A Melbourne homeowner has a promising rental appraisal, a project estimate and enough savings to cover the expected build. On paper, the return looks workable.
But after paying the deposit, how much cash should remain available?
The backyard may need additional drainage. First rent may arrive later than planned. A new tenant may not move in immediately. A repair may need attention at short notice. If the project is financed, repayments continue through every delay.
This is why a granny flat investment needs more than a positive ROI calculation. It also needs a cash buffer that suits the property, project and household.
A positive ROI does not mean the project is cash-ready
ROI compares a return with the money invested. It does not show whether the owner can pay an unexpected invoice next month.
A project can have a reasonable long-term return and still create short-term pressure if too much available cash is committed to construction.
Moneysmart advises property investors not to rely on rent to cover every mortgage and expense, because a property may be vacant. It also recommends considering whether the owner could meet costs for a period without a tenant.
That principle matters for a backyard dwelling because the risk starts before the first lease. Access, foundations, drainage and service connections can change the amount required to complete the project.
Separate three different numbers
Before setting a buffer, separate these three amounts.
1. Expected project cost
This should include the build and the known items needed to complete it. Depending on the property, that may include design, permits, site preparation, foundations, delivery, installation, service connections and external works.
A starting package price is not the same as a complete project cost. The Granny Flat Cost Victoria guide explains the difference.
2. Expected annual expenses
These belong in the return calculation. Examples may include management, insurance, maintenance, safety checks, owner-paid utilities and leasing costs.
3. Accessible cash buffer
This is money kept available for events that are uncertain in timing or amount. An unused buffer is not automatically an expense and should not simply be subtracted from annual return.
If part of the buffer is spent on the project or rental, the actual cost should then be recorded in the correct calculation.
Six risks your cash buffer may need to cover
There is no universal buffer that suits every Melbourne property. Build it from the risks that remain after the block and scope have been checked.
Unresolved site and service costs
The first bucket covers items that are not yet fixed.
A narrow access path, reactive soil, a long sewer connection, drainage work or an awkward foundation can affect the final amount. A Land Eligibility Check helps identify which questions need investigation before a design or budget is treated as settled.
Do not use a generic percentage to hide an unknown scope. Ask what is unresolved, what evidence will resolve it and whether the current amount is a quote, allowance or estimate.
Delay before first rent
Rent does not start when a frame arrives or when construction looks nearly complete.
Allow for required completion steps, final work, rental preparation, advertising, inspections, applicant checks and the agreed move-in date. The buffer should consider the cash gap between the expected completion date and the first rent actually received.
Vacancy after the first tenancy
An annual ROI model may already reduce income for vacancy. The cash buffer answers a different question: can the owner meet bills while that vacancy is happening?
Use the Rental Income Guide to establish a defensible rent range. Then test several unpaid-week assumptions instead of relying on 52 paid weeks.
Repairs and rental obligations
Rental providers need access to money when something stops working.
Consumer Affairs Victoria distinguishes urgent and non-urgent repairs. Its current guidance says urgent repairs must be handled immediately and explains circumstances in which a renter may arrange an urgent repair costing up to $2,500 when there has not been a prompt response.
That threshold is not a recommended reserve amount. It demonstrates why some rental costs are time-sensitive and why an annual maintenance percentage alone may not provide enough liquidity.
Finance pressure
If the project is financed, test repayments at the actual proposed loan terms.
Consider what happens if the first rent is delayed, the interest rate changes or the household temporarily loses other income. A broker can explain the loan structure, while an accountant can advise on tax treatment. Neither weekly rent nor a tax deduction removes the need to pay a bill when it falls due.
Household emergencies
A project reserve should not leave the household without its own safety net.
Moneysmart describes an emergency fund as accessible money for urgent or unexpected costs. It notes that a separate savings account or accessible offset account may help keep that money available. Its general guidance is not a prescribed granny flat buffer, but the separation principle is useful.
A worked cash-buffer stress test
The following numbers are hypothetical example assumptions only. They are not an MPLUS quote, rent appraisal, loan offer, forecast or recommendation.
| Buffer component | Example assumption | Example amount |
|---|---|---|
| Unresolved site and service allowance | Owner's provisional amount after an initial site review | $8,000 |
| Delay before first rent | 6 weeks at an assumed $520 per week | $3,120 |
| Later vacancy cash gap | 4 weeks at an assumed $520 per week | $2,080 |
| Urgent repair liquidity | Owner-selected working assumption | $2,500 |
| Finance and annual expense stress | Owner-selected working assumption | $3,000 |
| Illustrative project buffer | Sum of the assumptions above | $18,700 |
The arithmetic is:
$8,000 + (6 × $520) + (4 × $520) + $2,500 + $3,000 = $18,700
This does not mean every owner needs $18,700. A property with confirmed access, completed soil information, scoped connections and no finance may need a different amount. A complex site or highly leveraged household may need more.
The purpose of the table is to replace a vague contingency with visible assumptions.
Avoid double-counting the buffer
A cautious model can become misleading if the same risk is counted twice.
For example, if the complete project cost already contains a confirmed drainage item, do not add it again as an unknown site allowance. If annual net income already allows for four vacant weeks, keep that assumption in the ROI model, but still decide whether the household needs enough accessible cash to survive the vacancy.
Use the ROI Calculator for expected cost, rent, occupancy and annual expenses. Keep the liquidity stress test beside it rather than forcing every reserve dollar into the annual return formula.
Reduce uncertainty before increasing the buffer
A larger reserve is not the only answer. Better information can reduce the amount of uncertainty being funded.
Before committing:
- check access, usable backyard space, overlays, easements and services
- confirm what the project price includes and excludes
- identify provisional allowances and what could change them
- use a local rent range rather than one optimistic listing
- build a realistic completion and lease-up sequence
- test vacancy, repairs and finance separately
- keep household emergency money distinct from project money
Understanding how the build process works can also help identify when deposits, site work, completion steps and rental preparation may affect cash timing.
Start with the property, not a percentage
There is no responsible single percentage that answers how much cash every granny flat investor should keep.
The practical answer begins with the property. A block with simple access and confirmed connections creates a different risk profile from one with drainage, slope, easement or service uncertainty.
MPLUS can review the basic land factors before you rely on a project budget or return estimate. Start with a Free Land Check, then replace the example assumptions with information specific to your property, finance and rental plan.
