Use AI to Get Early Cost Estimates for Your Sydney Renovation — The Exact Prompts to Use

One of the most common questions we hear from homeowners at the start of a project is: “What’s it going to cost?”

It’s also one of the hardest questions to answer accurately at the early stage — before a designer has assessed your site, before a builder has seen the drawings, and before anyone really knows the full scope of what you’re planning.

But here’s something that’s changed recently: AI tools like ChatGPT, Claude, and Google’s Gemini have become genuinely useful for getting early-stage ballpark cost estimates. Not precise figures — not yet — but indicative ranges that can help you sense-check whether a project is broadly feasible at your budget before you spend money on design fees.

The key is knowing how to ask. A vague prompt gets a vague answer. A well-structured prompt — one that gives the AI the specific variables it needs — gets a much more useful response.

Below are the best prompts we’ve developed for four common residential project types. Copy them, fill in your details, and paste them into any major AI tool. Then read the results critically — and use them as a starting point for conversation with your building designer, not as a substitute for professional advice.

1. Additions & Alterations (First-Floor Addition)

This is the most nuanced category because costs vary significantly depending on your existing structure, the number of rooms, and the finish level. The prompt below gives the AI enough context to return a useful indicative range.

📋 THE PROMPT — Copy, fill in the brackets, and paste into your AI tool:

I’m planning a first-floor addition to my existing single-storey home in [SUBURB], Sydney, NSW. I want an indicative construction cost range to help with early budgeting. Here are the details:Existing home: [e.g. 3-bedroom 1970s brick veneer, approximately 140m² single storey] Proposed addition: First floor with [NUMBER] bedrooms, [NUMBER] bathrooms, [include/exclude] ensuite to master, [include/exclude] parents’ retreat or sitting room Approximate size of new upper level: [e.g. approximately 80m²] Level of finishes: [Choose one: Basic / Mid-range / High-end]Basic = builder’s grade fixtures, laminate benchtops, standard tiles Mid-range = stone benchtops, quality tapware, engineered timber floors, custom joinery High-end = bespoke cabinetry, premium stone, imported fixtures, architectural detailingStructural notes: [e.g. I believe the existing roof will need to be removed / I’m not sure if the existing structure can take an upper level / the home is brick and I’m unsure about the foundation capacity]Suburb: [SUBURB] — please factor in typical Sydney construction cost levels for this area.Please give me:An indicative construction cost range (excluding design, engineering, and council fees) An estimate of additional professional and approval costs (design, structural engineering, DA or CDC fees, certifier) A note on the key variables that could push this higher or lower A rough total project cost range including all of the aboveI understand this is an indicative estimate only and not a professional quote. Please be specific with numbers rather than giving ranges that are too broad to be useful.

What to do with the result:

Look at the midpoint of the range the AI returns — that’s your working number for early feasibility thinking. If the midpoint is well within your budget, you’re likely in a good position to proceed to a formal feasibility assessment with a building designer. If it’s significantly over, it’s worth having a conversation about scope before you commission design work.

Pay particular attention to what the AI flags as cost variables — structural unknowns, site access, and service relocations (particularly in older homes where plumbing and electrical are being moved) are the most common sources of budget blowout.

2. New Dwelling (Knockdown Rebuild or Vacant Block)

New dwelling costs are more predictable than alterations because you’re starting from scratch — but suburb, size, and specification still drive enormous variation. This prompt structures the question so the AI can return useful numbers.

📋 THE PROMPT — Copy, fill in the brackets, and paste into your AI tool:

