the new room count?
No. Once the room is built that wall separates two heated spaces, so it is no longer part of the thermal envelope (H1/AS1 Appendix B definitions and paragraph 2.1.2.4). It drops out of the comparison because of that definition, not as a concession, and cutting the doorway through it is not an alteration of envelope fabric.
What R-value do I use for the existing walls and roof?
None, if you are not touching them. MBIE’s method leaves unchanged existing elements out of both the proposed and the reference heat-loss equations, so their R-values never enter the calculation. Only elements the work alters need a before-state, and that comes from evidence of what is there or a stated conservative assumption.
Can a poorly insulated existing house make my addition fail?
No. Under MBIE’s method the addition is compared with a reference building made of the same new elements, so the existing house is on neither side of the comparison. If the addition fails, it is the addition that needs changing: better windows, more roof insulation or an insulated slab edge.
Which edition of H1/AS1 applies to an alteration?
Thermly assesses against H1/AS1 Sixth Edition, which took effect on 27 November 2025. The Sixth Edition itself says nothing about alterations; the method comes from the Building Act and MBIE’s guidance page, which was updated the same day. The Calculation Method is then run over the new work only.
06 · Sources
What this page is based on.
Every statement above traces to the Building Act, MBIE’s published guidance or a numbered clause of H1/AS1 Sixth Edition. The licensed standards are cited by clause number and not reproduced. Always check the sources for amendments before relying on them.
- 01
Building Act 2004
Section 17 (all building work must comply), section 42A (consent-exempt work), section 112 (alterations to existing buildings), section 115 (change of use) and Schedule 1 (exempt building work). Open the Act
- 02
MBIE: Building work repairs, replacements and alterations – H1 compliance
The guidance page the method rests on: repairs and replacements, new and enlarged openings, additions and extensions, and the Calculation Method under the Sixth Edition. Second edition, updated 27 November 2025. Open the guidance
- 03
LBP Codewords 113: Achieving H1 Energy efficiency compliance when altering an existing building
The practitioner-facing statement that replacement joinery must perform at least to the same extent as the joinery removed, and need not meet current levels (June 2023). Open the Codewords article
- 04
MBIE: exempt building work
Schedule 1 guidance, including the window and exterior-doorway exemption for homes of up to two storeys. Open the exempt-work guidance
- 05
Change of use: MBIE guidance and the 2005 regulations
How a change of use is defined, the household-unit rule, and Schedule 2’s use categories. Open the MBIE page · Open the regulations
- 06
H1/AS1 Sixth Edition
Clauses cited on this page: 2.1.2.4, 2.1.2.5, 2.1.2.6, 2.1.2.7, 2.1.2.11, 2.1.2.12, Appendix B, Appendix D and Appendix E (E.1.2.1 comment). The document contains no alterations method of its own. Open the Sixth Edition PDF
A note on advice
This page is general information, not legal, design or building-compliance advice. Use-category and change-of-use determinations rest with the territorial authority, and every consent application is assessed by the building consent authority on its own facts. For the wider H1 picture see the H1 compliance guide and the Fifth vs Sixth Edition comparison.