Duplex or dual occupancy on your block: the five things that decide it

"Can I put a duplex on it?" is the question we hear most. The answer is rarely a simple yes or no; it is five checks, and one failed check can close the door or change the pathway.

1. Is it permitted in the zone?

Dual occupancy has to be permissible in the property's zone under the council's local plan. Recent state reforms opened it up in more low-density zones across Sydney, but the local plan and any exclusions still have to be read for the specific block.

2. Is the block big enough?

Councils set a minimum lot size for dual occupancy, and it varies from one council to the next. The state complying-development code has its own minimums. A block a few square metres under the line is a block that cannot use that pathway, however good the design.

3. Is it wide enough?

Width at the building line matters as much as area. Narrow blocks that meet the area test can still fail the width test, especially for side-by-side layouts. Sometimes a front-and-back layout rescues a narrow block; sometimes nothing does.

4. What about height, floor space and overlays?

Height limits, floor space ratios, and overlays such as heritage, flood and bushfire decide how much can be built and whether the faster complying-development pathway is available at all. On a heritage-area block the answer is a council development application, whatever the numbers say.

5. Can it be built for what it will be worth?

A permissible duplex is not always a sensible one. Slope, access, drainage, the neighbours' windows and the cost of a basement or retaining walls can turn a "yes" on paper into a project that does not pay. This is the check most people skip, and the one that costs the most when skipped.