Methods

How the numbers are worked out.

What each measure covers, where the figures come from, the main assumptions, and how we show what we're unsure of. The detailed rate tables and formulas stay in our engine; everything they're built from is listed here.

Where it works today

retain.solutions currently covers existing houses in Greater Sydney, Australia. The estimates are calibrated to this region: Sydney construction costs, the NSW electricity grid, Sydney climate zones and NSW planning data. For homes outside Greater Sydney, or for apartments and other building types, the numbers won't hold, so during the pilot we only take Greater Sydney projects.

What goes in

Each comparison starts from a short intake about the house (type and era, floor area, storeys, condition, insulation, glazing, draughts, damp, structure), the client (goals, budget, how long they plan to stay, their reasons) and the site. We look up the site ourselves on the NSW Planning Portal: zoning, floor space ratio, height limit, minimum lot size, heritage, bushfire and flood. Where an answer is "Not sure", we assume the upgrade is needed and say so.

The seven measures

• Cost. Indicative 2025–26 Sydney rates for renovation, additions and new builds (Rider Levett Bucknall and trade cost guides), including GST and a 15% contingency on every option. When the kept house needs renovating inside, a new roof or new cladding, those are priced as extra lines, set by the kind of builder likely to do the work. Extend's cost per m² is for the new addition only. Excludes land, fees, furniture, finance, and new solar or hot water.
• Carbon. Embodied carbon (the upfront carbon in new building work and materials) plus running carbon (energy used in the home over the years the client plans to stay). Embodied carbon is built up from the materials in a typical Sydney house using the EPiC database (University of Melbourne): about 0.20–0.50 tCO₂e per m² of floor area for a new home and 0.20–0.55 t per m² for an addition. The low end is widened from EPiC's 0.30 so the range also covers built Sydney houses measured with process-based tools such as One Click LCA, which come in lower. We cross-check it against the GBCA's 2025 study, which comes out higher mainly because it also counts driveways, landscaping and solar panels, and measures per m² of heated and cooled floor area. Each option's total is shown as a band compared with Replace: very low, low, medium or high, with the tonnes underneath.
• Materials & waste. The estimated weight of the existing house, how much of it each option removes, the share kept, and the tonnes to landfill. Demolition waste is assumed 70–85% recycled (National Waste Report). Asbestos always goes to landfill.
• Floor area. Space gained in square metres, and a planning check of how much floor area the site allows under its floor space ratio.
• Running costs. Shown as a share of today's energy bills, so today is 100% and an option at 80% cuts bills by a fifth. Built from electricity use for the household size (AER benchmarks), with heating and cooling from NatHERS star bands for the comfort score, at 2026–27 default market offer prices plus daily supply charges. Where the client wants new work built above the BASIX minimum, the report also shows what that adds up front.
• Comfort & health. A score from 1 to 5, built from the house type, insulation, glazing, draughts, damp, condition and orientation, plus points for each upgrade. 4 meets today's minimum for a new home (BASIX, 7 stars) and 5 goes beyond it; a retrofit tops out at about 3.5. Half scores are shown. This score is internally assessed and has low accuracy.
• Delivery. Build time, whether the household can stay in the home, and the approval route. Council waits use each council's average days to a decision from the NSW council league table. Heritage areas add time and risk, and demolition may not be approved. Delivery also notes whether a BASIX certificate is likely: one is needed for a new home, and for alterations and additions of $50,000 or more that need approval.

BASIX

BASIX is the NSW energy and water standard checked when a project is approved. Its biggest pain is a fail found late, after the design is fixed, so the report flags the BASIX route early, at the keep-or-replace stage: none for Retain, likely for Retrofit when it needs approval and costs $50,000 or more, the alterations rules for Extend, and the full new-home rules (7 stars) for Replace. BASIX only looks at the new work; we also score the existing house's comfort and bills. The report doesn't issue certificates and isn't compliance advice: see the NSW Planning Portal.

The 20-year timeframe

Running carbon and energy bills are counted over the years the client plans to stay: about 10, 20, 30 or 50 years, or 20 years if not given. The sample uses 20 years. Over that time the NSW grid gets cleaner as coal closes: about 640 g CO₂e per kWh today, averaging 180–270 g over the next 20 years. Bills are at today's prices for every year, so they're not inflated or discounted.

How the suggested option is picked

We start with what the client wants. Each goal is a must-have, a nice-to-have or not needed: more space, better comfort, lower bills (at least 10% lower), keeping the original look and features, a better layout, lower carbon, and staying at home during the works. First, the options that meet every must-have, then the ones meeting the most nice-to-haves. Then the ones within budget, where an option counts as over budget if its middle cost is more than 15% above the top of the budget. Of those, we recommend the one with the lowest overall cost, carbon, waste to landfill and time, weighed equally, each as a share of the highest option. If two are level, the one that keeps more of the existing home wins, and a close call is flagged on the report.

Meets the brief shows the same check for every option: Meets (every goal), Meets the must-haves (but misses a nice-to-have) or Misses a must-have, with a note on what's missed or over budget. An option that misses a must-have stays on the report but is greyed out, so it reads as not really an option. When the client wants a better layout, the keep options include the cost of re-planning inside the existing walls.

How uncertainty is shown

• Every cost, carbon and bill figure is a low–high range, not a single number, and is rounded so it doesn't look more precise than it is.
• Carbon is shown as a band first, because the comparison between options is more reliable than the exact tonnes.
• Every rate in the engine carries a source ID and a reliability rating (below). Figures we've had to judge ourselves are marked "internally assessed" and are being checked through the pilot.
• Site potential is a quick check from public maps, written as "could" and "might", not a planning assessment.

How we check it

We test the engine against real projects, not just against its sources. Projects are anonymised and never named on reports or on this site.

• Built homes, final costs. 19 built Sydney and NSW homes with published or known final costs, adjusted to 2026 prices. 15 of the 19 land inside or overlap the engine's cost range. The other 4 cost more than the top of the range, by about 21–29%: two inner-city terrace renovations, a cottage extension near Wollongong, and a pair of three-storey townhouses, which the engine doesn't model yet. A cost loading for inner-Sydney terraces is next on our list.
• Quantity surveyor estimates. 27 QS cost reports on 25 houses from council DA records, mostly inner-city and south-west Sydney. 14 of the 27 land inside the engine's range and 19 within 5% of it. The engine can run high, by up to about a third, for suburban knock-down rebuilds at a standard finish, and low when a project adds a basement, a pool or a lift, which it doesn't price yet.
• Built homes, measured carbon. 5 built NSW homes with a measured upfront carbon figure (life cycle assessments). 3 land inside the engine's range and the other 2 are 2–3% below its low end.

An honest caveat: the cost rates were tuned using these same built homes, so these results show the engine fits them, not that it predicts every new project. The real test is finished pilot projects, and we'll publish those results as they come in.

Limitations

• Estimates are for typical homes of each type and era, not an inspection of a specific building. Real projects can differ significantly.
• Only Greater Sydney houses are covered for now. Apartments, townhouses and other regions come later (see the roadmap).
• Prices change, and some sources are older or secondary; the reliability column says which.
• The comfort score and some build-time figures are internal judgement calls with low accuracy.
• It is not a quote, an energy rating, or a structural, heritage, planning, bushfire or flood assessment. See the disclaimer.

Sources

The engine's current source list (engine v0.3.9, October 2026). Links open the original.

See it worked through

The full working behind the Croydon sample: what we were told, what we worked out about the house, what each option includes, and every total before rounding.

How we got here: the Croydon sample → The sample comparison → Five challenging scenarios →

Last updated 12 October 2026.