Solar and home extensions in Perth: planning for future changes

Planning a home extension or major renovation in Perth alongside — or after — your solar installation requires coordination between your solar system and the building work. Getting it right the first time avoids expensive rework later.

When the extension comes after the solar system

The most common scenario: solar was installed on the existing home, and you're now planning an extension that changes the roof or adds significant electrical load.

New roof area from the extension: If the extension adds a new roof section, consider whether that area should have additional panels. A rear extension may add a north-facing roof that currently doesn't have panels. This is the best time to add panels — minimal additional labour cost for panels on a new roof section that already has scaffolding and workers on-site.

Existing panels on the extension area: If the extension footprint requires building over, adjacent to, or connecting to a roof section that has panels, the panels may need to be temporarily removed, repositioned, or worked around. Coordinate with your solar installer during the building design phase, not during demolition.

Roof penetrations and conduit runs: During the extension's construction phase, additional conduit runs can be installed at a fraction of the cost of a retrofit (see the guide on solar and renovations for the specific savings). If you're planning to add panels to new extension roof area, run conduit during the build.

When solar comes after the extension

If you're renovating now and want to add solar afterward, the best time to prepare is during the building work:

Pre-install conduit routes: Ask your builder to install conduit runs from the roof to the switchboard location while walls are open. A 32mm conduit from the roof space to the meter box area, installed during wall framing, costs $100–$300 to the builder. The same cable route retrofitted after lining could cost $600–$1,500 depending on wall construction.

Reserve switchboard space: Ask your electrician to reserve dual-pole positions for the solar AC isolator and any future inverter circuit. Adding positions to a full switchboard later requires either a new switchboard or a sub-board — more expensive than leaving space during the initial upgrade.

Three-phase consideration: If your extension includes large loads (workshop machinery, multiple large air conditioners, EV charger planning), a three-phase supply upgrade now enables a larger solar inverter later without electrical infrastructure changes.

Switchboard upgrades during extensions

Home extensions frequently require a switchboard upgrade — older switchboards may be at capacity (full positions, old ceramic fuses, insufficient main fuse size for the new loads). A switchboard upgrade during an extension is an opportunity to:

1. Install a switchboard large enough for solar: minimum 24-position board recommended (positions for solar circuit, potential battery circuit, EV charger circuit, plus existing loads)
2. Upgrade to a 100A main switch (some older Perth homes have 63A mains) — required for larger solar + battery + EV combinations
3. Install a dedicated solar AC isolator position near the inverter location
4. Run a cable/conduit from the main switchboard to the planned inverter location

Load calculations for extension + solar

A home extension adds floor area, adds air conditioning zones, potentially adds a home office or workshop, and may add a pool or pool equipment. These additional loads change your solar sizing calculation:

• Before extension: Household uses 18,000 kWh/year, existing 10kW solar system
• After extension (new master suite, home office, additional AC zone): Projected 22,000–25,000 kWh/year

If you're planning solar at the same time as the extension, size for the post-extension consumption — not the pre-extension usage.

If solar is already installed, the extension may justify adding a second inverter and additional panels if the roof area and system design support it (your installer can assess whether your existing inverter can be expanded or a second inverter added).

Heritage and character areas in Perth

Some Perth suburbs have local planning policies that restrict visible modifications to the streetscape. Subiaco, Fremantle, Cottesloe, and parts of the inner-northern suburbs have heritage overlays or character area provisions.

For extensions in these areas:

• Planning approval may be required for the extension itself, and solar panel placement may be assessed as part of the approval
• Panels visible from the street (front roof sections) may require design consideration to meet character area requirements
• Rear-of-property panels are typically unrestricted even in character areas

If your extension requires planning approval, confirm with your local council (City of Subiaco, City of Fremantle, Town of Cottesloe, etc.) whether solar panel placement is part of the planning assessment.

Energy modelling for extension + solar combinations

Before committing to either the extension design or the solar system size, it's worth modelling the expected annual energy consumption and solar generation for the completed property. Key variables:

• Post-extension floor area and cooling zone count
• Planned occupancy (more people? home office?)
• New EV or pool (if applicable)
• Roof orientation and available area on both existing and extension sections

This modelling informs whether a 6.6kW, 10kW, or 13.3kW system is appropriate, and whether a battery makes sense given the post-extension load profile.

The single most cost-effective solar planning action during a Perth home extension is running conduit from roof space to switchboard while walls are open — a $200 investment that saves $600–$1,500 on a retrofit. Coordinate solar system sizing with the post-extension load profile, not the current one, to avoid under-sizing.