The Croydon sample is a family who could afford to rebuild. These five scenarios test the engine: heritage, redevelopment pressure, a close call, bushfire and a house past saving. Each is a real Sydney house with an invented brief, run through the same engine as the sample.
“We bought it knowing it was a wreck. We want to save the old front rooms, but we need a proper kitchen and a third bedroom, and it has to stop being damp and freezing.”
Goals: must have space, comfort, bills · staying about 20 years
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Estimates
Retain
Retrofit
Extend
Replace
Meets the brief
Misses a must-have
Misses a must-have
Meets, over budget
Meets, over budget
Cost
$65k–$240k
$190k–$600k
$750k–$1.6m
$870k–$1.4m
Carbon
Low
Low
Medium
High
Materials & waste
95%+ kept1–5 t to landfill
About 95% kept2–7 t to landfill
About 80% kept4–20 t to landfill
Under 10% kept17–55 t to landfill
Floor area
No change
No change
+25 m²
About 140 m²
Running costsEnergy bills vs today
Today: 100%
About 70–75% of today
About 80–85% of today
About 70–75% of today
Comfort & health
1 out of 5
3.5 out of 5
3 out of 5
4 out of 5
DeliveryTime, disruption, BASIX
High3–6 months, may need to move out during the repairs · No BASIX needed
High3–6 months, may need to move out during the repairs · BASIX likely if approval needed
High + heritage risk10–16 months, approval needed · BASIX for alterations
Highest, demolition may not be approved12–24 months, move out · Full new-home BASIX, 7 stars
Greyed out: misses a must-have. Extend figures cover the 25 m² addition plus ceiling insulation and draught sealing in the existing house. Extend includes a full repair of the structure; Retain and Retrofit include urgent structural repairs only.
Suggested option for this brief
Extend.
Extending this house may cost more than replacing it.
Adding a 25 m² extension with a light retrofit of the existing house is estimated to meet all three goals: more space, better comfort and lower bills, for roughly $750k–$1.6m. Especially since you need more space, extending looks like a good fit for this brief. That's about the same cost and about 70% of the carbon of replacing. The house is in poor condition with major structural problems: repairing it properly as part of an extension adds a lot, so get a structural engineer and a quantity surveyor to confirm the numbers. It keeps most of the existing home. At this cost, Replace is close enough that it's worth costing properly too. It's above the budget, as none of the options that meet the must-haves fit within it.
Your architect or building designer can design the addition to suit the site and the existing house, check what council will need, and find a builder experienced in additions.
What your client readsMoreLess
What your client reads · Retain
For your home, Retain means keeping the house and fixing only what's urgent: paint, floors and repairs plus urgent structural repairs (stumps, termites, damp and leaks). It doesn't include a full structural repair, a new roof or cladding, or renovating inside; a structural engineer can confirm what's urgent.
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It won't add the space you've asked for, but rethinking how the rooms you already have are used can feel like adding space without building more. It leaves the cold rooms and draughts as they are. Your architect or building designer can help you decide whether that's enough, and how to make the rooms you have work harder.
What your client reads · Retrofit
For your home, Retrofit means keeping the structure and improving how it feels to live in: ceiling insulation, insulated wall linings, underfloor insulation, draught sealing, double glazing the worst windows, efficient reverse-cycle heating and cooling and fixing the damp. It could make the home noticeably more comfortable and cut your energy bills by around a quarter. The cost also covers a new kitchen and bathrooms. It includes urgent structural repairs (stumps, termites, damp and leaks), not a full repair of the structure.
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It doesn't add space; Extend does. Your architect or building designer can bring in an energy assessor to pick the upgrades that give the best value.
What your client reads · Extend
For your home, Extend means keeping the existing house and adding about 25 m² at ground level: the extra living space you're after, while most of what's there stays. The cost also covers a new kitchen and bathrooms. Unlike Retain and Retrofit, it includes a full structural repair: new stumps and subfloor, frame repairs, a new roof and cladding, damp and termite work.
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While the builders are there, it adds ceiling insulation and seals draughts. A full retrofit (walls, windows) could add more comfort and lower bills. Your architect or building designer can test how the addition fits the site and what council will need.
What your client reads · Replace
For your home, Replace means demolishing the house and building a new home of around 140 m² to current energy standards: full control over the layout, and as comfortable as the build quality allows.
