Second Storey Addition Structural Engineer Melbourne: What You Need to Know

Adding a second storey to an existing Melbourne home is structurally more demanding than building a new two-storey house. The existing structure was designed for one storey of load. It may or may not have the capacity to carry a second. Getting that question answered correctly before design progresses saves significant time and money.

A structural engineer for a second storey addition in Melbourne assesses the existing footings, walls, and framing, then designs the new level so it performs safely and gets a building permit. This post explains what that process involves and what to expect.

Assessing the existing structure first

The structural engineer's first task on a second storey addition is not designing the new level. It is understanding what the existing house can and cannot support. This assessment drives the whole design approach.

• Review of existing drawings (if available) for footing type, reinforcement, and design loads
• Site inspection to assess visible structural elements: wall framing, subfloor framing, footing exposure where accessible
• Assessment of the building's age and likely construction standard
• Identification of any existing modifications, extensions, or structural deficiencies
• Determination of whether the existing footing system can accept second storey loads

Can the existing footings carry a second storey?

A single storey brick veneer house exerts roughly 5 to 8 kN per linear metre on the external footing strip (depending on roof type and brick density). Adding a second storey of similar construction doubles or more than doubles that load. Unreinforced strip footings designed for single storey loads may not have the bearing area to distribute the increased load to the soil without excessive settlement.

Whether the existing footings are adequate depends on:

• The footing type: unreinforced strip, reinforced raft, or pier and beam
• The footing dimensions: width and depth below ground level
• The soil class at the site under AS 2870
• The magnitude of the additional load from the second storey

If the existing footings are inadequate, options include widening the footing strip (requiring excavation adjacent to the existing footing), installing concrete piles, or redesigning the load path so the new second storey loads are carried on new footings rather than the existing ones. The structural engineer assesses each option against the site conditions and the architectural intent.

Existing wall framing adequacy

The ground floor walls of the existing house carry the second storey loads down to the footings. Those walls were designed for the original single storey configuration. The structural engineer checks whether the existing stud framing, top plate connections, and wall bracing are adequate for the increased load and, where the architectural design requires new openings or relocated walls, designs the modifications.

Brick veneer houses often have the brick skin as the external leaf only, with a timber stud frame carrying the floor and roof loads. The structural check focuses on that internal stud frame. Double brick and cavity brick construction carries loads through the masonry itself, which has different structural characteristics and may require different approaches for the second storey connections.

Structural design for the new level

The new second storey is designed as a complete structural system. PBE produces drawings covering:

• Floor structure for the new level: floor joists or engineered floor system sized and spaced to AS 1684
• Wall framing for the new level: stud size, spacing, and bracing wall layout for wind resistance
• Roof structure: rafter sizing, ridge support, ceiling joist design, and roof bracing
• Wind tie-down: continuous load path from roof to footing to AS 4055
• Lintel and beam design over all window and door openings in the new level

Stair opening in the existing floor

One of the specific structural challenges in a second storey addition is the stair opening through the existing ground floor ceiling and roof space. The engineer designs the trimmer joists and header beams around the opening to redirect the floor loads that were previously carried by the cut joists. The connection of this trimmer framing to the existing floor structure is detailed in the drawings.

Connection between new and existing structure

The junction between the new second storey floor and the existing roof or top plate of the ground floor is a critical structural detail. The new floor joists need to bear adequately on the existing or new top plate, the lateral connection must resist wind loads, and any point loads from the new upper floor posts or columns must be transferred into the ground floor wall framing in a controlled way.

Engineering fees for a second storey addition

These fees include the existing structure assessment, structural design for the new level, and the full permit documentation package. They exclude the building permit fee, soil report, and construction costs.

Project timeline

A realistic timeline for a second storey addition from engaging the structural engineer to starting construction:

• Structural engineer assessment and design: 3 to 5 weeks after architectural plans are confirmed
• Building permit (private building surveyor): 4 to 8 weeks from complete application submission
• Construction: typically 3 to 6 months depending on scope and builder

Total timeline from design engagement to construction start is commonly 3 to 5 months. Starting the structural engineering process as soon as the architectural drawings are close to final reduces the overall programme significantly.

Frequently asked questions

Do I need to assess the existing structure before designing a second storey addition?

Yes. The structural adequacy of the existing footings and wall framing must be confirmed before the new level can be designed. Skipping this step risks designing a second storey that cannot be built without significant and expensive structural modifications that were not budgeted.

How does the structural engineer know if the existing footings are adequate?

The engineer reviews any available drawings for the original building, inspects accessible footing elements (including subfloor piers where present), checks the soil report for the site classification, and calculates the load that will be imposed by the second storey. If the existing footings cannot carry the additional load at the applicable safety factors under AS 2870, the engineer designs a strengthening solution.

What if no original drawings are available for my house?

This is common for Melbourne homes built before 1980. Without drawings, the engineer works from a site inspection: measuring accessible structural elements, assessing the construction type, and making conservative engineering judgements where elements cannot be directly measured. In some cases, a small amount of selective investigation (such as exposing a footing edge or cutting a small opening in the ceiling to measure a joist) is needed to confirm key assumptions.

Can a second storey addition be built without a building permit?

No. A second storey addition is a Class 1 structural alteration in Victoria and requires a building permit under the Building Act 1993. The permit requires structural drawings and specifications prepared by a registered structural engineer. Proceeding without a permit creates serious liability issues at resale and may require demolition if the work is discovered by the council.

Is a second storey addition more expensive to engineer than a ground floor extension?

Yes, for two reasons. First, the existing structure assessment adds a scope that is not needed for a new single storey extension. Second, the structural design for a second storey level involves more elements (floor structure, wall framing, roof structure, wind tie-down through the full height) than a simple single storey extension. Engineering fees for a second storey addition are typically $1,500 to $3,000 higher than for a single storey extension of similar footprint.