How we got here
The working behind the Croydon sample.
This is the engine's own working page for the sample comparison: a 1920s bungalow in Croydon, Sydney (street number not shown), run through engine v0.4.4 on 28 October 2026. It shows the inputs, quantities and main assumptions behind each number. Rate tables stay in the engine; every figure carries a source ID from the methods page.
What we were told
- House type
- Interwar Californian bungalow (1915-1940)
- Floor area and storeys
- 110 m², 1 storey
- Condition and renovation history
- Good condition, original
- Shape, ceiling height, windows
- Rectangular, 3.0 m ceilings, average window
- Insulation: ceiling / walls / underfloor
- No / No / No
- Glazing, draughts, damp
- Single glazing, lots of draughts, damp: no
- Heating now, solar
- Gas, solar: no
- Orientation, structure, kitchens
- North; structure: none seen; kitchen and bathrooms: dated but working
- Planning
- Site 483 m² · R2 · FSR 0.55:1 · height limit 8.5 m · heritage: heritage conservation area, flood: no, bushfire: no
- Client goals and time in the home
- must have: more space, better comfort and lower bills; staying about 20 years; budget around $1.2m
- Scope settings
- Custom builder, mid finish; extension 80 m² ground floor; new house 190 m²; heating upgrade in Retrofit: no
What we worked out about the house
- Footprint
- 110 m²
- Perimeter
- 44 m (shape factor 1.06 x a square)
- External wall area, less windows
- 111 m²
- Window area
- 22 m² (20% of floor area)
- Windows double glazed in Retrofit
- 17 m²
- Weight of the existing house
- 88–132 t
- Asbestos risk for this house type
- Medium
- Comfort score today (1-5)
- 1.2
- Household electricity benchmark
- 9,008 kWh a year for 5 people (AER, S18)
What each option includes
| Retain | Retrofit | Extend | Replace | |
|---|---|---|---|---|
| Scope | Interior and exterior paint, new floor coverings, repairs (good condition) | Paint, floor coverings and repairs as Retain, plus ceiling insulation, wall insulation, underfloor insulation, draught sealing and double glazing the worst windows, asbestos allowance | Add 80 m² (ground floor, mid finish), tie-in, plus a deeper retrofit of the existing house | Demolish and remove asbestos, build about 190 m² (mid finish), site works and services |
Cost (AUD incl. GST)
| Retain | Retrofit | Extend | Replace | |
|---|---|---|---|---|
| Works | $12k–$28k | $35k–$84k | $329k–$489k | $662k–$1.08m |
| Renovation history allowance | – | – | – | – |
| Contingency (15%) | $2k–$4k | $5k–$13k | $49k–$73k | $99k–$161k |
| Total | $13k–$32k | $40k–$97k | $378k–$562k | $761k–$1.24m |
| Shown on the report (rounded) | $15k–$30k | $40k–$95k | $380k–$560k | $760k–$1.25m |
Carbon (t CO₂e)
| Retain | Retrofit | Extend | Replace | |
|---|---|---|---|---|
| Embodied (upfront building work) | 1–4 t | 7–17 t | 25–65 t | 40–100 t |
| Running (grid electricity and gas over the years staying) | 30–60 t | 25–45 t | 25–45 t | 25–40 t |
| Total | 30–60 t | 30–60 t | 50–110 t | 65–140 t |
| Band on the report (vs Replace) | Low | Low | Medium | High |
Materials and waste
| Retain | Retrofit | Extend | Replace | |
|---|---|---|---|---|
| Removed from the existing house | up to 2 t | 1–3 t | 3–13 t | 90–135 t |
| Share of the house kept | 100% | 100% | 90%–95% | 0% |
| Construction waste from new work | under 1 t | under 1 t | 2–6 t | 6–15 t |
| To landfill | up to 1 t | up to 2 t | 1–8 t | 15–50 t |
Running energy and bills
| Retain | Retrofit | Extend | Replace | |
|---|---|---|---|---|
| Comfort score after works (1-5) | 1.2 | 3.5 | 3.5 | 4.0 |
| Electricity used (kWh a year) | 6,521–7,617 | 6,355–7,300 | 6,381–7,351 | 6,486–7,549 |
| Gas for heating (MJ a year) | 6,789–12,683 | 1,546–2,888 | 2,381–4,448 | – |
| Energy bills ($ a year) | $2,578–$4,006 | $2,348–$3,311 | $2,385–$3,421 | $2,331–$3,222 |
| Bills over the years staying | $52k–$80k | $47k–$66k | $48k–$68k | $47k–$64k |
| Change vs today | – | About 10–15% lower than today | About 10–15% lower than today | About 10–20% lower than today |
Suggested option check
| Retain | Retrofit | Extend | Replace | |
|---|---|---|---|---|
| Meets 'more space' | No | No | Yes | Yes |
| Meets 'better comfort' | No | Yes | Yes | Yes |
| Meets 'lower bills' | No | Yes | Yes | Yes |
| Meets every must-have | No | No | Yes | Yes |
| Within budget | Yes | Yes | Yes | Yes |
| Cost, carbon, waste and time (0-100, lower is better) | 8 | 10 | 44 | 100 |
| Time used (months, incl. council wait) | 1 | 2 | 10 | 20 |
- How we chose
- Must-have goals first, then nice-to-haves, then budget (top figure $1.2m, well over = mid cost more than 15% above it), then the lowest overall cost, carbon, waste and time, each as a share of the highest option.
