COST GUIDE

Outbuilding Electrical Installation Cost UK 2025: Garden Office Wiring Prices

Detailed breakdown of outbuilding electrical installation costs in the UK for 2025 — garden shed wiring from £800, garden office from £1,500, SWA armoured cable and trenching costs, sub-board requirements, and Part P compliance.

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11 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

  • 1An outbuilding electrical supply and installation typically costs £800 to £3,000, with the primary cost driver being the length of the underground armoured cable run and the complexity of the trenching route.
  • 2All underground supplies to outbuildings require SWA (Steel Wire Armoured) cable buried at a minimum of 500mm depth, with sand bedding and cable covers — adding £20 to £50 per metre to the installation cost.
  • 3A dedicated sub-board (consumer unit) inside the outbuilding is required under BS 7671 and provides local overcurrent protection and isolation for the circuits within the outbuilding.
  • 4Garden offices with data points, multiple circuits, air conditioning, and high-speed internet connections cost more to wire than a simple garden shed — budget £1,500 to £3,000 for a well-specified garden office.
  • 5All outbuilding electrical work is notifiable under Part P of the Building Regulations. A competent person scheme electrician self-certifies and provides the required EIC and compliance certificate.
01 · Cost Guide

Outbuilding Electrical Supply — What Is Involved

Supplying electricity to a garden office, workshop, summerhouse, or outbuilding is one of the most common domestic electrical jobs. It transforms a cold storage space into a productive, comfortable work or leisure area. The cost depends primarily on the distance from the house, the nature of the ground between the two buildings, and the scope of the internal electrical installation.

  • Garden shed or summerhouse (simple) — £800 to £1,400. SWA cable, short garden run, small sub-board, lighting and socket circuits.
  • Garden office or workshop (medium) — £1,500 to £2,500. Longer cable run, more circuits, data points, heating circuit, and sub-board.
  • Large outbuilding or annexe (complex) — £2,500 to £3,500+. Long cable run, trenching under hard landscaping, larger sub-board, multiple circuits, and potentially a three-phase supply.
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02 · Cost Guide

Typical Costs by Outbuilding Type

The internal electrical fit-out varies significantly depending on how the outbuilding will be used.

  • Garden shed — £800 to £1,200 total. Typically one lighting circuit, four double socket outlets, and a consumer unit. Armoured cable run of up to 15 metres assumed.
  • Summerhouse or leisure room — £1,000 to £1,800. Lighting, multiple socket outlets, electric heater circuit, and data point for Wi-Fi extender or Cat6 data cable.
  • Garden office (home working) — £1,500 to £2,500. Multiple power circuits, dedicated data points (Cat6), USB outlets, electric panel heater or air source heat pump connection, exterior security light.
  • Workshop or hobby room — £1,500 to £2,800. Dedicated circuits for power tools and machinery, stronger overhead lighting (fluorescent or LED strips), potentially three-phase supply for larger machinery.
03 · Cost Guide

Supply Options: Underground vs Overhead

There are two routes for supplying electricity from the house to an outbuilding — underground or overhead. Both require SWA armoured cable.

  • Underground (preferred) — SWA cable buried at 500mm depth in a garden, on a bed of sharp sand with cable covers above. Invisible once reinstated. Requires trenching which adds £15 to £50 per metre depending on surface type.
  • Overhead catenary — SWA cable suspended on a catenary wire between the house and outbuilding. Minimum height of 3.5 metres above ground (5.2 metres over vehicle routes). Cheaper than going under a concrete driveway but visually intrusive.
  • Surface-mounted on fence or wall — SWA cable clipped to a fence or wall surface is acceptable in some situations and avoids both trenching and aerial installation. Must be protected from mechanical damage at low level.

The IET On-Site Guide and BS 7671 Appendix 4 provide the reference method and correction factors for armoured cable in various installation conditions. Burial depth correction factors must be applied to the cable's current-carrying capacity.

04 · Cost Guide

Sub-Board and Consumer Unit in the Outbuilding

A sub-board (local consumer unit or distribution board) is installed inside the outbuilding at the point where the armoured cable enters. It is required by BS 7671 and provides local protection and isolation.

