A dual-occupancy project can look straightforward on a plan: retain or remove one dwelling, create two well-designed homes and improve the value of the land. The reality is that unit development costs are shaped well before a slab is poured. Site constraints, planning controls, service connections and the level of finish all affect the final number.
For Melbourne property owners, the useful question is not simply, “What does it cost to build two units?” It is, “What must be allowed for to deliver compliant, durable homes without compromises appearing halfway through construction?” A sound budget answers that question from the outset.
What unit development costs actually include
A development budget is broader than the building contract. Construction is usually the largest component, but it sits alongside design, planning, consultant, authority, site and finance costs. Leaving any of these out can make an early feasibility figure look attractive while creating pressure later.
The key cost areas normally include site acquisition or the existing land value, demolition where required, architectural and engineering documentation, town planning, surveys, permits, construction, external works, service connections, legal and finance costs, taxes and a realistic contingency. If the project is intended for sale, holding costs and selling costs also need to be part of the model.
For an owner-occupier retaining one dwelling and selling the other, the priorities can be different from a pure development. The quality of the home you will live in, the functionality of private open space, acoustic separation and long-term maintenance may matter more than extracting every possible square metre. Good feasibility work accounts for the intended outcome, not just the number of dwellings on the site.
Start with the site, not a square-metre rate
Square-metre rates can help with early discussions, but they are not a reliable total for a unit project. Two sites with the same land size can require very different levels of work.
A relatively flat block with clear vehicle access, accessible services and straightforward planning controls is one proposition. A sloping site, narrow frontage, heritage overlay, drainage issue or restricted access is another. Soil classification can change footing and slab requirements. Existing trees may require arborist reports, protection zones or a revised building footprint. A stormwater solution may involve detention, drainage upgrades or careful coordination with council requirements.
Demolition also varies. Removing a simple weatherboard home is different from managing asbestos, substantial concrete, difficult access or neighbouring structures that need protection. Before treating demolition as a fixed allowance, make sure the scope identifies what is being removed, how materials will be managed and whether disconnections are included.
This is why an early site review is worth the effort. It identifies the items that do not show up in a glossy concept drawing but can materially affect the budget and build programme.
Design, planning and consultant costs
The first investment is usually in information. A properly coordinated project needs more than a floor plan. Depending on the site and proposal, it may require a feature and level survey, soil test, town planning assessment, architectural drawings, civil design, structural engineering, energy rating, landscape design, building surveyor input and specialist reports.
Town planning is often the most variable pre-construction stage. Whether a permit is required, how neighbours respond, the relevant planning overlay and council workload can all influence the timeframe and professional fees. A proposal that needs redesign after a request for further information is not necessarily a failed project, but it must be allowed for in both time and cost.
The cheapest design pathway is rarely the one with the lowest fee. Incomplete or poorly coordinated documentation tends to produce ambiguous pricing, variations and delays on site. Clear drawings and specifications let a builder price the same scope that you expect to receive. They also resolve practical matters early, such as window locations, waterproofing interfaces, drainage falls, fire separation and access for construction.
Construction costs are driven by scope and specification
Once permits and documentation are in place, the construction contract should define exactly what is being built. The main works generally cover demolition and site establishment, footings and slab, framing, roof, windows, cladding or brickwork, services, linings, joinery, finishes, landscaping and completion works.
The number of dwellings does not simply double the price of one home. Some costs are shared across the site, while others increase because dual occupancy requires separate services, fire and acoustic separation, additional kitchens and bathrooms, distinct outdoor areas, driveways and fencing. The geometry of the buildings matters as well. Simple forms are generally more efficient to build than designs with complex rooflines, extensive steel, large spans or many custom junctions.
Specifications make a substantial difference. Kitchen cabinetry, appliances, tapware, tiles, flooring, glazing, heating and cooling, façade materials and landscaping all have a price range. There is nothing wrong with a premium finish, provided it has been selected deliberately and included in the contract rather than left as a vague allowance.
The same principle applies to the elements you cannot see at handover. Proper flashing, waterproofing, bracing, insulation and drainage are not places to reduce scope. They protect the homes long after paint colours and fixtures have changed. A disciplined builder prices these details, coordinates the trades responsible and inspects work at the stages where defects can still be corrected.
External works and services are commonly underestimated
External works can be one of the largest gaps between an early concept budget and the final development cost. They include driveways, crossover works, paths, fencing, retaining walls, decks, letterboxes, landscaping, rainwater tanks and site drainage. On constrained sites, temporary protection, hoarding, traffic management or additional access arrangements can also be required.
Then there are authority connections and upgrades. Water, sewer, gas, electricity, telecommunications and stormwater connections need investigation early. Existing infrastructure may not be suitable for the proposed dwellings, and the location of easements, pits, poles and mains can dictate design decisions. Allowing a nominal figure without confirming requirements is a common source of budget shock.
If the homes will be separately titled, subdivision and plan registration introduce another workstream. Surveying, legal services and authority requirements should be mapped alongside construction, not treated as an administrative task for the end of the project.
Fixed pricing depends on complete information
A fixed-price building contract is valuable because it gives owners clarity over the construction scope. But a price can only be genuinely fixed where the documents, engineering, inclusions and site information support it. If selections remain unresolved or site conditions are unknown, allowances and provisional sums may be necessary.
That does not make a contract unreliable. It means the owner should understand which figures are fixed, which are allowances and what events may trigger a variation. Ask for clarity around soil conditions, service connections, owner-supplied items, council conditions, landscaping, demolition exclusions and any work described as provisional.
A transparent builder will explain the assumptions rather than bury them in fine print. At Builda Group, this is approached through detailed scope review, trade-led construction planning and documented inspections throughout the build. The objective is not to promise an unrealistically low starting figure. It is to establish a contract that reflects the job required.
Allow a contingency and protect the programme
Contingency is not a slush fund for indecision. It is a controlled allowance for risks that cannot be fully removed before work begins, particularly on renovation-heavy sites or older properties. Unexpected ground conditions, latent issues, authority requirements and necessary design changes can all arise despite careful preparation.
The appropriate contingency depends on the maturity of the documentation and the risk profile of the site. A fully documented new dual occupancy on a well-understood block may carry less uncertainty than a complex inner-Melbourne project involving demolition, retaining, difficult access and planning conditions. Your designer, builder and finance adviser should test the allowance against the actual project rather than applying a generic percentage without context.
Time also costs money. Interest, rates, insurance, rent and lost sale opportunities can accumulate when a programme extends. A realistic build sequence, timely selections and prompt decisions help protect both the schedule and the budget. Changes made after trades have commenced are typically more expensive than decisions made during documentation.
How to assess a unit development budget properly
A useful budget separates hard construction costs from soft costs, authority costs, holding costs and contingency. It also records what has been confirmed and what remains an estimate. That distinction lets you compare options honestly: retain and build behind, knock down and rebuild, modify the dwelling mix, or adjust the specification.
When comparing builder proposals, compare scope before comparing the bottom line. Check whether each price includes the same engineering, finishes, site works, drainage, services, external works and permits. A lower quote can be legitimate, but it can also reflect exclusions that will return later as variations. The aim is not merely to find the lowest number. It is to understand the number you are committing to.
A well-run unit development begins with a budget that acknowledges the real work behind two finished homes. Get the site understood, document the details, question every allowance and leave room for genuine risk. That is how a promising block becomes a properly built outcome, rather than an expensive lesson in what was missed.