Main residence knock-Down / Rebuild Duplex - Capital Gains and GST Consequences

 Author: NikkiATO(Community Moderator)Community Moderator

ATO Certified Response10 Feb 2026

Hi @newbiebuild,

Living in Side B for 3 months won’t automatically give you a full main residence exemption. Once you subdivide the land into two Torrens titles, you have two separate CGT assets. You can only have one main residence at a time, and moving between the two dwellings for short periods won’t change the overall tax outcome.

The 4‑year building rule only applies to one dwelling – the one you intend to live in as your main residence. If you build on land you already own, you can treat that land as your main residence for up to 4 years before you move in, as long as you move into the dwelling as soon as practicable after it’s finished and live there for at least 3 months. This rule doesn’t apply to both sides of a duplex.

When you subdivide, each side becomes a separate CGT asset from the date of subdivision. If you sell Side B after only a short stay, you’ll need to work out CGT based on:

• your cost base (your original land cost plus build costs split between the two titles)
• the capital gain (sale price minus cost base)
• the main residence exemption portion (days Side B was your home ÷ days you owned the subdivided lot)
• whether the 50% discount applies.

A brief period of occupancy will only give a partial exemption.

GST only applies if your project is an enterprise. A one‑off knock‑down rebuild may not be an enterprise, but subdividing and selling newly built property can be if the activity has a commercial nature. If you’re required to be registered, GST withholding rules apply at settlement.

Once you move into Side A and make it your home, it can be covered by the main residence exemption going forward, as long as you live in it and don’t use it to earn income. Earlier use of the land may still be relevant when you eventually sell.

Where you live during construction doesn’t change the main residence rules. The important part is that you move into the dwelling you plan to keep as soon as practicable after completion and live there for at least 3 months. This rule applies to one dwelling only.

You can use our CGT property exemption tool to help work out partial exemptions, and you should read about building or renovating your home and treating former home as main residence.

Given the mix of subdivision, building and selling, there are a few things that can change the outcome. It’s a good idea to speak with a registered tax professional for advice on your full plans, or you can ask for a private ruling if you want certainty before you begin.

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