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It's also the most expensive option, carries the most carbon and waste, and usually means moving out for 12–24 months. It can be the better choice when a home is beyond saving, or when keeping it can't give you what you need. In a heritage area, demolition may not be approved. Your architect or building designer can help confirm that, and plan the new home.
Site potential · quick estimate onlyMoreLess
Zoned R1 General residential. At an FSR of 1.25:1, this 103 m² site may allow about 129 m² of floor area, close to the current house. The desired new house is bigger than the FSR allows.
Carramar, Sydney
A post-war fibro cottage in Carramar.
Why it's hard: Retirees on a high-density lot, with developers knocking on the door.
“We're retired and happy here for another ten years or so. Developers keep knocking on the door, so we want to know whether fixing up the house is money well spent.”
Goals: must have comfort, bills · staying about 10 years
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Estimates
Retain
Retrofit
Extend
Replace
Meets the brief
Misses a must-have
Meets
Misses a must-have
Meets, over budget
Cost
$15k–$45k
$50k–$140k
$150k–$280k
$350k–$680k
Carbon
Low
Low
Medium
High
Materials & waste
About 90% keptup to 3 t to landfill
About 90% kept1–3 t to landfill
About 95% keptup to 3 t to landfill
Under 10% kept5–17 t to landfill
Floor area
No change
No change
+40 m²
About 105 m²
Running costsEnergy bills vs today
Today: 100%
About 70–75% of today
About 95% of today
About 70–75% of today
Comfort & health
1.5 out of 5
3 out of 5
2.5 out of 5
4 out of 5
DeliveryTime, disruption, BASIX
Low2–8 weeks, stay in the home · No BASIX needed
Moderate1–3 months, mostly stay in the home · BASIX likely if approval needed
Moderate to high4–9 months, approval needed · BASIX for alterations
Highest9–18 months, move out · Full new-home BASIX, 7 stars
Greyed out: misses a must-have. No extension size was given, so Extend assumes a 40 m² addition. Extend figures cover the 40 m² addition plus ceiling insulation and draught sealing in the existing house.
Suggested option for this brief
Retrofit.
Retrofitting the existing house is estimated to meet both goals, better comfort and lower bills, for roughly $50k–$140k. Especially since you don't need more space and the house is uncomfortable today, retrofitting looks like a good fit for this brief. That's less than half the cost and less than half the carbon of replacing. It keeps most of the existing home. The site may suit more than one home: see Site potential on page 2.
Your architect or building designer can plan the upgrades in the right order and bring in an energy assessor and a builder experienced in retrofits, so the work delivers the best value.
What your client readsMoreLess
What your client reads · Retain
For your home, Retain means keeping it as it is and refreshing how it works day to day: paint, floors and repairs. It's the quickest option, and keeps almost everything you already have.
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It leaves the cold rooms and draughts as they are. Your architect or building designer can help you decide whether that's enough, and how to make the rooms you have work harder.
What your client reads · Retrofit
For your home, Retrofit means keeping the structure and improving how it feels to live in: insulated wall linings, underfloor insulation, draught sealing, double glazing the worst windows and efficient reverse-cycle heating and cooling. It could make the home noticeably more comfortable and cut your energy bills by around a quarter.
MoreLess
You can stay in the home for most of the work. Your architect or building designer can bring in an energy assessor to pick the upgrades that give the best value.
What your client reads · Extend
For your home, Extend means keeping the existing house and adding about 40 m² at ground level while most of what's there stays.
MoreLess
While the builders are there, it adds ceiling insulation and seals draughts. A full retrofit (walls, windows) could add more comfort and lower bills. Your architect or building designer can test how the addition fits the site and what council will need.
What your client reads · Replace
For your home, Replace means demolishing the house and building a new home of around 105 m² to current energy standards: full control over the layout, and as comfortable as the build quality allows. The zoning might also allow more than one home on the site.
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It's also the most expensive option, carries the most carbon and waste, and usually means moving out for 9–18 months. It can be the better choice when a home is beyond saving, or when keeping it can't give you what you need. Your architect or building designer can help confirm that, and plan the new home.
Site potential · quick estimate onlyMoreLess
Zoned R4 High density residential. At an FSR of 1.25:1, this 657 m² site may allow about 821 m² of floor area, 7.8 times the current house. At 657 m² in R4 with a height limit of 13 m, this site could suit apartments. Check the council's minimum lot size first.
Ryde, Sydney
A mid-century brick house in Ryde.
