6 Hidden Costs in a Dual Occupancy (That Most People Don’t See Coming)

Dual occupancies can be a great way to unlock value from a site — but they’re also where we see the biggest cost surprises. Not because people are careless, but because certain costs don’t show up clearly until it’s too late to control them. Below are six of the most common hidden costs we see on dual occupancy projects, based on real builds we’ve been involved in.

1. Site Costs That Sit Outside the Quote

Earthworks, access constraints, retaining walls, rock removal, and restricted machinery access are often loosely defined early on. If site conditions aren’t clearly documented and allowed for upfront, these costs surface during construction — usually as variations.

2. Services Upgrades Triggered by Dual Occupancy

Dual occupancies almost always place greater demand on existing services. Sewer upgrades, new stormwater points, electrical upgrades, and authority requirements can add significant cost if not identified during the design and approval stage.

3. Stormwater and OSD Requirements

On-site detention systems, pump-outs, and council-specific drainage requirements are one of the most underestimated cost items. When these aren’t designed early, they often become expensive late-stage additions.

4. Incomplete or Uncoordinated Documentation

Missing internal elevations, joinery details, electrical layouts, or window coordination leads to assumptions being priced. Those assumptions usually change once construction starts — and that’s where costs increase.

5. Holding Costs During Delays

Every additional month on site comes with holding costs: interest, rates, insurance, and sometimes rent. Small delays can quietly erode profit if they aren’t factored into the overall feasibility.

6. Late Design Changes

Changes that seem minor on paper — moving a wall, resizing windows, upgrading finishes — are amplified on dual occupancies. Because you’re building two homes, every change is effectively doubled.

The takeaway is simple: most dual occupancy cost blowouts aren’t unavoidable — they’re the result of risks not being identified early. Clear documentation, realistic allowances, and builder input before construction starts can make a significant difference to the outcome.