What makes a duplex feasibility different from a single-house build

Three structural differences matter. (1) Yield: you're producing TWO sale events not one, so revenue is roughly double the comparable freestanding house price. (2) Stamp duty: still paid once on the parent-lot purchase, but if you Torrens-subdivide before settlement it can change the structure (talk to a property lawyer). (3) Margin discipline: most developers target 18–22% margin on cost for duplex. Under 15% and the project doesn't pay you for the risk; over 25% and you've either underpriced the land or overpriced the build.

Class 1a vs Class 2 — and why duplexes win

Most Sydney duplexes are Class 1a (attached dwellings of not more than two storeys, each with its own egress and not sharing a common wall above a fire-resistant separation). Class 1a is exempt from the NSW Design and Building Practitioners Act (DBP Act 2020), which means none of the $45k+ in registered designer/practitioner documentation that apartments need. As soon as you go above two storeys, share a basement, or wrap the units around common property, you can flip to Class 2 — adding 1.5–2.5 months and significant soft cost to the program. Confirm the classification with your designer + private certifier before lodging DA.

GRV — how to price the dwellings

GRV (gross realisation value) is the most important number in the feasibility and the easiest to get wrong. Don't anchor on a single 'comp from down the road' — pull 6–12 months of comparables within 1km, within 15% size variance, sold in similar market conditions. Sydney duplex pricing in 2025–26 typically lands $1.4M–$3.2M per dwelling for inner-ring suburbs, $1.0M–$1.8M for middle-ring, $750k–$1.3M for outer Sydney. Eastern Suburbs, Inner West, and Lower North Shore command 20–40% premiums.

How much equity do I need for a duplex?

Standard developer finance in NSW for an experienced developer with a credible build team is typically 65–75% LTC (loan-to-cost) on a duplex. So for a $2.5M total development cost (TDC) you'd need roughly $625k–$875k of equity — plus interest reserve and a 7–10% contingency outside the loan. First-time developers without a track record will see 50–60% LTC, so equity goes to $1.0M–$1.25M. The calculator below shows the exact required equity and peak debt for your inputs — adjust the 'finance profile' selector to match your situation.