How to Estimate a Granny Flat (Secondary Dwelling Cost Estimate)

Even at 60 to 90 square metres, a granny flat still runs through every structural stage of a house build, so estimate each stage on its own measured quantities rather than folding them into a single square-metre figure.

Slab and footings still need excavation, formwork, reinforcement and the slab pour, plus a stepped or suspended slab where the block falls away. Frame and roof cover carpentry, trusses or rafters, and roof covering sized to the actual roof plan. Lock-up brings in windows, external doors, cladding and insulation to close the building in. Fit-out is usually the largest single cost driver in a granny flat, because a kitchen, bathroom and laundry are compressed into a small footprint but still need full joinery, tiling, waterproofing and fixtures.

• Slab & footings — excavation, formwork, reinforcement, concrete, waterproofing, termite protection
• Frame & roof — carpentry, trusses or rafters, roof covering, fascia and guttering
• Lock-up — windows, external doors, cladding, insulation
• Fit-out — kitchen, bathroom, laundry, internal linings, painting, floor coverings, fixtures

Every granny flat has to tie back into the existing house’s services, and that connection work is a distinct scope that deserves its own line items rather than a rough allowance. Sewer, water, power and often stormwater all have to run from the new dwelling to an existing connection point, and the distance and depth involved on your specific block will drive the cost far more than the size of the granny flat itself.

Sewer connections depend on fall — a gravity connection to an existing line is straightforward to price, but a low point on the block may need a pump-out system, which changes both the trade scope and the item list. Water and power tie-ins usually mean trenching across the yard, a new sub-board or metering arrangement, and reinstating whatever the trench disturbs along the way.

• Sewer — confirm fall to the existing connection point; price a pump system separately if gravity drainage is not available
• Water — new tie-in point, trenching length, backflow requirements
• Power — trenching, new sub-board or separate metering, distance from the existing switchboard
• Stormwater — connection point and any detention requirements for the added hard surface

A granny flat is built on a block that already has a house, a driveway, fences and gardens on it, and that changes how the job is priced compared with a vacant new-home site. Restricted access affects plant selection, material handling and how much work has to be done by hand — all of which belongs in preliminaries rather than being buried inside individual trade rates.

Narrow side access might rule out a standard excavator or concrete pump, pushing the job toward smaller plant, barrow runs or a concrete boom positioned from the street, all of which cost more than open-site access. Protecting the existing dwelling, driveway and any established landscaping during the build is a real cost, not a courtesy, and should be allowed for explicitly.

• Confirm access width and any overhead or side clearance before assuming standard plant can be used
• Price protection of the existing driveway, fences and gardens as its own item
• Allow for extra handling or smaller plant where wide-access assumptions do not hold
• Check bin and skip placement, and whether delivery trucks can reach the site at all

Council approval and certification costs are a real part of a granny flat build and need to sit in your estimate as clear allowance line items — but the pathway and the fees themselves depend on your council and state, so an estimate is not the place to make planning or legal calls.

Many secondary dwellings can go through a complying development or exempt pathway rather than a full development application, but eligibility depends on block size, zoning, setbacks and other local planning rules that vary by council. Certification, inspections and any contributions are genuine costs — allow for them as a line item and confirm the actual figures and pathway with your local council or a private certifier before committing to a number.

• Include an allowance for private certifier or council fees as its own line item
• Note in the estimate which approval pathway has been assumed, and flag it as unconfirmed if it has not been checked
• Never treat an estimate line item as confirmation that a design is approvable — that call sits with the council or certifier

Per-square-metre rates are usually built from full house data, where the cost of the kitchen, bathroom and services connection is spread across a much larger floor area. Apply that same rate to a granny flat and it will consistently under-price the job, because the fixed-cost trades — kitchen, bathroom, laundry, services connection and preliminaries — take up a much bigger share of a smaller build.

The table below illustrates how those stages typically weigh against each other on a small dwelling. It is an example breakdown only, not a quote — the real proportions on your job depend on the design, the block and your own current rates.