Garden Office Electrics: Supply and Wiring
A garden office needs a properly designed sub-main: the right SWA size for the load and run, correct burial, a small consumer unit, and careful attention to earthing.
Garden offices, studios and home gyms have made outbuilding supplies a routine job. Done properly it's a new circuit — usually a buried SWA sub-main from the house to a small consumer unit in the outbuilding — designed for the load, the distance, and the earthing arrangement. It's notifiable work that needs full testing and certification.
This guide covers sizing the supply, cable type and burial, the outbuilding consumer unit, and the earthing considerations that catch people out, particularly on TN-C-S (PME) supplies. It assumes design and installation to BS 7671, Amendment 2, with safe isolation before work at the origin.
Key takeaways
- A garden office is a new circuit: a designed sub-main to a small outbuilding consumer unit.
- Size the SWA for the load and — critically — the voltage drop over the run length.
- Bury SWA at an adequate depth with marker tape, or protect it where surface-run.
- Earthing needs care on PME supplies — an earth electrode may be needed for outdoor risks.
- It's notifiable work requiring full testing and an Electrical Installation Certificate.
Sizing the supply
Work out the anticipated load — lighting, sockets, heating, IT, maybe a small kitchenette — apply diversity, and size the circuit for the design current. Then, because outbuilding runs are long, check voltage drop: a cable that carries the current fine can still drop too much voltage over 20 or 30 metres, so the run length often drives the cable size more than the load does.
The result is usually steel wire armoured (SWA) cable, sized from the load, the length and the installation method, protected by an appropriately-rated device at the house end.
Voltage drop drives the size
On a long garden run, voltage drop — not current capacity — is often the deciding factor. Calculate it for the actual length before you settle on a cable size.
Cable type and burial
SWA is the standard choice: it's robust, suitable for direct burial, and its armour can serve as the CPC. Buried cable should be laid at an adequate depth, on and covered with sand or sifted soil, with marker tape above it to warn future diggers, and ideally routed away from where people will dig.
Where the cable runs on the surface or up a wall, protect it from mechanical damage and UV. Provide a proper gland at each end and don't rely on the armour as an earth without checking its cross-section is adequate for the fault current.
Doing this on a job? Electrical Installation Certificate — part of TradePlanr. Free to start, no card needed.
The outbuilding consumer unit and earthing
At the outbuilding, fit a small consumer unit with RCD protection for the final circuits. This gives local isolation and protection and keeps the outbuilding's circuits tidy and testable.
Earthing is the detail that trips people up. On a TN-C-S (PME) supply, extending the PME earth to an outbuilding — particularly one with outdoor exposed metalwork or an EV point — can be problematic, and it may be necessary to establish a separate earth electrode and treat the outbuilding as TT for some or all of its circuits. Assess this on the specific installation rather than assuming.
Certify the new circuit
Issue the Electrical Installation Certificate from TradePlanr for the sub-main and outbuilding circuits, with the design and test results, so the notifiable work is properly documented.
Testing and notification
Test the sub-main and the outbuilding's circuits in full — continuity, insulation resistance, polarity, Zs and RCD operation — and confirm the earthing arrangement performs as designed. This is notifiable work, so it requires an Electrical Installation Certificate and notification to building control, usually via your competent person scheme.
Hand over the certificate and a clear description of the earthing arrangement, especially if you've established a local electrode, so any future work is done on an accurate picture.
From guidance to action
Frequently asked questions
What cable do I need for a garden office?
Usually steel wire armoured (SWA), sized from the load, the run length and the installation method. Because runs are long, voltage drop often drives the cable size more than the current-carrying capacity does.
How deep should the SWA be buried?
At an adequate depth to avoid damage, bedded and covered in sand or sifted soil with marker tape laid above it to warn future excavation. Where it can't be buried deep enough, provide additional mechanical protection.
Is there an earthing problem with a garden office on a PME supply?
There can be. Extending a TN-C-S (PME) earth to an outbuilding, especially with outdoor metalwork or an EV point, can be problematic and may require a separate earth electrode, treating the outbuilding as TT. Assess it on the specific installation.
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