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Financing a granny flat

From Modular Living Australia — the independent knowledge base Last updated: 31 Jul 2026

Financing a granny flat is not financing a house. There is no separate loan product for it, the security is an existing property rather than a new one, and — the point most often missed — the completed dwelling is rarely worth to a lender what it cost to build. This guide sets out the two funding structures in use, what a construction loan requires, and the valuation gap that determines whether a project is feasible at all.

Two structures §

There is no single granny flat loan in Australia. Projects are funded either by accessing existing equity — through a line of credit or a cash-out — or by a construction loan, under which the total is not advanced at once but released in progress draws as construction stages are completed^[1].

Equity access is simpler and generally cheaper to arrange. Its limit is arithmetic: most lenders allow borrowing up to 80 per cent of the property's current value, less the remaining mortgage^[2]. On a property worth A$850,000 with a A$600,000 loan, that produces roughly A$80,000 of usable equity — enough for a modest project and not enough for most of the ranges published in the state cost guides on this site.

A construction loan removes that ceiling but adds process, and the process is the subject of the next two sections.

What a construction loan requires §

Lenders usually fund 70 to 80 per cent of total project cost, and generally require a quantity surveyor's report and full council-approved plans before approval^[3]. Loan-to-value ratios for construction are more conservative than for established purchases, where up to 90 to 95 per cent is available^[3].

Two consequences follow for anyone planning the sequence of a project.

The first is that approvals come before finance, not after. Council-approved plans are an input to the loan application, which means the design, engineering and approval costs are incurred before the loan is certain. Those costs are the owner's regardless of whether the loan proceeds.

The second is the clock. Most construction loans cap the build period at 12 months, and delays can trigger costly extensions^[3]. That is comfortable for a straightforward project and less so where an approval pathway is slow or a builder's queue is long — an HIA builder survey in April 2025 found members expected to construct ten times more secondary dwellings in 2026 than in 2022, which is already pushing build timelines out in some areas^[4].

Not all lenders are comfortable with these projects, and specialist or second-tier lenders sometimes offer better terms than a major bank^[3].

The valuation gap §

This is the section that matters most, and it contradicts most of what is published about granny flats as an investment.

Banks rarely value a newly built secondary dwelling dollar-for-dollar against its construction cost. Lenders typically value the addition at only 50 to 70 per cent of build cost — meaning a A$150,000 granny flat may add only A$75,000 to A$105,000 to the bank's assessed property value^[5].

Three practical conclusions follow.

• Do not fund the build on the assumption of a post-completion refinance. The source states this directly^[5]: the equity released after completion may not cover what was spent, and a plan that depends on it can leave an owner unable to repay a short-term facility.
• A cash buffer is not optional. Sufficient existing equity or cash is needed to absorb the gap^[5].
• Value uplift and rental return are separate questions. A secondary dwelling can improve cash flow through rent while adding materially less to the valuation than it cost. Marketing that conflates the two is describing one benefit and pricing it twice.

Most lenders will not formally value the dwelling until it is complete, though some provide a preliminary valuation based on plans and specifications^[3]. Where a project's feasibility depends on the outcome, obtaining that preliminary view before signing a building contract is worth the effort.

Rental income and serviceability §

The additional rental income is generally recognised by lenders on refinancing after completion, which improves the borrower's position for future lending^[3].

Before completion it is a different matter: some lenders will not recognise projected rental income from a dwelling that does not yet exist^[3]. A serviceability assessment made on current income alone may therefore be tighter than the finished position, and it is the assessment that determines whether the loan is approved.

Whether a specific lender will count secondary dwelling rent, and at what discount, is a policy question that varies between institutions and is not covered by the sources compiled here. It is the single most useful question to put to a broker early.

Insurance is not automatic §

Adding a secondary dwelling requires updating the primary home's building insurance. An existing policy will almost certainly not cover the new structure automatically^[5].

This is easy to overlook because the dwelling sits on land already insured, and expensive to overlook because the exposure runs from the start of construction. Where the dwelling is to be rented, landlord cover for that tenancy is a further and separate question.

What is not covered here §

No lender's published policy documents have been used for this guide; the figures above come from finance and property publications describing market practice. Individual lender policy on secondary dwelling rental income, on maximum loan size for an addition, and on acceptance of prefabricated or modular construction is not covered by these sources — the last is dealt with generally in Can you get a mortgage on a modular home?

Tax treatment is not covered here. A rented secondary dwelling has income tax, capital gains tax and, in some states, land tax consequences that depend on individual circumstances and should be taken to a registered tax agent.

References §