How Much Does It Cost to Build a Granny Flat?

A granny flat can be one of the smartest additions you make to a property. Whether you’re creating space for family, building a private home office, or adding a second dwelling to generate rental income, it’s a project that can deliver long-term value — if it’s planned properly.

But the first question most homeowners ask is the big one: how much does it cost to build a granny flat?

The honest answer is: it depends on the size, design, inclusions, site conditions, and approvals. In this guide, we’ll break down the real-world cost ranges, what drives the price up or down, and what to consider before you start.

Average Cost to Build a Granny Flat in Australia

As a general guide, most granny flats fall into these rough price ranges:

• Basic granny flat (simple finish, standard inclusions): $120,000 – $180,000
• Mid-range granny flat (better layout, upgraded finishes): $180,000 – $250,000
• High-end granny flat (premium design, high-spec materials): $250,000 – $350,000+

These figures typically cover a turnkey build, but not always. Some quotes include everything from approvals through to landscaping. Others may exclude items like site works, service connections, retaining walls, driveways, or upgrades needed to meet council requirements.

How Size Impacts Granny Flat Cost

Size is one of the biggest cost drivers, but not always in a perfectly linear way. A 30m² granny flat isn’t half the price of a 60m² one — because the “fixed costs” still exist.

Here’s what often happens:

• Smaller granny flats can look expensive per square metre because approvals, services, kitchen, bathroom, and connections still need to happen.
• Larger granny flats usually offer better value per square metre, but total project cost increases as you add rooms, materials, and finishing.

Most granny flats are somewhere between 40m² and 70m², with 60m² being a common sweet spot for a comfortable 2-bedroom layout.

What’s Included in a Granny Flat Build?

If you’re comparing quotes, it helps to know what “build cost” usually includes.

A well-scoped granny flat build may include:

• Design and drafting (or plans supplied by you)
• Engineering (as required)
• Building approvals and certification
• Slab or footings
• Framing, external cladding/brickwork
• Roof and gutters
• Windows and external doors
• Internal walls, plastering, and paint
• Kitchen and bathroom fit-out
• Electrical and plumbing rough-in and fit-off
• Flooring
• Basic fixtures and fittings
• Final handover and compliance documentation

But here’s where costs can blow out: site works and services. These are often the biggest difference between a “cheap-looking” quote and the actual total cost once you start.

The Biggest Cost Factors That Affect Granny Flat Pricing

1. Site Works and Ground Conditions

Site works are the hidden costs people don’t see coming. Factors that can increase site costs include:

• Sloping blocks needing retaining walls
• Poor soil requiring additional excavation or piering
• Limited access (tight driveways, narrow side access)
• Tree removal
• Drainage requirements
• Existing services being difficult to connect

If the site is flat, accessible, and has easy connection points, your build is usually more straightforward.

2. Service Connections (Water, Sewer, Power)

Connecting a granny flat to services can range from simple to expensive.

Costs vary depending on:

• Distance from the main dwelling to the granny flat
• Whether sewer connection is nearby or needs extension
• Whether the existing switchboard can handle extra load
• Stormwater requirements and where water needs to discharge

Sometimes, upgrades are required to meet modern standards — especially if the existing dwelling is older.

3. Design Complexity

A simple rectangular footprint is typically cheaper to build than a design with multiple offsets, feature rooflines, or complicated layouts.

Design choices that often add cost:

• Higher ceilings
• Raked ceilings
• Large spans that require structural beams
• Multiple roof pitches
• Extra glazing, stacker doors, or feature windows
• Custom joinery and built-ins

The more complex the design, the higher the labour and material costs.

4. Inclusions and Finish Level

Your selections can shift your budget quickly.

Upgrades that add noticeable cost include:

• Stone benchtops
• Higher-end appliances
• Premium tapware and bathroom fittings
• Tiling to ceilings
• Feature lighting
• Custom cabinetry
• Upgraded flooring (engineered timber, high-end vinyl plank, etc.)
• Air conditioning
• Acoustic insulation upgrades

A granny flat can look incredible without being over-the-top, but it helps to decide early where you want to invest and where you want to keep things standard.

5. Council and Compliance Requirements

Rules vary by location, and compliance can impact design and cost. Requirements can include:

• Setbacks and siting restrictions
• Private open space
• Fire separation distances
• Bushfire requirements (BAL ratings)
• Flood zones
• Stormwater management and drainage
• Parking requirements (in some cases)

These factors can affect not only approvals, but also what you need to build — such as upgraded windows, fire-rated materials, or additional drainage.

