INSTALLATION GUIDE

Outbuilding Electrical Installation: Garden Office Wiring Guide UK

Everything you need to know about running electricity to an outbuilding — SWA armoured cable requirements, trench depths under paths and drives, sub-board installation, 30mA RCD protection, and why TT earthing is the correct earthing system for separate outbuildings.

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12 min readUpdated 2026-05-18Andrew Moore, Founder of Elec-Mate

Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.

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Key Takeaways

  • 1SWA (steel wire armour) armoured cable is the correct cable type for underground runs from the house to an outbuilding. Standard twin and earth flat cable must never be buried directly in the ground.
  • 2The minimum trench depth for buried armoured cable under a garden (where there is no risk of mechanical damage) is 500mm, and 600mm or more under paths, driveways, or other areas subject to digging or mechanical damage, in accordance with BS 7671 guidance.
  • 3An outbuilding receiving a separate supply requires its own sub-distribution board (sub-board) or consumer unit, with its own main isolator, circuit breakers, and RCD protection for each circuit.
  • 4TT earthing is the correct earthing system for most electrically separate outbuildings in the UK. The outbuilding requires its own earth electrode (typically a copper earth rod driven into the ground near the outbuilding) under Regulation 542.1.2.3 of BS 7671.
  • 5All outbuilding electrical installation work is notifiable under Part P of the Building Regulations 2010. An Electrical Installation Certificate (EIC) must be issued on completion.
01 · Installation Guide

SWA Armoured Cable: The Right Cable for Underground Runs

The cable used to supply an outbuilding from the main house must be suitable for direct burial in the ground. Standard PVC-insulated twin and earth flat cable (6242Y) is not suitable for underground use — it must never be buried directly in the ground, even in conduit, without additional armoured protection for runs in areas subject to digging.

  • SWA armoured cable — steel wire armour cable (to BS 5467 or BS 6724) has a PVC outer sheath, copper conductors, PVC insulation, and a layer of galvanised steel wire armour. The armour provides mechanical protection against spades and accidental digging, and also serves as the circuit protective conductor (CPC) when properly terminated using armour clamp glands.
  • Cable sizing — the cross-section of the SWA cable is determined by the total load of the outbuilding circuits, the length of the run, and the permissible voltage drop. For a garden office or workshop sub-board, 6mm² three-core SWA is typical for runs up to 25–30 metres. For runs over 30 metres or higher loads, 10mm² or 16mm² cable should be used to keep voltage drop within the 3% limit for lighting circuits and 5% for power circuits under BS 7671 Section 525.
  • Termination — SWA cable must be terminated at both ends using proper armour clamp glands. At the house end, this is typically a gland into the main consumer unit or an outdoor IP-rated junction box. At the outbuilding end, it terminates into the sub-board. Correct armour termination is essential — the armour must be clamped and connected to the earth bar at both ends.
  • Alternative — MICC cable or conduit — where the route is entirely in conduit with draw boxes (accessible for cable replacement), standard cable in rigid conduit to the appropriate depth is also acceptable. However, SWA armoured cable is the most common and practical solution for outbuilding supplies.
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02 · Installation Guide

Trench Depth for Buried Cable Under Paths and Drives

The depth at which the armoured cable is buried determines the level of mechanical protection it receives from accidental excavation. BS 7671 Appendix 4 and the IET On-Site Guide provide guidance on minimum depths.

  • Under open garden — 500mm minimum — where the cable runs under lawn or garden beds that will only be disturbed by hand tools (spades and forks), a minimum depth of 500mm to the top of the cable is the accepted practice. The cable should be laid on a bed of soft sand or fine soil, with cable cover tiles or marker tape placed 150mm above it before backfilling.
  • Under paths and drives — 600mm minimum — where the cable crosses a path, driveway, or any surface that may be broken up for repair or alteration, a minimum depth of 600mm to the top of the cable is required. Heavy-duty cable cover tiles must be laid above the cable at this depth. Where mechanical excavation is likely, additional protection in steel conduit or a cable duct is advisable.
  • Record the route — the cable route must be recorded on a drawing with measurements from fixed reference points (walls, fence posts). This drawing should be kept with the property's electrical installation documentation. Without a route record, any future excavation in the garden risks damaging the cable.
03 · Installation Guide

Sub-Board for the Outbuilding

An outbuilding with more than one circuit requires a sub-distribution board (consumer unit) at the outbuilding end of the supply cable. This provides a local main isolator and circuit protection for each circuit in the outbuilding.