- Result
- Extend, with a deeper retrofit. Adding an 80 m² extension and a deeper retrofit of the existing house is estimated to meet all three goals: more space, better comfort and lower bills, for roughly $380k–$560k. Especially since you need more space and the existing house is uncomfortable today, extending with a deeper retrofit looks like a good fit for this brief. That's less than half the cost and about 65% of the carbon of replacing. A light retrofit alongside the extension would fall short of the goals, so this includes a deeper retrofit of the existing house: insulation, better windows and draught sealing. It keeps most of the existing home, with one round of building work, not two.
- Why a deeper retrofit
- With only a light retrofit (ceiling insulation and draught sealing), Extend would cost about $350k–$500k and cut bills by about 9%, short of the 10% the bills goal needs. The deeper retrofit adds about $25k–$60k and cuts bills by about 12%, so it meets every goal and still fits the budget.
Site potential
- Site check
- Zoned R2 Low density residential. Depending on planning regulations and council controls, at an FSR of 0.55:1 this 483 m² site could allow about 266 m² of floor area, 2.4 times the current house. At 483 m² in R2 with an 8.5 m height limit, the site might suit a dual occupancy if it meets the council's minimum lot size (450 m² assumed here). In a heritage conservation area, more homes on the site and demolition are much harder to get approved, so the zoning alone doesn't show what's possible.
- Numbers used
- Floor area allowed 266 m²; minimum lot size 450 m² (assumed for R2)
Main assumptions
Contingency 15% on every option. No renovation allowance (original house). Costs from RLB Riders Digest 2026 (S01) and Sydney trade guides (S02-S08).
Electricity 28–34c per kWh plus $1.41–1.85 a day (2026-27 default market offer, S19), at today's prices for every year.
NSW grid carbon averages 180–270 g CO₂e per kWh over the next 20 years as coal closes (S21, S22). Today it is 640 g.
Heating and cooling come from NatHERS star bands for the comfort score (S17); 35–50% of that load is assumed actually used, as households heat and cool only part of the house (internally assessed, calibrated to S18).
New-build carbon 0.20–0.50 t CO₂e per m² (EPiC, S25, with the low end widened to cover measured built houses, S10; cross-checked against GBCA 2025, S09); additions 0.20–0.55 t per m².
Demolition waste 70–85% recycled (National Waste Report, S23). Asbestos always goes to landfill.
Comfort score: house-type baseline, adjusted for insulation, glazing, draughts, damp, condition and orientation, plus points for each retrofit upgrade (internally assessed, low accuracy). 4 = the BASIX minimum (7 stars), so a new home shows 4, or 5 if built above minimum; a retrofit tops out at about 3.5.
Values marked IA are internally assessed (low accuracy) and are being checked through the pilot.
Sources
All 25 sources, with links and reliability ratings, are listed on the methods page.
How the numbers are worked out → Back to the sample →
Illustrative example based on a real house, not an assessment of it. Indicative, order-of-magnitude estimates for early feasibility discussion. Not a quote, and not a structural, planning or valuation assessment. Full disclaimer.