A Melbourne homeowner sees a granny flat advertised at an attractive starting price. The backyard is level, the design looks simple and the figure appears to fit the budget.
Then the questions begin. Does the price include foundations? What about delivery, service connections, permits, paths and drainage? Is the kitchen allowance enough for the selected finish? The original number has not necessarily changed. The scope being compared has.
That is the central issue with granny flat building costs. A price can be genuine and still be incomplete for your property.
The short answer: building cost is not always project cost
Homeowners often encounter three different numbers:
- Building or product price: the dwelling or nominated construction package.
- Building contract price: the work and allowances documented in the signed contract.
- Complete project budget: the contract price plus site-specific, authority and owner-paid items needed to make the dwelling usable.
These figures should not be used interchangeably.
For building-permit purposes, Victoria's Building and Plumbing Commission uses a formal cost-of-building-work calculation that includes labour and materials associated with the permitted work. That technical figure is not automatically the same as a marketing price or the amount a homeowner will spend across the complete project.
The Granny Flat Cost Victoria guide explains the main cost categories. This article focuses on how to decide whether the number in front of you is complete enough to compare.
What usually sits inside granny flat building costs?
The core building scope may include the structural frame, external walls, roof, windows, internal linings, kitchen, bathroom, electrical work, plumbing within the dwelling and selected finishes.
But labels such as turnkey, fully finished or installed are not universal definitions. One supplier may include transport and foundations. Another may stop at the factory door, at lock-up or at a nominated handover point.
Ask for a written inclusions schedule that identifies:
- the exact floor plan and measured area
- structural system and foundation scope
- windows, doors, insulation and energy specification
- kitchen, bathroom, flooring and appliance selections
- electrical, plumbing and heating or cooling inclusions
- delivery, lifting, installation and on-site finishing
- painting, testing, cleaning and handover documents
If an item is not named, do not assume it is included.
The costs created by the block
Two homeowners can choose the same granny flat and receive different project budgets because the properties are different.
The site may affect:
- demolition and clearing
- soil investigation and foundation design
- excavation, retaining or drainage
- side access for workers, machinery or modules
- crane position and transport planning
- distance and route to sewer, water, power and communications
- protection or reinstatement of fences, paths and landscaping
- work around easements, trees or existing services
A level-looking lawn does not prove simple ground conditions or clear service routes. A Land Eligibility Check should happen before a generic package price is treated as property-specific.
The costs that may sit outside the building contract
Some necessary expenses may be arranged by the builder. Others may remain with the owner or a separate consultant.
Common categories include surveys, soil information, engineering, energy documentation, planning advice where relevant, the building permit process, service-authority work and external completion items.
Victoria still requires a building permit for a small second dwelling. Whether a planning permit is required depends on the land and proposal. The Council Approval Guide can help separate these approval pathways without assuming the same process applies to every block.
Keep an owner-side cost ledger beside the builder's quotation. This prevents an excluded item from disappearing simply because it will be paid to someone else.
Use a price-certainty ladder
A useful way to review granny flat building costs is to ask how much evidence sits behind each number.
1. Advertised or from price
This is an early reference point. It may describe a standard design, specification and assumed site. It is not a final property budget.
2. Preliminary estimate
The provider may know the design and suburb but still rely on assumptions about soil, access, services and approvals.
3. Site-informed estimate
The property has been reviewed and major cost drivers are becoming clearer. Unresolved items should be labelled as allowances or exclusions.
4. Contract price
Plans, specifications, inclusions, exclusions, prime cost items and provisional sums should be documented. The number is more useful, but only for the scope actually contracted.
5. Current complete-project forecast
This combines the contract with known owner-paid work and approved written changes. It should be updated when a real assumption is replaced by verified information.
The objective is not to demand a fixed figure before the site is understood. It is to know which stage of certainty you are looking at.
Compare quotes on one page
Before choosing a provider, place each quote into the same comparison table.
| Cost area | Quote A | Quote B | What to verify |
|---|---|---|---|
| Building and finishes | Included, allowance or excluded | Included, allowance or excluded | Same plan and specification |
| Foundations and site work | Soil, levels and design basis | ||
| Delivery and installation | Access, crane and on-site completion | ||
| Service connections | Route, distance and authority scope | ||
| Design and approvals | Documents, consultants and permit responsibility | ||
| External works | Paths, drainage, fencing and reinstatement | ||
| Owner-paid items | Named supplier and working allowance |
This process often reveals that the cheaper quote covers less, not that the same work costs less.
If you want to understand how design, factory work, site preparation and installation connect, review How We Build before comparing payment milestones.
Watch allowances and variations
An allowance is not the same as a confirmed price.
Prime cost items usually relate to selected fixtures and fittings that are not fully specified. Provisional sums cover work where the final amount cannot yet be determined. Both need enough detail to show what has been assumed.
Consumer Affairs Victoria advises that permitted changes to a domestic building contract price include documented variations, prime cost items and provisional sums. Agreed variations should be recorded in writing with the price and timing consequences before the work proceeds, subject to the applicable rules.
The practical lesson is simple: make selections early, investigate predictable site conditions and keep every change attached to the current total. A verbal upgrade can affect more than the item itself if it changes drawings, engineering, labour or programme.
A better budgeting sequence
Before relying on a granny flat build price:
- Confirm the likely building area and access on the property.
- Choose a realistic design and written specification.
- Identify site, foundation and service assumptions.
- Separate fixed items, allowances, exclusions and owner-paid work.
- Check responsibility for design, permits and authority coordination.
- Compare quotes using the same categories.
- Keep a contingency for unresolved risks, not a substitute for missing investigation.
- Update the complete-project forecast whenever verified information changes.
This gives you a budget that becomes more reliable as the project becomes more defined.
Start with the property, not the headline price
The most useful granny flat building cost is not the lowest advertised number. It is the clearest current estimate for a documented design on a reviewed block.
MPLUS can review the initial property context, usable space, access and obvious site constraints before you commit to a standard package. Check My Land to start with the block, then compare the building costs on a scope that makes sense for it.
