Why Outbuilding Wiring Is a Specialist Job
A garden office, detached garage, or large workshop is legally a separate structure to the house. That changes several things: the cable running underground between buildings is exposed to physical damage, groundwater, and root intrusion; the outbuilding needs its own earthing arrangements; and the whole installation is notifiable under Part P of the Building Regulations. It is not a job for plug-in extension leads or surface-run two-core cable.
The correct approach is steel wire armoured (SWA) cable buried at the right depth, a small consumer unit or RCBO board inside the outbuilding, and proper bonding of any metalwork in the building. When those three elements are right, the supply is safe, weatherproof, and ready for decades of use.
Cable Type: Why SWA Is the Right Choice
Steel wire armoured cable is the standard choice for underground runs to outbuildings. The armour layer provides mechanical protection against spades, fork tines, and ground movement. It also provides a robust earth path. The outer sheath is PVC or LSOH (low smoke zero halogen), resistant to UV, moisture, and most soil chemicals.
Common sizes for garden office supplies:
- 4mm² 2-core SWA — suitable for a small garden office with lighting, sockets, and a small panel heater (up to approximately 32A, protected by a 32A RCBO at the house end)
- 6mm² 2-core SWA — preferred for longer runs or where an EV charger, air source heat pump, or electric heating load is planned (up to approximately 40–45A)
- 10mm² 2-core SWA — used for larger workshops or where a sub-board will feed multiple heavy loads simultaneously
The correct cable size depends on the load (total wattage of everything that might run simultaneously), the cable run length (voltage drop increases with distance), and the rating of the RCBO or MCB protecting the circuit at the main consumer unit. We calculate this at survey and specify accordingly.
Burial Depth Requirements
BS 7671 and IEC 60364 specify minimum depths for buried cables based on the ground surface above them:
| Ground Type | Minimum Depth (SWA) |
|---|---|
| Garden / lawn / flower beds | 500mm |
| Paths, patios (pedestrian only) | 600mm |
| Driveways (light vehicles) | 600mm |
| Roads / areas with regular vehicles | 900mm |
Cable warning tape (bright yellow, printed with “Caution: Electric Cable Buried Below”) is laid in the trench at approximately 150mm depth, above the cable. This warns anyone digging in the future. We use mechanical cable route markers at each end of the run and note the route in the electrical installation certificate.
Can I dig the trench myself?
Yes. The groundworks — digging the trench, laying conduit or duct if desired, and backfilling after cable installation — are not Part P notifiable and can be done by the homeowner or a landscaper. We just need the trench to be the right depth and clear of any obstructions before we arrive. This can reduce the overall cost if you are comfortable with the groundworks.
Earthing Arrangements for Outbuildings
This is the part most homeowners and general handymen get wrong. An outbuilding fed from the main house supply is a “separate installation” under BS 7671 Section 705. The earthing rules are different from a standard socket circuit in the house.
The most important point: you cannot rely solely on the earth conductor in the SWA cable armour as the only earth path for the outbuilding. If the cable is ever damaged or disconnected, the outbuilding would become unearthed. The safe approach depends on whether there are animals in the outbuilding:
- No livestock: A separate earth conductor alongside or within the SWA supply cable is usually sufficient, with the outbuilding consumer unit earthed back to the main house earth terminal
- Livestock present (stables, chicken coops, kennels): An additional local earth electrode (earth stake driven into the ground outside the outbuilding) is required, with special equipotential bonding rules under BS 7671 Section 705 to avoid step potential voltage gradients that can harm animals
For the typical garden office with no livestock, a standard earth back to the house is correct — but we always install this as a dedicated earth conductor rather than relying on the cable armour alone.
The Outbuilding Consumer Unit
Any outbuilding with more than one circuit needs its own small consumer unit or RCBO board. For a typical garden office, this is usually a 4-way or 6-way unit with a main isolator switch and individual RCBOs for each circuit. Having a local board means:
- You can isolate the outbuilding independently without touching the main house board
- A fault in the office (tripped RCBO) is obvious and reset-able from inside the outbuilding
- Future additions (EV charger, extra sockets, air conditioning) are straightforward
- The installation looks professional and will pass any future EICR
A typical outbuilding board for a garden office might include: a 20A RCBO for the socket ring or radial, a 6A RCBO for lighting, and a spare way for any future heating circuit. If an EV charger is planned, a 40A RCBO way for that purpose can be included at installation stage — much cheaper than adding it later.
