Budgeting for Your House Extension: Costs, Scope & Getting an Accurate Quote

Introduction

Budgeting isn’t just about pulling a figure from the sky—it’s about setting a realistic foundation for your extension that reflects your goals, your site conditions, and your available funds. House extensions are major financial undertakings, and too often they stall or blow out because the budget wasn’t matched to the actual scope. Whether you’re adding a rear extension, going up with a second-storey addition, or building a detached granny flat, having clarity on costs at every stage is the key to delivering a stress-free renovation.

This guide will take you deep into what really drives house extension costs, how scope and size impact price, and why working backwards from budget (rather than design) is the smarter approach. With AIreno, both homeowners and trades get the advantage of a transparent, automated, and data-driven quoting engine that turns confusion into confidence.

What Does a House Extension Cost in Australia?

It depends on where you live, what you’re building, and how high you want to go with finishes. Here’s a general guide to what most extensions cost around Australia:

• Single-storey extension: $2,500 to $4,000 per square metre
• Second-storey extension: $3,000 to $4,500 per square metre
• Granny flat or separate dwelling: $120,000 to $300,000+

Keep in mind:

• These figures generally include full construction but exclude design, approvals, landscaping, and major site works.
• Prices in major metros like Sydney and Melbourne tend to sit at the higher end, especially where access is difficult or permits are restrictive.
• Sloping blocks, excavation, stormwater management, bushfire zones, heritage overlays—all can drive the price up fast.

AIreno tip: Our pricing engine automatically adjusts for location, site type, and complexity. It’s like having a QS in your pocket, minus the spreadsheet.

What Really Impacts the Cost of an Extension?

There are dozens of variables that influence price, but here are the most critical:

1. Size of the Extension:
   • Every square metre adds materials, labour, engineering, insulation, and finishing.
   • Bigger isn’t always better—be strategic with space. Sometimes removing a wall is more effective than building out.
2. Layout & Function:
   • Wet areas (bathrooms, laundries, kitchens) are the most expensive per sqm.
   • Open-plan spaces cost more to engineer and can require steel beams for support.
3. Level of Finish:
   • A basic finish might use laminate benchtops and floating floors. A luxury finish includes custom joinery, stone, designer lighting, etc.
   • Your finishes can make up 20-40% of your build cost.
4. Access to Site:
   • Is there a driveway? Can materials be delivered easily? Will scaffolding or cranes be required?
   • Builders factor in time and effort—tight sites mean higher labour costs.
5. Structural & Planning Considerations:
   • Adding a second storey? You may need reinforcement of existing structures.
   • On a heritage block or in a flood/fire zone? Expect extra approvals, materials, and delays.
6. Type of Approval Required:
   • CDC (Complying Development Certificate) is faster and cheaper than a full DA (Development Application), but not always possible.
   • AIreno can help you identify your pathway early—saving months.

Aligning Budget With Scope: Plan Backwards to Avoid Blowouts

Too many homeowners start by designing their dream extension, only to find it’s double their budget. The smarter way is to start with your number, then shape the design to fit.

AIreno flips the process on its head.

• Enter your ballpark budget
• Choose your preferred type of extension
• Instantly see what size and finish level fits your number

You can then adjust scope (reduce square metres, simplify layout, change finish level) in real time until everything lines up.

For builders and trades:

• Every lead you get has already been budget-aligned
• No more quoting on unrealistic designs
• You walk into conversations with educated clients and clear scopes

How AIreno Delivers Instant, Accurate Quotes

AIreno removes the friction from the quoting process. Instead of waiting days for a builder to estimate, you get a detailed quote in minutes.

The process for homeowners:

1. Select your project type (rear, second storey, garage conversion, etc.)
2. Enter rough dimensions, site conditions, and finish level
3. AIreno instantly generates a breakdown by stage (site prep, structure, fitout, etc.)
4. A planner helps refine the scope and remove any gaps or oversights
5. You get matched to builders ready to quote, already aligned to your brief

The result:

• No more conflicting estimates
• No hidden costs
• No chasing trades for weeks

What’s Actually Included in a Full Extension Quote on AIreno?

Unlike many rough calculators, AIreno gives you a complete cost overview, including:

• Site establishment & demolition
• Foundations and structural framing
• Roofing, insulation, and cladding
• Internal fitout: walls, flooring, painting
• Plumbing, electrical, and HVAC rough-in
• Fixtures, fittings, cabinetry, and appliances (if applicable)
• Council fees, private certifiers, and compliance costs
• Final clean-up and rubbish removal

You also get optional inclusions: landscaping, driveways, decks, and smart home upgrades.

Why Builders Love AIreno: Faster Quotes, Better Clients

AIreno isn’t just built for homeowners—it’s a secret weapon for smart builders.

Here’s how it works:

• You set your real pricing by location, scope, and type
• AIreno uses this to create instant estimates for homeowners using your pricing
• By the time they reach out, they’re already pre-sold on your rate, scope, and timeline

You get:

• Fewer tyre-kickers
• More conversions
• Less time quoting, more time building

AIreno essentially acts like your digital sales team—educating, qualifying, and delivering scoped jobs on autopilot.

Conclusion

Budgeting right is the foundation of a successful house extension. With AIreno, you get clarity on costs from the start, tools to keep your scope realistic, and support to make smarter choices. Whether you’re a homeowner planning the next stage of your life or a contractor looking to grow your pipeline without the admin, AIreno gives you the edge.

Your budget isn’t just a number—it’s your blueprint. Build with it.

Next up in our House Extension series: Designing Your Extension: Layout, Functionality & Choosing Materials.