Duplex vs Granny Flat: Which Pays Off Faster in Sydney?

If you own a 600 to 800 square metre block in Western Sydney, you have probably been pitched both options: build a duplex and double your potential, or add a granny flat for fast rental income. Both have their place, but the financial math is very different. Here is the honest 2025 comparison.

Capital outlay

This is where the two options diverge most. A granny flat costs $140,000 to $200,000 turn-key for a 60 sqm secondary dwelling in Western Sydney. A duplex (two dwellings on the same block, side by side) typically runs $750,000 to $1.3 million for a complete build.

If you already own the block, the duplex requires you to either knock down the existing house or buy a vacant block, while the granny flat goes into the existing backyard. That makes the granny flat dramatically lower friction to start.

Rental yield, gross and net

A 2-bedroom granny flat in Western Sydney rents for $450 to $650 per week in 2025. On a $180,000 build that is gross yield of 13 to 19 percent. Net yield (after rates, insurance, maintenance, agent fees) is typically 10 to 14 percent.

A new 4-bedroom dwelling in a duplex rents for $700 to $950 per week. On a $500,000 build cost per side, that is 7.3 to 9.9 percent gross. Net is around 5 to 7 percent after holding costs.

On a yield basis, the granny flat wins comfortably. On dollar income, the duplex wins (one side alone generates more rent than a granny flat).

Capital growth

This is where the duplex pulls ahead. A duplex sits on land that is suitable for subdivision into two separate titles. Once subdivided, each side is independently sellable, and the combined sale value is typically 20 to 35 percent higher than the cost of building (in 2025 Western Sydney markets).

A granny flat does not get its own title. It stays as a secondary dwelling on your existing block. The capital gain when you sell is bounded by what an extra 60 sqm of accommodation adds to the price of a property: typically $100,000 to $180,000 over the long term, less than what you spent to build it (especially after holding costs).

Approval friction

Granny flats are usually approved via the fast-track CDC pathway under the State Environmental Planning Policy, in 8 to 10 weeks.

Duplexes are more often DA (14 to 30 weeks), although in some councils they can go through the Low Rise Housing Diversity SEPP for faster approval. Heritage areas, bushfire areas and corner lots all complicate duplex approvals.

If you want to start construction soon, granny flat wins on speed.

Block size requirements

Granny flat: minimum 450 sqm.

Duplex: typically 600 to 700 sqm minimum.

This means a granny flat is feasible on most established Western Sydney blocks, while a duplex requires you to be on a wider or larger lot. If your block is under 600 sqm, you may not even have the duplex option.

The hold-and-sell strategy

The most lucrative play we see in Western Sydney right now is: buy a 700 to 800 sqm block in a growth suburb (Box Hill, Schofields, Marsden Park, Austral), knock down the existing house, build a duplex with subdivision, sell one side to fund the equity in the other. The owner ends up holding a brand-new dwelling for net cost of around $300,000 to $450,000 once the second side is sold, in an area where the dwelling itself is worth $850,000 to $1.1 million.

This is not a granny flat play. Granny flats are passive income on a property you intend to keep.

Tax considerations

Both granny flats and duplexes can be depreciated under ATO rules and provide tax benefits. Duplexes allow capital works deductions on the full structure plus plant and equipment. Granny flats follow similar rules but on a smaller asset base.

One specific 2021 tax change: granny flat arrangements between family members no longer trigger capital gains tax under specific written arrangement conditions, making them more attractive for elderly parents or adult children.

Resale impact on the main dwelling

A granny flat at the rear of an existing property has mixed impact on resale of the main dwelling. Some buyers see it as added income potential. Some see it as a loss of garden space and added rates liability. In our experience selling alongside agents, the net effect on the main house price is roughly neutral, the granny flat sells with the main as a package.

A duplex sells as two independent dwellings, which gives flexibility in market timing. Both halves do not need to sell at the same time.

Which one suits you

Pick a granny flat if:

• Your block is under 600 sqm.
• You want passive income within 12 months.
• You want low capital outlay and high yield.
• You plan to live in the main dwelling long term.
• You want minimal approval friction.

Pick a duplex if:

• Your block is 600 sqm or larger.
• You are willing to deploy $750,000 to $1.3 million.
• Your goal is capital growth, not just yield.
• You want the option to sell one or both sides independently.
• You can wait 14 to 30 weeks for approval.

Final word

There is no universally right answer here. We have built both for clients within a month of each other on neighbouring blocks, and both were the right call for that client. The wrong call is to assume one is automatically better. Run the numbers for your specific situation.