Dual occupancy development—building two dwellings on a single block that previously held one—has become a popular strategy for both owner-occupiers wanting to live in one and rent the other, and investors looking to add value. The arithmetic seems simple: buy a house on suitable land, knock it down, build two townhouses, sell or rent them for more than you spent.

In practice, the numbers work less often than people think. I’ve watched enough dual occupancy projects to know where the assumptions break down and where people consistently underestimate costs or overestimate final values.

The Purchase Price Trap

People pay too much for the acquisition. The target property needs to be cheap enough that the land value makes sense for two dwellings, not one. If you pay full market value for an existing house, you’re essentially paying for the existing dwelling that you’re going to demolish.

The land component needs to be the valuable part. Look for run-down houses on good blocks rather than renovated houses on the same blocks. You want to pay for location and land size, not someone else’s renovation work you’re demolishing.

Rule of thumb: if the property is presented as a “renovated home in great condition,” you’re probably paying too much for dual occupancy development. Look for “potential development site” or “renovators delight” instead.

Construction Costs Keep Rising

Dual occupancy budgets consistently underestimate construction costs. People plan on $2500-3000/sqm and actual costs come in at $3200-3500/sqm once everything is included.

This kills the margins. If your pro forma assumed $2800/sqm and actual costs are $3400/sqm, you’ve added $80,000+ to costs on a pair of 130sqm townhouses. That probably eliminates most of your expected profit.

Builder quotes are often incomplete. They might not include landscaping, driveways, fencing, connection fees to utilities, or allowances for dealing with unexpected site issues. All these add up.

Get fixed-price contracts with detailed specifications. Provisional sums and cost-plus elements are where blowouts happen. A $550,000 building contract with $80,000 in provisional sums isn’t really a $550,000 contract.

Holding Costs During Construction

The project takes longer than expected, which means you’re paying interest on borrowed money for longer than budgeted. Construction delays are standard—weather, material delays, builder scheduling, permit issues.

Plan for 12-14 months from demolition to completion and sale, not the 9 months your builder optimistically suggested. Every extra month of holding costs is another $3000-5000 in interest on a typical development loan.

If you borrowed $900,000 for land and construction at 8% interest, that’s $6000/month in interest during the build. Three extra months is $18,000 you didn’t budget for.

Council and Professional Fees

DA costs for dual occupancy in Sydney or Melbourne run $15,000-25,000 for consultants (architect, surveyor, planning consultant if needed) plus council fees. Some areas are more expensive if they have strict design requirements or if you need to go through panel review.

Building permit fees, certifier costs, and inspection fees add another $8,000-12,000. Then there’s engineering, soil testing, bushfire assessment if applicable, and potentially tree protection or ecological reports.

Budget $30,000-40,000 minimum for all professional and approval costs. First-time developers often budget half this and get shocked when the bills come in.

Infrastructure Contributions

Some councils charge significant infrastructure contributions for dual occupancy development. These can be $20,000-$50,000+ depending on council area and specific zoning.

Check this early in feasibility assessment. Infrastructure charges that weren’t budgeted can completely eliminate profit on marginal projects.

Sydney and Melbourne councils publish contribution rates, but calculating the exact amount for your specific project requires understanding the formulas. Get professional advice rather than guessing.

The End Value Reality

People overestimate what each townhouse will sell or rent for. They look at comparable sales but fail to account that the best comparables are established homes with gardens and backyards, not brand new townhouses with minimal outdoor space.

Dual occupancy townhouses typically sell for 10-20% less per sqm than equivalent detached houses in the same suburb. Buyers discount the smaller land component and shared walls.

Rental yields are often okay but not exceptional. Two 3-bed townhouses might rent for $550-600/week each in an area where a house rents for $750/week. You’re not doubling the rental income of the original house—you’re increasing it by maybe 50-70%.

Capital Gains Tax Erosion

For investors, CGT on the uplift is a major cost that’s often ignored in early feasibility calcs. If you buy for $1M, spend $800,000 on demolition and construction, and sell two townhouses for $1M each, you’ve made $200,000 pre-tax.

But CGT on that $200,000 (assuming you held less than 12 months so no 50% discount) at your marginal tax rate might be $90,000 or more. Your actual after-tax profit is $110,000.

After holding costs, professional fees, and selling costs (2% agent fees on $2M is $40,000), that $200,000 gross profit might be $50,000-70,000 net profit. For 18 months of stress and risk, the return isn’t spectacular.

When It Actually Makes Sense

Owner-occupier living in one, renting the other makes the numbers work better. You’re not paying CGT on your principal residence portion, you’re not paying rent elsewhere during construction, and the rental income offsets holding costs.

Areas where land values are high relative to building costs favor dual occupancy. Inner-ring suburbs where land is $1200-1500/sqm and building is $3000/sqm work better than outer suburbs where land is $600/sqm and building is still $3000/sqm.

Projects where you’re not demolishing a perfectly good house improve margins. Buying a derelict house for land value and building two townhouses works better than buying a renovated house and knocking it down.

The Mistakes I See Repeatedly

Underestimating total costs by 15-25%. This is almost universal. First-time developers budget too optimistically on almost every line item.

Overestimating sale prices or rental yields by 10-15%. People look at aspirational comparables rather than realistic ones.

Failing to account for CGT, selling costs, and the opportunity cost of capital tied up during the project.

Not getting fixed-price contracts with adequate detail. Cost overruns kill more projects than any other single factor.

Assuming everything will go smoothly and on schedule. It won’t. Budget for delays and complications.

What Actually Works

Conservative feasibility modeling with 20% buffer on costs and 10% discount on expected sale prices. If the numbers work with that pessimistic scenario, the project is probably viable.

Fixed-price building contracts with experienced builders who’ve done dual occupancy before. Don’t pick the cheapest quote—pick an experienced builder with a solid quote.

Adequate finance buffer. Don’t borrow exactly what your budget says you need. Have 15-20% extra capacity for cost overruns and delays.

Professional team from the start. Good architect, good builder, experienced certifier. This isn’t where you want to cheap out with unlicensed friends or the lowest bidder.

The Realistic Assessment

Dual occupancy can work but margins are thinner than people expect. In current market conditions (March 2026), many projects break even or make modest profits that don’t justify the risk and stress.

Owner-occupier scenarios work better than pure investment because you’re capturing the personal use value and avoiding CGT on half the project.

If the numbers look marginal based on realistic cost assumptions, don’t proceed hoping for the best. Markets aren’t rising fast enough to bail out marginal projects like they did in past cycles.

Do the feasibility work properly upfront with professional input. Spending $5,000 on feasibility study beats losing $50,000 on a project that never made sense.

Dual occupancy development isn’t passive investing—it’s an active project requiring constant oversight, decision-making, and problem-solving. Make sure you’re willing to commit the time and mental energy before starting.