Can My Home’s Electrical Supply Support a Granny Flat?
Yes, many existing homes can supply a granny flat, but this cannot be confirmed from the size of the switchboard or the rating printed on the main switch alone.
The answer depends on the combined electrical demand of both dwellings, the capacity and condition of the incoming supply, and how the new load will be distributed and managed.
Why the supply question should come first
A granny flat may be compact, but electrically it behaves like a second home. It may have its own hot-water cylinder, cooking appliances, heat pump, laundry equipment, lighting and general power.
Those loads can operate at the same time as the appliances in the main house. During a Dunedin winter evening, for example, both homes could be heating while cooking, hot water and laundry loads are also running.
Assessing the supply before the design is finalised allows those demands to be planned for properly. It can also prevent late changes to switchboards, cable routes, trenching or the network connection.
Start with the capacity you actually have
The main switch rating does not tell you how much spare capacity is available.
A proper assessment may need to consider:
The type and size of the existing network connection
The service line and mains supplying the property
Metering and main-switch arrangements
The condition and capacity of the existing switchboard
The electrical demand already created by the main home
Any future plans for EV charging, solar, batteries or further electrification
Maximum demand is not calculated by simply adding every appliance rating together. It considers which loads are reasonably likely to operate at the same time.
That makes the proposed appliance schedule for the granny flat an important part of the assessment.
What will the granny flat use?
An all-electric granny flat with electric hot water, an oven, induction cooking, a heat pump and a clothes dryer will place a different demand on the property from a simple studio with modest appliances.
Future plans matter too. If the occupant may eventually charge an EV, or if solar and battery equipment could be added later, these should be considered before the electrical layout is locked in.
Clear information allows the supply and distribution design to be based on facts rather than broad assumptions.
Shared supply or a separate connection?
A granny flat can sometimes be supplied through a dedicated submain from the existing home. This typically means both dwellings share the same network connection and electricity account.
The other option is a separate network connection, meter and Installation Control Point. This can provide greater independence but may involve additional network work, metering requirements and cost.
The right arrangement can depend on intended use, billing, access, isolation, future subdivision and how independently the two dwellings need to operate. In Dunedin, a new or altered connection may also require coordination through Aurora Energy’s connection process.
This decision is best made early because it influences the electrical design and often the physical route used to bring power to the new dwelling.
Capacity is only one part of the foundation
Even when the incoming supply has enough capacity, the rest of the electrical foundation still needs to support the new dwelling.
This may include a correctly sized submain, suitable protection, clear fault separation, compliant earthing and isolation, and a dedicated distribution board for the granny flat. The condition and layout of the existing main switchboard may also affect what can be added safely and cleanly.
A modern-looking switchboard does not automatically prove the supply is sufficient. Equally, an older supply does not automatically mean a major upgrade is required.
What if the existing supply is constrained?
A constrained supply does not always lead directly to a full mains upgrade.
Depending on the findings, the practical solution might involve:
Load management to prevent certain high-demand appliances operating together
Controlled hot water or coordinated EV charging
Targeted switchboard or mains improvements
A three-phase or higher-capacity connection
A separate connection for the granny flat
The goal is to choose the simplest solution that supports the actual plan without creating another limitation later.
What about the new granny flat exemption?
Since January 2026, certain new, standalone, single-storey granny flats of up to 70 square metres may qualify for a building consent exemption when all conditions are met. However, the exemption does not remove the requirement for compliant and properly documented electrical work. MBIE’s granny flat guidance explains the wider conditions, while WorkSafe confirms the electrical certification requirements.
When should the supply be assessed?
Ideally, before the final layout, appliances, trenching and construction price are confirmed.
A useful assessment should establish what the property has now, what the granny flat will require, and whether the two can work together as one coordinated electrical system. It should also consider Capacity, Protection, Distribution, Condition and Future Ready planning.
A Power Integrity Review™ can provide that clarity and identify whether the property can support the granny flat as it stands, needs targeted modernisation, or requires a supply alteration.
The takeaway
Your home may already have enough capacity for a granny flat, but the answer should come from an assessment rather than an assumption.
Understanding the electrical foundation early gives you the best chance of choosing the right supply arrangement, avoiding repeated work and building a system that supports both homes properly.