Why it's hard: A solid house, a big brief and a close call between extending and replacing.
“Solid old brick house, but it's dark and the layout doesn't work for three teenagers. Half our friends say renovate, half say knock down and build. We just want a straight answer.”
Goals: must have space, comfort, bills · staying about 30 years
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Estimates
Retain
Retrofit
Extend
Replace
Meets the brief
Misses a must-have
Misses a must-have
Meets
Meets, over budget
Cost
$80k–$210k
$110k–$280k
$630k–$1.1m
$910k–$1.45m
Carbon
Low
Low
Medium
High
Materials & waste
95%+ kept1–6 t to landfill
95%+ kept1–5 t to landfill
About 90% kept2–13 t to landfill
Under 10% kept17–55 t to landfill
Floor area
No change
No change
+110 m²
About 240 m²
Running costsEnergy bills vs today
Today: 100%
About 80–90% of today
About 80–90% of today
About 80–90% of today
Comfort & health
1.5 out of 5
3.5 out of 5
3.5 out of 5
4 out of 5
DeliveryTime, disruption, BASIX
Moderate3–6 months, may need to move out · No BASIX needed
Moderate to high3–6 months, may need to move out · BASIX likely if approval needed
Moderate to high6–12 months, approval needed · BASIX for alterations
Highest9–18 months, move out · Full new-home BASIX, 7 stars
Greyed out: misses a must-have. Extend figures include a deeper retrofit of the existing house, as a light retrofit would fall short of the goals.
Suggested option for this brief
Extend, with a deeper retrofit.
Adding a 110 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 $630k–$1.1m. 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 about 75% of the cost and about 70% 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.
Your architect or building designer can design the extension and the upgrades as one project, so the whole home works better, with one round of building work.
What your client readsMoreLess
What your client reads · Retain
For your home, Retain means keeping the house and renovating it, with no new space or energy upgrades: paint, floors and repairs, plus a new kitchen and bathrooms. It keeps the structure and most of what you already have.
MoreLess
It won't add the space you've asked for, but rethinking how the rooms you already have are used can feel like adding space without building more. It leaves the cold rooms and draughts as they are. Your architect or building designer can help you decide whether that's enough, and how to make the rooms you have work harder.
What your client reads · Retrofit
For your home, Retrofit means keeping the structure and improving how it feels to live in: ceiling insulation, wall insulation, underfloor insulation, draught sealing, double glazing the worst windows and efficient reverse-cycle heating and cooling. It could make the home noticeably more comfortable and cut your energy bills by around a fifth. Shading the west-facing living areas could also help in summer. The cost also covers a new kitchen and bathrooms.
MoreLess
It doesn't add space; Extend does. Your architect or building designer can bring in an energy assessor to pick the upgrades that give the best value.
What your client reads · Extend
For your home, Extend means keeping the existing house and adding about 110 m² upstairs: the extra living space you're after, while most of what's there stays. The cost also covers a new kitchen and bathrooms.
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It includes bringing the existing house up to standard, so the whole home is more comfortable, with more building work up front. A second storey usually means moving out for a while. Your architect or building designer can test how the addition fits the site and what council will need.
What your client reads · Replace
For your home, Replace means demolishing the house and building a new home of around 240 m² to current energy standards: full control over the layout, and as comfortable as the build quality allows. The zoning might also allow more than one home on the site.
MoreLess
It's also the most expensive option, carries the most carbon and waste, and usually means moving out for 9–18 months. It can be the better choice when a home is beyond saving, or when keeping it can't give you what you need. Your architect or building designer can help confirm that, and plan the new home.
Site potential · quick estimate onlyMoreLess
Zoned R2 Low density residential. At an FSR of 0.50:1, this 599 m² site may allow about 300 m² of floor area, 2.5 times the current house. At 599 m² in R2 with a minimum lot size of 580 m² and a height limit of 9.5 m, this site may suit a dual occupancy.
Chatswood West, Sydney
A weatherboard cottage in Chatswood West.
Why it's hard: The builder says knock it down, but the structure hasn't been checked and the site is bushfire-prone.
Weatherboard cottage90 m²Single storeyPoor conditionBushfire-proneSite 531 m²R2 zoneBudget Up to $1m
“We paid for the bush outlook, not the house. Our builder says knock it down, but we'd like to know if keeping some of it is realistic before we decide.”
