- You can't treat any other property as your main residence (except for up to 6 months if you are moving house).
Just spotted this edit, so will answer here.
Keen eye. Good on you.
So what this is saying is that if you claim a main residence exemption on the old property, you can't claim the main residence exemption on the new property for that period. So you're basically shifting your exemption from one property to another for the period.
Both your properties have had some CG over the 18 months. You need to have a think about and plot out the trajectory of your new home. Is it a forever home, is it an interim step before your forever home, which property has had the bigger CG in the 18 months, do you want more cash flow now, or when/if you sell the new place, are you ever going to sell the new place etc.
There's lots of variables.
However the correct approach is varied. If the new place is your forever home, take the 6 year rule and don't pay CGT on the old place as you'll never be liable for the 18month "non exempt" period on the new place.
If it is an interim home, you plan to move it on in a few years and the GC was greater on the new place, you may wish to pay the CGT onthe old property to circumvent a bigger tax chunk down the road on the new place. Or vice versa.
You may have need for the cash flow today, rather than when you sell your new home down the track and pay the premium for the access to funds now - there is a potential for lost opportunity cost if you had say wanted to invest the funds rather thN using them to pay a tax etc.
The approach I've taken, is the new place I built, I over capitalised significantly so that the CG of the next few years doesn't catch up to the market value as its a long term home. I'm basically gaming a situation where I can take the six year rule and have negligabl3 CG on the new place over the first few years, limiting the tax on the early part of the second CG event. Given the market conditions, I'm laughing.
However it is worth noting that the tax office has a short memory. It only expects you to keep records for several years, as such its a fair assumption that their records are time limited also (even though its technically not true, they have everything forever). So even though you can't technically claim the main residence exemption on two properties for the 18month period, depending on when you dispose of the new place (ie if you do so in 10-15 years), you sort of can. The ATO wont be cross referencing data for 10-15 years ago. The "oops it was 10 years ago" excuse will get you off the hook without any penalties or fines and is a legitimTe excuse if youre ever questioned. They dont have the computing power to even cross reference everyone's current records for the last year, which is why they do targeted reviews each year they give you fair warning about. Disclaimer, this is not advice and I am not advocating tax fraud, just hilighting how the system works and how some people abuse it to their advantage.
Hope that covers the question. In anycase, if you're not 100% sure, I'd see a tax accountant and have a chat.