Most Australians never think about capital gains tax on the family home. The main residence exemption usually wipes it out. Then someone converts the garage into a short-stay unit, or builds a granny flat for a tenant, and that assumption stops being safe.
A recent two-part tax series looks at exactly this, and the first instalment lays out how earning income from part of a home can chip away at the exemption. The detail that matters most is a rule that can treat the home as bought at market value on the day it starts producing income. That makes a capital gains tax valuation the single most useful document you can hold.
Why income from your home changes the tax picture
The exemption works because the home is your main residence and nothing else. Once part of it earns income, the tax system starts treating that part as an income-producing asset.
Where a fully exempt home first begins producing income, a special rule can deem it acquired at its market value on that date. In plain English, your original purchase price and the costs you piled up before that date largely drop out of the calculation. Only the growth from the valuation date onward is in play.
That cuts both ways. If you have owned the home for decades, the old gain is effectively ignored, which helps. But only if you can prove what the home was worth on day one.
Two examples with real numbers
The series runs two illustrations, and both use today’s 50 per cent discount.
The garage conversion. A $30,000 conversion takes up 15 per cent of the floor area. The home is valued at $1.03 million when it starts earning. Short-term accommodation at $200 a night and 50 per cent occupancy brings in about $36,500 a year. A decade later the house sells for $1.5 million.
- Taxable portion of the gain: $70,500
- After the 50 per cent discount: $35,250
- Tax at 39 per cent: roughly $13,750
- Income earned over the period: about $365,000
The granny flat. A $200,000 secondary dwelling takes a 25 per cent share. It is let at $600 a week. Starting value is $1.2 million and the sale price is $1.8 million.
- Taxable share of the gain: $150,000
- After the discount: $75,000
- Tax: about $29,250
- Rent collected: $312,000
In both cases the tax is a fraction of the income earned. This is not an argument against renting out space. It is an argument for knowing the cost before you start.
The catch: records and valuation
The writer’s advice is blunt. Get a market valuation when the change happens, and keep good records from then on. A previous family cut its CGT bill by almost $14,000 using about $75,000 of retained costs.
Without a valuation, you may struggle to defend the starting value if the tax office asks. For anyone searching for how to handle a CGT valuation, the order is simple: value first, then keep costs, receipts and dated floor-area calculations. Our piece on the capital gains tax valuation deadline covers a related timing trap, and our negative gearing, CGT and investor tax guide sets out the wider rules.
Quick take
- The exemption can shrink the moment part of your home earns income
- A deemed market-value start date can erase your old cost base
- A valuation at that date is the evidence you will need later
- Keep every receipt for costs you may later claim
The unintended consequence for rentals
Governments say they want more homes available to tenants. Renting a spare room or building a granny flat is one of the cheapest ways to add supply, because no new land is needed.
But a tax rule that bites on that behaviour, and is hard to follow without a valuation and detailed records, can make some owners think twice. Compliance cost and uncertainty act like a small tax on top of the real one. Some will go ahead anyway. Others will leave the room empty.
That is a judgement, not a measured finding. The source figures show modest tax relative to income, so the deterrent may be more about confusion than cost. Clearer guidance would likely help more than any change to the maths.
What to do before you list the room
If you are weighing a conversion or a short-stay setup, a few steps keep the bill predictable:
- Book a market valuation dated close to the day income starts.
- Work out the floor-area share used for income, and write it down.
- Keep invoices for construction and ongoing costs from day one.
- Speak to a registered tax agent before you sign a lease or list online.
The next step is simple: get the valuation done before the first guest or tenant arrives, not after.
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General info, not financial advice.