  • Main isolator — the sub-board must have a main switch (double-pole isolator) so that all circuits in the outbuilding can be isolated safely for maintenance. The main switch rating must be equal to or greater than the maximum demand of all circuits in the outbuilding.
  • Circuit breakers and RCDs — each circuit in the outbuilding must have appropriate overcurrent protection (MCB) and RCD protection. An RCBO (combined MCB and RCD) for each circuit provides the best arrangement — a fault on one circuit does not trip all circuits. Alternatively, a dual-RCD board with MCBs is acceptable.
  • Enclosure suitability — the sub-board enclosure must be appropriate for the outbuilding environment. In a dry, enclosed building, a standard metal consumer unit is acceptable. In a workshop, garage, or agricultural outbuilding where dust or moisture may be present, an IP-rated enclosure (IP44 or higher) is required. Under Regulation 421.1.201 of BS 7671, consumer units in domestic premises must have non-combustible enclosures — a metal enclosure is standard.
04 · Installation Guide

RCD Protection for Outbuilding Installations

RCD (Residual Current Device) protection is essential for outbuilding electrical installations. The risk of electric shock in an outbuilding environment (where the user may be working outdoors or with garden equipment) is higher than in a domestic living room.

  • 30mA RCD on all socket circuits — under Regulation 411.3.3 of BS 7671, all socket-outlet circuits in an outbuilding must be protected by an RCD with a rated residual operating current not exceeding 30mA. This requirement applies regardless of whether the socket will be used to power garden equipment.
  • RCD protection for outdoor socket-outlets — any socket-outlet intended for use with equipment used outdoors must be protected by a 30mA RCD under Regulation 411.3.3. This includes sockets on the exterior wall of the outbuilding used for mowers, hedge trimmers, or power tools.
  • RCD operating time — the RCD must operate within 300ms at its rated residual current and within 40ms at 5× the rated residual current. This must be verified by testing during the commissioning inspection and recorded on the EIC schedule of test results.

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05 · Installation Guide

Earthing System for Outbuildings: Why TT is the Right Choice

The earthing system for an outbuilding is one of the most important design decisions in the installation. In the UK, most domestic properties use TN-C-S earthing (also called PME — Protective Multiple Earthing). However, PME earthing must not be extended to a separate outbuilding in most circumstances.

  • Why PME cannot be extended to outbuildings — in a TN-C-S (PME) system, the earth and neutral are combined in the supply cable. If the neutral conductor is broken between the transformer and the property, the PME earth terminal can rise to a dangerous voltage (up to 230V). This is safe in a house because all metalwork is bonded together, but in a separate outbuilding — where a person may simultaneously touch the earthed metalwork and true earth (the ground) — a risen PME earth is extremely dangerous. Regulation 8 of the Electricity Safety, Quality and Continuity Regulations 2002 restricts extension of PME earths to separate buildings.
  • TT earthing — the correct solution — in a TT earthing system, the outbuilding has its own independent earth electrode (typically a 1.2m copper earth rod driven vertically into the ground near the outbuilding) connected to the earth bar of the sub-board. The outbuilding earth is completely independent of the house supply neutral. Under BS 7671 Regulation 542.1.2.3, a TT earth electrode must achieve a resistance to earth of 200Ω or less for the RCD to operate within acceptable limits — in practice, most installations achieve well under 100Ω.
  • Earth electrode test — the resistance to earth of the electrode must be measured during commissioning and recorded on the EIC schedule of test results. If the measured value exceeds 200Ω, a second or longer earth rod must be installed, or specialist electrodes used.

The three-core SWA cable from the house provides the live, neutral, and CPC conductors. In a TT system, the CPC (armour) connects to the earth bar in the outbuilding sub-board, and the earth bar also connects to the local earth electrode. The house end of the armour connects to the main earthing terminal in the house — this is correct and required under BS 7671.

06 · Installation Guide

Part P Notification for Outbuilding Electrical Work

Running electricity from the house to an outbuilding is notifiable under Part P of the Building Regulations 2010. This applies whether the outbuilding is a garden shed, a workshop, a summer house, or a purpose-built garden office.

  • Use a competent person scheme electrician — a NICEIC, NAPIT, or ELECSA registered electrician can self-certify the work and notify the scheme on your behalf. You receive a completion certificate confirming the notification.
  • EIC on completion — an Electrical Installation Certificate (EIC) covering the entire outbuilding installation (cable route, sub-board, all circuits, and earthing) must be issued on completion. Keep this with your property records. Use the Elec-Mate EIC app to complete and issue the certificate on site.
07 · Installation Guide

For Electricians: Outbuilding Electrical Installations

Outbuilding electrical installations are satisfying, technically interesting jobs that combine cable routing, sub-board installation, earthing design, and full commissioning testing. These jobs generate consistent revenue and repeat business from homeowners upgrading their properties.

Complete the EIC Including Earth Test

Use the Elec-Mate EIC certificate app to record all test results including the earth electrode resistance value — a common omission that can cause issues if the installation is ever inspected.

Quote for the Full Installation

Use the quoting app to itemise SWA cable, glands, sub-board, earth rod, circuits, and testing. Clients appreciate a detailed quote that explains what they are getting — it also protects you from scope creep if additional work is requested later.

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