Goals: must have space, comfort, bills · staying about 30 years
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Estimates
Retain
Retrofit
Extend
Replace
Meets the brief
Misses a must-have
Misses a must-have
Meets
Meets
Cost
$90k–$250k
$120k–$330k
$400k–$700k
$790k–$1.3m
Carbon
Low
Low
Medium
High
Materials & waste
About 70% kept2–8 t to landfill
About 85% kept1–5 t to landfill
About 85% kept1–7 t to landfill
Under 10% kept5–19 t to landfill
Floor area
No change
No change
+70 m²
About 200 m²
Running costsEnergy bills vs today
Today: 100%
About 80% of today
About 90% of today
About 80–85% of today
Comfort & health
1 out of 5
3.5 out of 5
3 out of 5
4 out of 5
DeliveryTime, disruption, BASIX
Moderate3–6 months, may need to move out · No BASIX needed
Moderate to high3–6 months, may need to move out · BASIX likely if approval needed
Moderate to high4–9 months, approval needed · BASIX for alterations
Highest9–18 months, move out · Full new-home BASIX, 7 stars
Greyed out: misses a must-have. Extend figures cover the 70 m² addition plus ceiling insulation and draught sealing in the existing house.
Suggested option for this brief
Extend.
Adding a 70 m² extension with a light retrofit of the existing house is estimated to meet all three goals: more space, better comfort and lower bills, for roughly $400k–$700k. Especially since you need more space, extending looks like a good fit for this brief. That's about 50% of the cost and about 60% of the carbon of replacing. It keeps most of the existing home.
Your architect or building designer can design the addition to suit the site and the existing house, check what council will need, and find a builder experienced in additions.
What your client readsMoreLess
What your client reads · Retain
For your home, Retain means keeping the house and renovating it, with no new space or energy upgrades: paint, floors and repairs, plus a new kitchen and bathrooms. It keeps the structure and most of what you already have.
MoreLess
It won't add the space you've asked for, but rethinking how the rooms you already have are used can feel like adding space without building more. It leaves the cold rooms and draughts as they are. Your architect or building designer can help you decide whether that's enough, and how to make the rooms you have work harder.
What your client reads · Retrofit
For your home, Retrofit means keeping the structure and improving how it feels to live in: ceiling insulation, wall insulation, underfloor insulation, draught sealing, double glazing the worst windows, efficient reverse-cycle heating and cooling and fixing the damp. It could make the home noticeably more comfortable and cut your energy bills by around a fifth. The cost also covers a new kitchen and bathrooms.
MoreLess
It doesn't add space; Extend does. Your architect or building designer can bring in an energy assessor to pick the upgrades that give the best value.
What your client reads · Extend
For your home, Extend means keeping the existing house and adding about 70 m² at ground level: the extra living space you're after, while most of what's there stays. The cost also covers a new kitchen and bathrooms.
MoreLess
While the builders are there, it adds ceiling insulation and seals draughts. A full retrofit (walls, windows) could add more comfort and lower bills. Your architect or building designer can test how the addition fits the site and what council will need.
What your client reads · Replace
For your home, Replace means demolishing the house and building a new home of around 200 m² to current energy standards: full control over the layout, and as comfortable as the build quality allows.
MoreLess
It's also the most expensive option, carries the most carbon and waste, and usually means moving out for 9–18 months. It can be the better choice when a home is beyond saving, or when keeping it can't give you what you need. Your architect or building designer can help confirm that, and plan the new home.
Site potential · quick estimate onlyMoreLess
Zoned R2 Low density residential. At an FSR of 0.50:1, this 531 m² site may allow about 266 m² of floor area, 3.0 times the current house. At 531 m², the site is under the 580 m² minimum lot size, so more than one home is unlikely.
Bankstown, Sydney
A post-war fibro cottage in Bankstown.
Why it's hard: A run-down fibro with termites and asbestos on a big block, and a family that wants two homes where there is one.
Post-war fibro cottage95 m²Single storeyPoor conditionTermites and dampSite 884 m²R2 zoneBudget around $1.5m
“The house has termites and asbestos, and it's falling apart. My parents are moving in with us, so we'd like two homes on the block: one for us, one for them. We have about $1.5m. Is there any point keeping it?”