Circuits Inside the Garden Office
Once the supply arrives at the outbuilding board, the individual circuits are similar to any domestic installation:
- Socket circuit: A 20A radial circuit in 2.5mm² twin and earth is standard for a garden office of normal size. For a larger workshop with power tools, a 32A ring or a dedicated 16A radial for machinery may be appropriate.
- Lighting: A 6A circuit in 1.5mm² twin and earth, typically feeding LED downlights, a surface fitting, or external security light.
- Heating: A dedicated circuit for an electric panel heater or infrared heater. A 2kW panel heater draws about 9A — a dedicated 16A radial with 2.5mm² cable is clean and avoids overloading the socket circuit when the heater and a computer are running simultaneously.
- Data / network: Not electrical per se, but we often run Cat6 alongside the SWA supply in the same trench (in separate conduit) to give the office a wired network connection back to the house router. Far more reliable than Wi-Fi across a garden.
Part P: What It Means for Your Garden Office
Running a new circuit from the main house consumer unit to an outbuilding is notifiable work under Part P of the Building Regulations 2010 (England). This means it must either be carried out by a registered competent person (such as a NAPIT-registered electrician, who self-certifies) or notified to the local authority Building Control before work starts.
In practice, using a registered electrician is the standard route. We self-certify the installation, issue an Electrical Installation Certificate (EIC), and notify the relevant authorities. You receive a copy of the certificate. This is the document that proves the work was carried out to the correct standard — it matters when you sell the property, when you claim on insurance, or when a future EICR is carried out.
Work carried out without Part P certification is not compliant, may invalidate your buildings insurance, and will be flagged as a code C2 (potentially dangerous) or C3 (improvement recommended) on any future EICR — with the cost of remediation falling to the property owner.
How Much Does Garden Office Wiring Cost?
| Scope | Typical Cost (ex VAT) |
|---|---|
| Supply to small outbuilding, 10–15m cable run, sub-board, sockets + lighting | From £680 |
| Supply to garden office, 15–25m run, 4-way board, socket radial, lighting, heating circuit | From £880 |
| Longer run (25–40m) or larger cable size (6mm² or 10mm²) | From £1,100 |
| Including data (Cat6 in separate conduit, same trench) | Add £120–£200 |
| Consumer unit upgrade at house if needed to add supply way | From £380 |
| All prices exclude groundworks — trench cost varies by length and ground conditions | — |
Groundworks are usually the wildcard. A 15m run through a lawn is straightforward; the same run through tree roots, under a patio, or through a retaining wall is a different job. We quote groundworks separately and can recommend local groundwork contractors we regularly work alongside.
Plan for what you might want in five years
Adding an EV charger to a garden office supply later — after the trench is filled and the garden is reinstated — is expensive. If there is any chance you will want a charger, EV-ready 6mm² cable and a spare RCBO way in the sub-board costs very little extra now and saves a significant bill later. Same logic applies to air conditioning circuits.
Adding an EV Charger in a Detached Garage
A detached garage fed from the house is one of the most common locations for a home EV charger in Mid Somerset. The supply arrangements are the same as for any outbuilding — SWA cable, sub-board, correct earthing — with the additional requirement that the EV charger circuit must be protected by a Type A RCBO (minimum) and the charger must have its own 7kW (32A) dedicated circuit.
OZEV-approved chargers such as the Ohme ePod, Easee One, or Wallbox Pulsar Plus all require a dedicated circuit and an earth electrode if a PME supply is used. We assess the earthing arrangement at survey stage and confirm the correct solution for your installation. See our EV charger installation guide for full details.
What We Cover in Mid Somerset
DS Electrical carries out outbuilding and garden office wiring across Mid Somerset and BANES — Wells, Shepton Mallet, Bath, Frome, Radstock, Midsomer Norton, Street, Bruton, Castle Cary, Cheddar, Wincanton, and surrounding villages. We supply and install armoured cable, outbuilding consumer units, and all associated circuits, and issue the Part P certificate on completion.
Call 07889 334849 or use the button below to get a quote for your garden office or outbuilding supply.