How to add a double storey extension to a single storey home

Before adding another level to a single-storey house, a structural engineer must determine whether the existing building can safely support the additional load. Their assessment helps the builder identify any reinforcement required before the renovation process begins.

Foundations and structural assessment

An engineer’s assessment typically considers the foundations, concrete slab, footings, load-bearing walls and overall condition of the existing home. They may also review original plans and arrange further testing where structural details are unclear.

Not every house is structurally ready for a double-storey extension in its current form. However, this does not necessarily rule out the project. The engineer and builder can recommend practical upgrades to strengthen the ground level and support the proposed addition.

Reinforcement and roof reconstruction

Depending on the assessment, the existing home may require reinforced walls, beams, columns or upgraded footings. Plumbing and electrical services may also need to be relocated or extended to accommodate the new floor.

The builder will generally remove part or all of the existing roof before constructing the new level and installing a replacement roof above it. This work must follow the approved engineering plans and building approval requirements through to inspection and completion.

Before you convert a single-storey home, consider who will manage the project, which approvals apply and how the new level will connect with the ground floor. Resolving these matters early can reduce delays, expensive design changes and disruption during construction.

Choosing a registered builder

A double-storey extension is a complex addition, so choose a registered builder with experience extending existing homes rather than constructing only new homes. In Victoria, a registered builder or building company is generally required for domestic building work valued at more than $10,000.

Ask prospective builders about:

• Their experience with second storey extensions
• Who will coordinate the architect, engineer and trades
• How they protect the existing house during construction
• Their approach to timelines, variations and communication
• Examples of comparable projects and homeowner references

One integrated team can make it easier to coordinate design, permissions and construction from concept through to completion. With more than 50 years’ experience in home extensions and renovations, Supa Group understands how to coordinate design, approvals and construction around an existing home.

Plans, permits and insurance

Your plans should account for structural requirements, local council rules, energy-efficiency standards, and the development's effect on neighbouring properties. Depending on the site and applicable planning controls, you may need a planning permit, a building permit or both. A required planning permit must generally be obtained before the building permit.

The proposed height, setbacks, overlooking, overshadowing and neighbourhood character may influence what the council will approve. These limits vary by property, so homeowners should confirm the requirements with their local council rather than relying on a general rule.

Your builder, architect or designer may manage applications on your behalf, but responsibilities should be clearly documented. Appropriate contracts and domestic building insurance should also be confirmed before work starts.

Staircase placement and layout

A staircase occupies space on two levels, so its location can significantly affect the existing and proposed floor plans. Placing it poorly may reduce usable living space, interrupt natural light or create awkward circulation between rooms.

The design team should consider how the staircase connects shared areas downstairs with bedrooms or private spaces upstairs. Its position must also allow for suitable headroom, structural support and safe access without compromising the functionality of the original home.

The cost of a second storey extension varies considerably because every existing home, design and site presents different requirements. Rather than relying on a broad price-per-square-metre estimate, homeowners should seek a project-specific quote that covers design, engineering, permits, structural work, finishes, and construction.

Size, design and finishes

The total floor area and number of rooms will influence the cost, but size is only one factor. A straightforward addition with bedrooms and a living area will generally involve different costs from a design with bathrooms, custom joinery or premium finishes.

Key cost drivers may include:

• Number and type of rooms added
• Complexity of the floor plan and roofline
• Bathroom, plumbing and electrical requirements
• Windows, flooring, cabinetry and other finishes
• Access limitations and site conditions
• Changes required to the existing ground floor

Structural and energy-efficient upgrades

Structural reinforcement can add to the budget where the existing footings, walls or framing need strengthening. Costs may also increase if the project involves extensive roof reconstruction, service upgrades or repairs uncovered after work begins.

Energy-efficient features such as improved insulation, high-performance glazing and efficient heating or cooling may require greater upfront investment. However, including them during the extension can be more practical than retrofitting them later and may help reduce long-term energy use.

A clear scope and fixed-price contract can give homeowners greater certainty before construction starts. 

You may be able to remain in your home during parts of a second storey extension, but it depends on the scope of work, access arrangements and how well the property can be kept safe and weatherproof.

Roof removal, structural changes and service interruptions can make some stages noisy, dusty or unsuitable for occupation. Your builder should explain when water, power or access may be affected and whether temporary relocation is recommended.

Before construction begins, confirm:

• Which areas of the home will remain accessible
• How dust, noise and site safety will be managed
• Whether temporary roofing will protect the existing home
• When plumbing or electrical services may be disconnected
• Whether children, pets or vulnerable household members can remain safely on site

For many homeowners, moving out during the most disruptive stages can reduce stress and help the construction team work more efficiently.

A double storey extension can take several months from initial design to final completion. The timeframe depends on the size and complexity of the build, structural upgrades, permit approvals, material availability and site conditions.

The process generally includes:

• Initial consultation and site assessment
• Design, engineering and cost planning
• Council and building approvals
• Product and material selection
• Structural work and construction
• Inspections, finishing and handover

In Australia, planning and building approval timeframes can vary between properties and councils. Approval is not simply a rubber stamp, particularly where the design may affect neighbouring homes or local planning controls.

Allowing enough time before construction helps reduce rushed decisions and unexpected delays. For families deciding between selling and buying a new property or choosing to expand their current home, the full project timeline should be considered alongside moving costs, lifestyle needs and long-term plans.

Adding another level is a major decision, but the right team can make the process far clearer. Supa Group manages second storey extensions from concept and design through to approvals, construction and completion, giving Melbourne homeowners one point of contact throughout the project.

With more than 50 years of continuous operation, fixed pricing, confirmed timelines and a 10-year structural guarantee, Supa Group helps families expand the homes they already love with greater confidence.