Imagine a Melbourne homeowner comparing two-bedroom granny flat designs for a clear-looking backyard. One agent suggests the completed dwelling could rent for $570 a week, based on nearby two-bedroom units. Another suggests $510 because the proposed granny flat has shared driveway access, no dedicated parking and less private outdoor space.
That $60 weekly difference changes the annual income assumption by thousands of dollars. Yet neither number is useful until the homeowner can see what evidence and property features sit behind it.
The practical answer
A pre-build rental appraisal should give you a supported range, not a confident single number. It should identify the proposed dwelling, likely tenant, recent comparable rentals and the adjustments made for privacy, parking, access, size, condition and inclusions.
Use the appraisal as one input in an investment model, not as a guarantee. The final rent will depend on the completed property and the market when it is actually offered for lease.
The Rental Income Guide explains the main features that can affect demand, but an appraisal should apply those factors to the specific design and location.
A rental appraisal is not always a formal valuation
A property manager may provide an appraisal as an estimate of achievable rent. This can be useful for early planning, especially when the agent manages similar properties nearby.
It is not automatically the same as an independent rental valuation prepared for a lender, legal matter or another formal purpose. If a bank, broker or adviser needs rental evidence, ask what report format and provider qualifications they will accept before ordering anything.
The Australian Property Institute’s rental-valuation guidance highlights the importance of supporting market evidence, stated assumptions, limitations and clear reasons for the conclusion. Those principles are useful even when reviewing an informal agent appraisal.
Choose comparables that behave like the proposed granny flat
The nearest two-bedroom apartment is not necessarily the best comparable.
A useful comparison should consider:
- the same suburb or a genuinely similar nearby rental market
- recent achieved leases where available, not only current asking rents
- bedroom count, bathroom, internal area and usable layout
- detached or attached form
- private or shared access
- dedicated parking and practical street access
- private outdoor space and separation from the main home
- heating, cooling, laundry, storage and fixed inclusions
- furnished or unfurnished condition
- whether utilities are separately metered or included in rent
- likely lease length and tenant group
A compact granny flat with a private entrance and well-screened courtyard may compare differently from one that opens directly onto the main home’s entertaining area. The adjustment should be explained, not hidden inside a final number.
Ask for the evidence behind the range
When speaking with a local property manager, ask:
- Which three to five recent rentals are most comparable?
- Were those figures advertised rents or achieved rents?
- How long were the properties available before leasing?
- What makes the proposed granny flat better or worse than each comparable?
- Does the range assume parking, private access or included utilities?
- Which tenant group is expected to apply?
- How might a different bedroom count or layout change the range?
- How long is the appraisal intended to remain useful?
Two appraisals can be more informative than one, particularly when each agent shows their evidence. If the ranges differ materially, do not simply average them. Find the assumption causing the difference.
Build three rent scenarios, not one
A practical ROI model can use:
- Conservative case: lower supported rent and a longer leasing gap
- Expected case: the middle of a well-supported appraisal range
- Stronger case: only where evidence supports a higher rent and stable demand
This prevents the project from depending on the top of an appraisal range. The ROI Calculator can organise rent, vacancy, operating costs and total project cost without treating 52 weeks of advertised rent as collected income.
Worked example using assumptions only
The following figures are hypothetical planning assumptions. They are not a Melbourne rental appraisal, construction quote or return forecast.
| Item | Expected case | Conservative case |
|---|---|---|
| Weekly rent | $535 | $500 |
| Occupied weeks | 50 | 48 |
| Collected annual rent | $26,750 | $24,000 |
| Annual cash operating costs | $4,500 | $4,500 |
| Pre-finance operating cash | $22,250 | $19,500 |
| Complete project cost | $220,000 | $220,000 |
| Simple pre-finance return | 10.1% | 8.9% |
The difference between the two rent and vacancy assumptions reduces annual pre-finance operating cash by $2,750. It also changes the simple return by about 1.2 percentage points.
This example does not include finance, tax or future capital value. It shows why an appraisal range should be tested rather than selecting the highest weekly figure.
The complete project-cost denominator must also be verified. The Granny Flat Cost Victoria guide explains why foundations, access, services, approvals and selections may sit outside a headline build price.
Broad Melbourne data is context, not the answer
The Victorian Government Rental Report provides useful market context. Its latest published page at the time of checking reported a metropolitan Melbourne median weekly rent of $580 for new lettings in the September 2025 quarter.
That figure should not be copied into a granny flat ROI model. It combines different locations and dwelling types and is based on the broader private rental market. A specific granny flat may sit above or below it depending on its suburb, size, privacy, access, condition and tenant appeal.
Suburb medians have the same limitation. Use them to understand the market, then return to genuinely comparable properties.
Check the proposed design, not an imaginary finished home
A pre-build appraisal is only as good as the information supplied.
Give the agent the proposed floor plan, bedroom sizes, parking arrangement, access path, outdoor area, laundry, heating and cooling, storage, utility structure and expected finish. Ask the agent to state which features remain assumptions.
Site feasibility matters too. A design may need to change after checking easements, access, services, setbacks or other property constraints. The Land Eligibility Check helps identify these issues before the rent model becomes fixed, while How We Build explains how site review connects to design and construction.
What a useful appraisal should leave you with
Before relying on the result, confirm that you have:
- a rent range rather than one unsupported number
- recent comparable evidence
- clear adjustments for granny-flat-specific features
- an expected tenant and lease strategy
- vacancy and operating-cost assumptions
- a date and list of limitations
- conservative and expected ROI scenarios
- a separately verified complete project cost
The goal is not to predict the exact weekly rent years in advance. It is to find out whether the project still makes sense when realistic rental evidence replaces the optimistic headline.
MPLUS can review the usable backyard, access, planning context and obvious site constraints before you rely on the proposed design or rental model. Check My Land to test whether the property behind the appraisal is workable.
Sources checked
- Consumer Affairs Victoria, Challenging rent increases or high rent
- Department of Families, Fairness and Housing Victoria, Rental report
- Australian Property Institute, Rental Valuations and Advice