  • 4-way split-load board — £250 to £400 fitted. Two MCB slots each side of a dual RCD. Suitable for shed or simple garden room (lighting + sockets).
  • 6–8 way RCBO board — £350 to £550 fitted. Individual RCBO per circuit for better discrimination. Preferred for garden offices where a single fault should not cut power to everything.
  • Isolation facility — the main switch on the sub-board must be lockable or the sub-board door must be lockable, to satisfy the requirements of Regulation 537.2 (isolation and switching) of BS 7671.
05 · Cost Guide

Garden Office Electrical Requirements

A well-specified garden office requires more than a simple power circuit. Working from home means reliable power, good data connectivity, adequate lighting, and comfortable heating year-round. Planning the electrical installation carefully from the outset avoids expensive remedial work later.

  • Data connectivity — Cat6 data cable from the house router or a dedicated Wi-Fi access point. Running a single Cat6 cable underground in the same trench as the power cable adds very little cost at the installation stage.
  • Heating — electric panel heater (500W to 2kW depending on floor area), infrared heating, or air source heat pump. Dedicated circuit per heater. Smart thermostat and timer strongly recommended.
  • Power circuits — minimum of six double socket outlets in a typical home office, plus USB outlets. Separate circuit for IT equipment (computers, monitors, printers) is good practice.
  • Lighting — LED downlights or a high-quality LED strip for even task lighting. Bias lighting behind monitors reduces eye strain. External security light with PIR sensor at the outbuilding entrance.

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06 · Cost Guide

Trenching Costs for Outbuilding Supplies

Trenching is frequently the most variable cost element in an outbuilding electrical installation. The ground conditions, surface type, and length of the run all affect the price significantly.

  • Through lawn or garden (per metre) — £15 to £25 per metre, including digging, cable laying, sand bedding, cable covers, and reinstatement. A 15-metre run adds £225 to £375.
  • Through block paving (per metre) — £25 to £40 per metre. Blocks must be carefully lifted, stored, and relaid. More labour-intensive than grass reinstatement.
  • Under concrete or tarmac driveway (per metre) — £40 to £60 per metre. Requires breaking out, core drilling, or boring. Concrete reinstatement adds significant materials and labour cost. Consider an overhead alternative.
  • Mini-digger hire (long runs) — for runs over 20 metres through a lawn, hiring a mini-digger for half a day (£150 to £300) can reduce labour time significantly and produce a cleaner trench.
07 · Cost Guide

Building Regulations and Part P

Installing a new electrical supply to an outbuilding is notifiable electrical work under Part P of the Building Regulations (England and Wales). This applies to all new outbuilding supplies, regardless of the size or type of outbuilding.

  • Competent person self-certification — a registered NICEIC, NAPIT, or ELECSA electrician notifies building control and issues the Part P compliance certificate without a separate building control application.
  • Electrical Installation Certificate — the EIC documents the full installation including the supply cable, sub-board, and all circuits. Essential for property sale conveyancing.
  • Earthing — the outbuilding must have its own earth electrode (earth rod) if the supply protective conductor (earth) cannot be confirmed as effective at the outbuilding. This is assessed at the time of installation and tested with an earth rod resistance test.
08 · Cost Guide

For Electricians: Outbuilding Electrical Jobs

Outbuilding supplies are a consistently popular domestic job. The scope is well-defined, the work is satisfying, and there are natural upsell opportunities (data cable, security light, EV charger) that add value without significantly extending the installation time.

Quote Trenching Separately

Always quote trenching as a separate, itemised line. Surface type (lawn, block paving, concrete) makes an enormous difference to cost. Customers understand why a run under their concrete driveway costs more when it is clearly explained and priced transparently.

Offer Data Cable in the Trench

Always offer to pull a Cat6 data cable in the same trench as the armoured power cable. The materials cost is minimal, the trench is already open, and the customer saves significant money versus having it done separately. Use the quoting app to add it as an optional extra.

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