Goals: must have space, comfort, bills · staying about 30 years
Scroll sideways to see all four pathways →
Estimates
Retain
Retrofit
Extend
Replace
Meets the brief
Misses a must-have
Misses a must-have
Misses a must-have
Meets
Cost
$60k–$180k
$160k–$450k
$1.05m–$1.65m
$1.15m–$1.75m
Carbon
Very low
Low
High
High
Materials & waste
About 80% kept1–5 t to landfill
About 80% kept1–5 t to landfill
About 45% kept5–25 t to landfill
Under 10% kept6–25 t to landfill
Floor area
No change
No change
+205 m²
About 300 m²
Running costsEnergy bills vs today
Today: 100%
About 80–85% of today
About 95% of today
About 85% of today
Comfort & health
1 out of 5
3.5 out of 5
3 out of 5
4 out of 5
DeliveryTime, disruption, BASIX
High3–6 months, may need to move out during the repairs · No BASIX needed
High3–6 months, may need to move out during the repairs · BASIX likely if approval needed
High10–16 months, approval needed · BASIX for alterations
Highest9–18 months, move out · Full new-home BASIX, 7 stars
Greyed out: misses a must-have. Replace is costed as 300 m² of new building: room for two homes of about 150 m² each (a dual occupancy). Extend is sized to the same floor area, a 205 m² addition plus ceiling insulation and draught sealing in the existing house.
Suggested option for this brief
Replace.
Replacing the house with a new 300 m² home is estimated to meet all three goals: more space, better comfort and lower bills, for roughly $1.15m–$1.75m. Especially since the house has major structural issues and the new home would be much bigger than today, replacing the house looks like a good fit for this brief. It carries the most carbon of the options, so a well-built, long-lasting home matters.
Your architect or building designer can help work out, with a structural engineer, whether the house is worth saving, and if not, design a new home that is efficient, long-lasting and makes the most of the site.
What your client readsMoreLess
What your client reads · Retain
For your home, Retain means keeping the house and fixing only what's urgent: paint, floors and repairs plus urgent structural repairs (stumps, termites, damp and leaks). It doesn't include a full structural repair, a new roof or cladding, or renovating inside; a structural engineer can confirm what's urgent.
MoreLess
It won't add the space you've asked for, but rethinking how the rooms you already have are used can feel like adding space without building more. It leaves the cold rooms and draughts as they are. Your architect or building designer can help you decide whether that's enough, and how to make the rooms you have work harder.
What your client reads · Retrofit
For your home, Retrofit means keeping the structure and improving how it feels to live in: ceiling insulation, insulated wall linings, underfloor insulation, draught sealing, double glazing the worst windows and fixing the damp. It could make the home noticeably more comfortable and cut your energy bills by around a fifth. Shading the west-facing living areas could also help in summer. The cost also covers a new kitchen and bathrooms. It includes urgent structural repairs (stumps, termites, damp and leaks), not a full repair of the structure.
MoreLess
It doesn't add space; Extend does. Your architect or building designer can bring in an energy assessor to pick the upgrades that give the best value.
What your client reads · Extend
For your home, Extend means keeping the existing house and adding about 205 m² at ground level: the extra living space you're after, while most of what's there stays. The cost also covers a new kitchen and bathrooms. Unlike Retain and Retrofit, it includes a full structural repair: new stumps and subfloor, frame repairs, a new roof and cladding, damp and termite work.
MoreLess
While the builders are there, it adds ceiling insulation and seals draughts. A full retrofit (walls, windows) could add more comfort and lower bills. Your architect or building designer can test how the addition fits the site and what council will need.
What your client reads · Replace
For your home, Replace means demolishing the house and building a new home of around 300 m² to current energy standards: full control over the layout, and as comfortable as the build quality allows. The zoning might also allow more than one home on the site.
MoreLess
It's also the most expensive option, carries the most carbon and waste, and usually means moving out for 9–18 months. It can be the better choice when a home is beyond saving, or when keeping it can't give you what you need. Your architect or building designer can help confirm that, and plan the new home.
Site potential · quick estimate onlyMoreLess
Zoned R2 Low density residential. At an FSR of 0.50:1, this 884 m² site may allow about 442 m² of floor area, 4.7 times the current house. At 884 m² in R2 with a minimum lot size of 450 m² and a height limit of 9 m, this site may suit a dual occupancy.
Illustrative examples based on real houses, with invented briefs. Street numbers are not shown, and none of this is an assessment of these homes. Indicative, order-of-magnitude estimates for early feasibility discussion, calibrated to Sydney. Not a quote, and not a structural, planning or heritage assessment. Full disclaimer.