I’m planning to build a new home in [SUBURB], Sydney, NSW. I want an indicative construction cost range for early budgeting purposes. Here are the details:Project type: [Knockdown rebuild / New build on vacant land] Site: Approximately [SITE AREA]m², [flat / gently sloping / steep] site, [standard street frontage / corner block / battle-axe block] Proposed dwelling: [NUMBER] bedrooms, [NUMBER] bathrooms, [single storey / double storey], approximately [TOTAL FLOOR AREA]m² of gross floor area (if known)Key spaces to include:[e.g. Open plan kitchen, dining, living] [e.g. Separate media room] [e.g. Double garage] [e.g. Covered alfresco] [e.g. Home office]Level of finishes: [Basic / Mid-range / High-end]Basic = project home level, standard inclusions, off-the-shelf fixtures Mid-range = custom design, stone benchtops, quality joinery, engineered timber floors, good quality tapware and fittings High-end = fully bespoke design, premium imported materials, architect-level detailing, landscaping includedAdditional considerations: [e.g. Heritage Conservation Area / bushfire prone land / flood planning area / steeply sloped site requiring significant excavation — include any that apply, or note “none identified”]Please provide:An indicative construction cost range for the new dwelling only (excluding site costs, demolition, design, and council fees) If it’s a knockdown rebuild — an estimate of demolition costs Estimated design, engineering, and approval costs (building designer or architect, structural engineer, CDC or DA fees, certifier) A note on the key cost drivers and what would move this up or down An indicative total project cost range including all of the abovePlease be specific with numbers rather than overly broad ranges. I understand this is indicative only and not a professional quote.

What to do with the result:

For a knockdown rebuild, compare the total project cost against the cost of buying an already-renovated equivalent property in the same suburb — including stamp duty. In many Sydney suburbs the numbers are closer than people expect, and in some, the rebuild is the more cost-effective path to the home you want.

For a new build on vacant land, the AI’s estimate combined with your purchase price gives you a rough total cost of the finished asset — which you can compare against comparable sales to sense-check whether the project stacks up financially.

3. Secondary Dwelling (Granny Flat)

Secondary dwellings have relatively defined parameters under the NSW Housing Code — maximum 60m² for CDC approval — which actually makes early cost estimation more reliable than for larger projects. Use this prompt to get a useful indicative number quickly.

📋 THE PROMPT — Copy, fill in the brackets, and paste into your AI tool:

I’m planning to build a secondary dwelling (granny flat) on my residential property in [SUBURB], Sydney, NSW. I want an indicative cost range for early budgeting. Here are the details:Site: Approximately [SITE AREA]m² total lot area, [flat / sloping] site Main dwelling: [e.g. Existing 3-bedroom house occupying approximately 180m²] Proposed secondary dwelling:[Detached / Attached to main dwelling / Above garage] [NUMBER] bedrooms [NUMBER] bathrooms Approximately [SIZE]m² in floor area (maximum 60m² for CDC approval in NSW) [Include / Exclude] covered outdoor areaLevel of finishes: [Basic / Mid-range / High-end]Basic = standard kitchen and bathroom, tiled floors, laminate benchtops, builder’s grade fixtures Mid-range = stone benchtops, quality tapware, engineered timber or quality tile floors, custom joinery High-end = premium finishes throughout, bespoke cabinetry, high-specification appliancesIntended use: [Rental income / ageing parent / adult child / Airbnb / flexible use]Site considerations: [e.g. Existing concrete slab / need to demolish existing structure / sloping site requiring cut and fill / limited site access for machinery — note any that apply, or “none identified”]Please provide:An indicative construction cost range for the secondary dwelling Estimated design and approval costs (building designer, CDC certifier, council contributions if applicable, utility connections) An indicative total project cost An estimated weekly rental yield range for a secondary dwelling of this size and specification in [SUBURB] A rough payback period based on rental income vs. total project cost The key variables that most affect cost for this type of projectI understand this is indicative only. Please be specific rather than overly broad.

What to do with the result:

The rental yield and payback period estimates are particularly useful here — they give you a rough financial case for the investment that you can pressure-test against your own circumstances. Note that the AI’s rental estimates should be cross-checked against current listings in your suburb on Domain or realestate.com.au before you rely on them for financial planning.

Also pay attention to what the AI says about utility connections — a separate electrical meter, water connection, and gas connection for the secondary dwelling are real costs that are often excluded from builder quotes and can add $10,000–$20,000 to the total.

4. Dual Occupancy (Duplex)

Dual occupancy is the most complex of the four project types to cost early, because it involves the most variables — site configuration, the relationship between the two dwellings, the structure, and whether subdivision is intended. This prompt gives the AI the context to return a useful range.

