How will building a granny flat work out tax wise?

Hi Margaret,

We own a five acre block near Darwin with a four-bedroom house under renovation.

My hubby is a 65 year-old self-employed tradesman and as we are not overly endowed with super, had the idea that if he built a small residence (granny flat) on same block with separate power, water etc, we could base ourselves there and rent out the main dwelling for income.

We would also caretake the block, e.g. mow lawns, remove palm frond,s etc. (There are a lot of properties this size in this area which build granny flats /dependant units and rent those out seemingly with no trouble planning-wise it seems.) We can’t subdivide five acres here.

Can you please advise if our idea is even feasible tax wise before we look into it further? I realise it is a pretty broad query!

Many thanks,



Dear Jane,

This is an interesting question and one which is not often asked, and so I referred to Julia Hartman, a great expert on these more obscure scenarios.

She has said that there are many things to consider, the main one being the capital gains tax exemption for your principal place of residence. Once the granny flat is built, if you moved into it immediately, then from a tax point of view this would be treated as two separate properties. Once you rent out the former family home, the cost base of that property is considered to be the market value at the time. The granny flat and the land underneath it would also be given a market value.

At this point you would have to choose which one you wanted covered by the main residence capital gains tax (CGT) exemption – and ahead of time it can be really hard to work out which of the two is likely to have the greatest amount of growth.  If you choose the former family home to have the exemption, then the granny flat and its land will have CGT from its determined market value, and if you choose the granny flat, then the family home will attract CGT from the market value established upon you moving out.

If you don’t want  the property to be treated like two separate assets,  then you need to live in both the granny flat and the house, combined as your home, for the first three months at least.  This way, the two houses are seen as one dwelling,  and for CGT purposes the treatment would be just the same as if you were renting out rooms or part of your home.  This of course exposes the whole property to CGT from the market value of the whole property at the time the granny flat is built, but only for the portion of the home which was rented out – and so it’s done on a pro rata basis.

As you can see – its complex! It’s going to be really hard to actually work out which pathway is the best one to take, since you can’t really estimate future values.  Make sure that you have an accountant who can help you work out some ballpark figures and don’t make any decisions until you have a clear understanding of what could potentially happen in the future.



Margaret Lomas is a best-selling author and writes and hosts the popular Property Success With Margaret Lomas and heads up the panel on Your Money, Your Call, both on Sky News.

She is the founder of Destiny.

Have a property question? Ask Margaret!





Margaret Lomas

Margaret Lomas

Margaret Lomas is a best-selling author and writes and hosts the popular Property Success With Margaret Lomas and Your Money, Your Call, both on Sky News. She is the founder of Destiny.

Granny Flats

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