Granny Flat Cost Breakdown: Where the Money Goes

While every project is different, a typical cost breakdown might look like this:

• Design, approvals, and engineering: 5% – 10%
• Site works and slab/foundations: 10% – 20%
• Structure (frame, roof, external shell): 25% – 35%
• Services (plumbing, electrical): 10% – 15%
• Internal finishes (plaster, paint, flooring, tiling): 10% – 20%
• Kitchen and bathroom fit-out: 10% – 20%
• External works (paths, stairs, deck, landscaping): 5% – 15%

Site works and external works are the areas most likely to vary significantly.

Cost Differences: One-Bedroom vs Two-Bedroom Granny Flats

A one-bedroom granny flat is often cheaper, but only by a margin — because the kitchen, bathroom, approvals, and connections remain similar.

As a rough guide:

• 1-bedroom granny flat: $140,000 – $220,000
• 2-bedroom granny flat: $170,000 – $280,000+

A two-bedroom layout is often popular for resale value and rental flexibility, but it depends on your goals and the size limits that apply to your property.

Prefab vs Custom Build: Is Prefab Cheaper?

Prefab granny flats can be a cost-effective option, but “cheaper” isn’t guaranteed — especially once you factor in everything outside the unit itself.

Prefab might suit you if:

• You want a standard design
• Site access is good for delivery
• You’re happy with limited customisation
• Timelines are tight and you want a quicker build process

Custom builds might be better if:

• Your block has tricky access or slope
• You want the granny flat to match the existing home
• You want a specific layout, look, or inclusion level
• You want a higher-end finish

In many cases, the real difference comes down to site works and services, not whether the structure arrives as a prefab unit or is built on-site.

How Much Extra for Site Works, Driveways, and Landscaping?

This is one of the most overlooked areas of budgeting. A granny flat might be complete, but if it needs access, privacy, drainage, and outdoor areas, you’ll want to plan for those costs.

Potential extras include:

• Concrete paths or driveway extensions
• Decks, stairs, or ramps
• Fencing and screening
• Turf or landscaping
• Retaining walls
• Extra drainage pits or soak wells
• External lighting

Depending on the site, external works can add anywhere from $5,000 to $50,000+.

Hidden Costs to Watch Out For

Even with a detailed quote, there are common areas that catch people out:

• Upgrades to existing power supply or switchboard
• Sewer connection upgrades
• Stormwater requirements
• Rock excavation
• Tree removal permits and arborist reports
• Acoustic upgrades if close to boundaries
• Bushfire or flood compliance requirements
• Temporary fencing and site access costs

A good builder will flag these early and allow for them in the scope — or at least explain what needs confirming before final pricing.

Can a Granny Flat Increase Property Value?

In many cases, yes — but it depends on your market and how the granny flat is designed.

A well-built granny flat can:

• Create rental income potential
• Add flexibility for multi-generational living
• Improve buyer appeal for families
• Increase usable living space on the property

If rental income is part of your plan, it’s worth considering privacy, separate access, storage, and a layout that feels like a true home — not an add-on.

How Long Does It Take to Build a Granny Flat?

Timeframes vary depending on approvals and build type, but a typical project may look like:

• Design + approvals: 4 – 12+ weeks
• Construction: 10 – 18+ weeks

Site works, weather, and material lead times can shift this, but a well-planned project should run smoothly with the right builder and a clear scope.

How to Get an Accurate Granny Flat Quote

If you want accurate pricing (not just a rough estimate), here’s what helps:

• A basic concept plan or layout
• Intended size (m²) and bedroom/bathroom count
• Preferred finish level (standard, mid-range, high-end)
• Any must-haves (air con, deck, separate access)
• Site details: slope, access, existing services, soil type (if known)

The more clarity you provide upfront, the more accurate the quote will be — and the fewer surprises you’ll have later.

Build Your Granny Flat With Coast Construct Co

A granny flat should feel like a proper home — not a compromise. At Coast Construct Co, we take a straight-up, organised approach to granny flat builds, with clear scope, quality workmanship, and communication that keeps you in the loop from start to finish.

If you’re thinking about building a granny flat and want honest advice on costs, design options, and what your site will realistically allow, get in touch with Coast Construct Co.