📋 THE PROMPT — Copy, fill in the brackets, and paste into your AI tool:

I’m planning a dual occupancy development on my residential property in [SUBURB], Sydney, NSW. I want an indicative cost range for early feasibility. Here are the details:Site: Approximately [SITE AREA]m² total lot area, [flat / gently sloping / steep] site, [standard street frontage / corner block / battle-axe] Existing structure: [e.g. Existing 3-bedroom house to be demolished / existing house to be retained as Dwelling 1 / vacant land]Proposed development type: [Choose one]Attached dual occupancy (two dwellings sharing a common wall — duplex style) Detached dual occupancy (two separate dwellings on the same lot)Dwelling 1: [NUMBER] bedrooms, [NUMBER] bathrooms, approximately [SIZE]m², [single / double storey] Dwelling 2: [NUMBER] bedrooms, [NUMBER] bathrooms, approximately [SIZE]m², [single / double storey]Level of finishes (both dwellings): [Basic / Mid-range / High-end]Basic = project home level, standard inclusions Mid-range = custom design, stone benchtops, quality joinery and fixtures High-end = premium bespoke finishes, high specification throughoutSubdivision intent: [Yes — intending to Torrens title subdivide and sell separately / No — retaining both dwellings / Strata title]Additional context: [e.g. Demolition of existing dwelling required / Heritage Conservation Area / bushfire prone land / any known planning constraints — or “none identified”]Please provide:An indicative construction cost range for both dwellings combined If demolition is required — estimated demolition cost Estimated design, engineering, and approval costs (building designer, structural engineer, DA application, certifier, survey, subdivision costs if applicable) An indicative total project cost If subdivision is intended — an indicative end value of each lot and therefore gross development margin The key cost drivers and risks for this type of project A note on likely DA timeframes for a dual occupancy in [SUBURB]I understand this is indicative only and not a professional quote or planning advice. Please be specific rather than overly broad.

What to do with the result:

For a dual occupancy, the gross development margin estimate is the critical number — the difference between your total project cost (land plus construction plus all fees) and the combined end value of the two dwellings. In Sydney’s current market, viable dual occupancy projects typically target a gross margin of 20–25% or more. If the AI’s rough numbers suggest a tighter margin than that, it’s worth a detailed feasibility assessment before committing to design.

How to Use These Results Responsibly

AI cost estimates are a starting point, not an endpoint. Here’s how to treat them:

Use them to test feasibility, not to set your budget. If the AI returns a range of $380,000–$520,000 for your first-floor addition, the useful information is that you’re probably in the $400,000–$500,000 ballpark — not that your budget of $280,000 will work if you negotiate hard.

Cross-check across multiple AI tools. Run the same prompt through ChatGPT, Claude, and Gemini. If the ranges are broadly consistent, you have more confidence. If they vary significantly, the project has higher-than-usual cost uncertainty and you should be conservative.

Update the prompt with new information as you get it. When a structural engineer tells you the existing footings need underpinning, add that to the prompt and see how it affects the estimate. AI tools are most useful as a running check as your understanding of the project grows.

Use the result to have a better conversation with your designer. Walking into a first meeting with an AI-generated indicative range — and the reasoning behind it — is far more productive than asking “how much do you think it’ll cost?” with no context. It helps your designer calibrate the brief, identify where the risks lie, and have an honest conversation about budget from day one.

The One Thing AI Can’t Tell You

AI tools have no knowledge of your specific site — your soil conditions, your existing structure, your council’s current assessment culture, or the planning controls that apply to your exact lot. They can give you a useful market-rate estimate. They cannot tell you whether your project is approvable, whether your existing structure can support an upper level, or whether there’s a sewer easement running through the spot you want to build on.

That’s what a feasibility assessment from an experienced building designer is for. And it’s almost always the most valuable thing you can spend your first few hundred dollars on.

At Plan Land, our feasibility assessments cover exactly what AI can’t — the site-specific constraints, the applicable planning controls, the realistic approval pathway, and an honest conversation about budget based on your actual brief and your actual block.

Ready to take the next step? Start with the prompts above, get a feel for the numbers, then come and talk to us.

📍 Plan Land — residential building design across Sydney, NSW. planland.com.au/contact

Disclaimer: AI-generated cost estimates are indicative only and should not be relied upon for financial, planning, or construction decisions. Always seek advice from a qualified building designer, builder, and financial adviser before committing to any project.