Granny flat electrical: what a new supply connection involves.

The two supply options

Branch from the existing switchboard

The simpler and cheaper approach is to run a new sub-circuit or set of sub-circuits from your main house switchboard out to the granny flat. The flat shares the main metered supply with the house. There is one electricity account, one meter, and one bill that covers both dwellings.

This works fine when the granny flat is occupied by family members who are not paying separately for power. It is also the right choice when the main switchboard has capacity and the cable run is short.

The limitation is obvious: if you ever want to rent the flat to a separate tenant, you cannot split the electricity bill. You also cannot cut power to the main house without cutting power to the flat, and vice versa.

Separate metered supply via a DNO application

A separate metered supply means the granny flat gets its own electricity account: its own meter number, its own bill, its own connection point with the Distribution Network Operator (in the eastern suburbs, that is AusNet Services). The flat is electrically independent from the main house.

This requires an application to AusNet through a licensed electrical contractor. The process involves submitting a Notice of Intention to Connect, coordinating the new metering point installation with the network, and in some cases installing a new service pillar at the property boundary. The DNO approval has its own timeline, so this is not something you can arrange mid-construction.

The cost of a separate supply is higher than branching from the existing board. For most homeowners, it is worth it when the flat will be rented commercially, when the flat has a different owner from the main dwelling, or when you want a clean separation for body corporate or estate reasons.

Cable run from the main house to the flat

Regardless of which supply approach you choose, if the granny flat switchboard or sub-board is fed from the main house, you need a cable run between the two buildings. Almost always, this goes underground. The same sub-main approach powers detached workshops, where a sub-board with its own circuits sized for tool loads is the standard arrangement.

Underground conduit

Above-ground cable runs between buildings are rarely acceptable for permanent dwellings. The standard approach is a PVC conduit trench, typically 500 mm deep for low-voltage cabling under a garden area, deeper under driveways and hardstand. The conduit is run, backfilled with clean sand bedding, and a cable-cover warning tape is laid above it before the trench is refilled.

Cable sizing depends on two things: the load the flat will draw, and the length of the run. A typical granny flat (two bedrooms, kitchen, bathroom, reverse-cycle air conditioning) might have a peak demand of 15–20 A continuous. But the voltage drop across the cable run matters. A 16 mm² cable that is perfectly adequate for a 20-metre run may not be adequate for a 60-metre run if you want to stay within the 5% voltage drop limit in AS/NZS 3000. We calculate the run length and the anticipated load together before specifying the cable.

If a separate metered supply is chosen, the cable from the service pillar to the granny flat switchboard is the service entrance cable for that dwelling and is sized accordingly.

Smoke alarm requirements for a separate dwelling

The NCC (National Construction Code) and Victoria’s specific smoke alarm requirements treat a granny flat as a separate Class 1a building once it has its own kitchen and bathroom. That means smoke alarm requirements apply to the flat independently of the main house.

In Victoria, interconnected photoelectric smoke alarms are required. In a new granny flat build, they must be hardwired with battery backup, not just battery-only. They must be installed in each bedroom, in corridors adjacent to bedrooms, and on each storey. The interconnection requirement means that if one alarm triggers, all alarms in the dwelling sound.

If the flat is part of a complete new build, this is straightforward to include in the rough-in. If it is a conversion of an existing garage or studio, it often means running new wiring through finished walls; factor this into the scope.

RCD protection for a separate dwelling

For new work, AS/NZS 3000 requires RCD protection on the final sub-circuits in Class 1 buildings, and a granny flat is a Class 1 building. Plan for an RCD or RCBO on each granny flat circuit, including power, lighting and air conditioning.

The granny flat switchboard should include a safety switch for every outgoing circuit. The switchboard itself should be sized with spare ways for future circuits (air conditioning, an EV charger, whatever the tenant might add).

When a switchboard upgrade at the main house is triggered

Adding a granny flat supply from the main switchboard can trigger the need to upgrade the main board, for a few reasons.

First, capacity. If the existing board is fully loaded and you are adding a sub-board feed that might draw 40 A under peak conditions, the main board’s main switch and the incoming supply may not have the headroom. We check the main switch rating and the incoming supply fuse at the service pillar before committing to a branch approach.

Second, RCD coverage. If the main board is an older ceramic fuse board or an early circuit breaker board without RCDs, adding a new sub-circuit to it compounds the problem. A board upgrade that adds RCDs to all circuits, including the new granny flat sub-circuit, is often the right outcome.

Third, metering. If the main meter is old and the network requires a meter upgrade as part of the DNO connection process, the metering enclosure and associated infrastructure may need replacement at the same time.

None of this is unusual or alarming; it is part of the scope assessment that needs to happen before construction starts.

Practical notes for the eastern suburbs

Doncaster, Mitcham and Ringwood have a lot of late-1970s and 1980s homes on 700–900 m² blocks. The main switchboard on many of these homes is a 1980s or early-1990s board with limited spare capacity and often no RCDs or obsolete ceramic fuse protection. Adding a granny flat to one of these properties commonly involves:

• Switchboard upgrade at the main house (new enclosure, circuit breakers, RCDs, main switch)
• Underground conduit run across the block, sometimes 40–70 metres depending on the block layout
• Granny flat switchboard installation
• All granny flat circuits rough-in and fit-off
• Smoke alarm installation and interconnection
• DNO application if a separate supply is required

The switchboard upgrade alone is a meaningful cost, but it is work that most of these properties need regardless of the granny flat. Combining it with the flat project is usually more efficient than doing it twice.

Get the scope locked in before construction starts

The single biggest mistake I see is the granny flat builder starting work before the electrical scope is confirmed. The builder puts up the frame, insulates the walls, and then calls an electrician, who now has to route cables through insulated walls instead of open framing, and cannot adjust the switchboard location because the cabinet has already been built in. Agree the electrical layout, the supply method, and the switchboard location before the first timber goes up.

If you are planning a granny flat anywhere in Melbourne and want to work out the right electrical approach for your property, talk to us about the project before construction gets underway.