Do I need a sub-board in my garden room?

If the garden room has more than one circuit — for example, separate lighting and socket circuits — a sub-distribution board (consumer unit) in the garden room is required. The sub-board provides a local main isolator, individual circuit protection, and RCD protection for each circuit. It also makes fault-finding and maintenance much simpler. For a very simple garden room with only one circuit (for example, a single lighting and socket circuit on one MCB with RCD protection), a single-way consumer unit or a fused connection unit with RCD is technically acceptable, but a proper sub-board is always the better solution.

How far can electricity travel to a garden room without voltage drop issues?

The maximum permissible voltage drop from the consumer unit to any point in a circuit is 3% for lighting and 5% for power under BS 7671 Section 525. In practice, for a 6mm² cable feeding a garden room sub-board with a 32A protective device, the maximum run before voltage drop becomes an issue is approximately 35 metres. For longer runs, 10mm² or 16mm² cable is required. Your electrician will calculate the correct cable size for your specific installation based on the distance, the load, and the protective device rating.

Can I add an EV charger to my garden room electrical installation?

Yes, but it must be planned from the start. A 7kW EV charger draws up to 32A continuously, which is a significant load. The SWA cable from the house must be sized to supply both the garden room loads and the EV charger — this typically means 16mm² or 25mm² SWA rather than 6mm². The EV charger requires a dedicated 32A circuit with its own RCBO in the garden room sub-board. EV charger installations are also subject to BS 7671 Section 722 (Electric Vehicle Charging Installations) and must comply with The Electric Vehicles (Smart Charge Points) Regulations 2021.

What internet connection options work for a garden room home office?

The most reliable option is a hardwired ethernet connection — CAT6 cable run in conduit alongside or within the SWA cable trench. At the house end, the cable connects to the router or a network switch. At the garden room end, it terminates in a standard ethernet socket. This provides gigabit-speed internet without the reliability issues of Wi-Fi. Wi-Fi extenders and powerline adapters are alternatives but are less reliable for video conferencing and large file transfers. Run the CAT6 conduit during electrical first fix — it costs very little extra.

Do I need planning permission to wire a garden room?

The electrical installation is controlled by Part P of the Building Regulations, not planning permission. Whether the garden room structure itself requires planning permission depends on its size, height, proximity to boundaries, and whether the property is in a conservation area or has Article 4 directions in force. Many garden rooms are permitted development. The electrical installation requires Part P notification regardless of whether planning permission was needed for the building.

Can I use a normal extension lead to power my garden room temporarily?

No — not even temporarily. Extension leads are not designed for permanent outdoor use, are not compliant with BS 7671 for fixed installations, and present a real fire and electric shock risk. A temporary supply while the garden room is being built can be provided using a properly terminated outdoor socket on the house, but this must be a fixed outdoor socket, not a socket extended into the garden via a cable run across the ground. The permanent installation must be completed before the garden room is occupied.

What is the best consumer unit for a garden room?

A modern metal consumer unit with RCBO protection for each circuit (rather than a dual-RCD board) is the best choice for a garden room. RCBOs provide individual RCD and overcurrent protection per circuit — if one circuit trips, the rest of the board remains live. This is particularly useful in a home office where a fault in the electric heater should not also take out the lighting and computer circuits. The consumer unit must have a non-combustible enclosure under Regulation 421.1.201 of BS 7671 — metal is the standard choice.

INSTALLATION GUIDE

Garden Room Electrical Installation: Home Office Wiring Guide UK

Everything you need to know about wiring a garden room or home office — dedicated circuit vs sub-board, cable sizing for long runs, EV charging integration, internet and data infrastructure, insulation considerations, consumer unit options, and Part P compliance.

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13 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

1A garden room used as a home office or studio requires a properly designed electrical installation — not a socket from the house on an extension lead. SWA armoured cable must be buried in a trench between the house and the garden room.
2For garden rooms up to approximately 30 metres from the house with modest electrical loads (lighting, sockets, no heavy heating), 6mm² three-core SWA is typically the correct cable. Beyond 30 metres, 10mm² or larger is required to keep voltage drop within BS 7671 limits.
3If EV charging is planned at the garden room (for example, in a driveway adjacent to the garden room), this must be included in the electrical design from the start — a 7kW EV charger requires a dedicated 32A circuit and significantly increases the cable and sub-board specification.
4Internet connectivity for a garden room home office should be planned at first fix — CAT6 ethernet cable and conduit for future cabling runs cost very little when installed during the electrical first fix but are expensive and disruptive to add retrospectively.
5All garden room electrical installation work is notifiable under Part P of the Building Regulations 2010. A TT earthing system with a local earth electrode is required in virtually all cases, and an Electrical Installation Certificate (EIC) must be issued on completion.

01 · Installation Guide

Dedicated Circuit from the House or Sub-Board in the Garden Room?

The first design decision for a garden room electrical installation is whether to run a single circuit from the house to the garden room (powering everything from one protective device) or to install a sub-board in the garden room with individual circuits for lighting, sockets, and heating.

Dedicated single circuit — when appropriate — for a very small garden room (a simple writing shed or storage room with minimal electrical requirements), a single circuit with RCD protection feeding one or two sockets and a lighting point may be sufficient. However, this is the minimum acceptable solution and limits future flexibility significantly.
Sub-board — the recommended approach — for any garden room intended as a home office, studio, or habitable space, a sub-board is the correct solution. It provides individual circuit protection (so a heater fault does not take out the lighting), a local main isolator, and room to add circuits in the future without returning to the house consumer unit.
Typical circuits in a garden room sub-board — a well-specified garden room installation includes: one lighting circuit, one or two socket-outlet circuits, a dedicated heating circuit (for electric panel heaters or underfloor heating), and a spare way for future use. If an EV charger is planned, an additional 32A circuit is required.

The sub-board in the garden room must be protected by the main protective device in the house consumer unit. The SWA cable feeding the sub-board must be rated to carry the total maximum demand of all circuits in the garden room simultaneously, with the rating determined by the cable's current-carrying capacity under BS 7671 Appendix 4.

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02 · Installation Guide

Cable Sizing for Distance: Getting the Voltage Drop Right

Voltage drop is the reduction in voltage that occurs as current flows through a cable. In a long cable run from the house to a garden room, voltage drop can reduce the voltage at the garden room to below the acceptable minimum, causing equipment to operate inefficiently or fail to start.

BS 7671 voltage drop limits — under Section 525 of BS 7671, the voltage drop from the origin of the installation (the main consumer unit) to any point must not exceed 3% (6.9V) for lighting circuits and 5% (11.5V) for power circuits. For a garden room sub-board, the 5% limit applies to the SWA feed cable.
Practical cable sizes by distance — for a typical garden room with a 32A sub-board feed (lighting, sockets, heating), the minimum cable sizes are approximately: up to 20 metres — 6mm² SWA; 20 to 35 metres — 10mm² SWA; 35 to 60 metres — 16mm² SWA. These are indicative values — your electrician must calculate the exact voltage drop for the specific installation load and cable route.
Earth fault loop impedance — as well as voltage drop, the cable length affects the earth fault loop impedance. For longer runs, the measured Zs may exceed the maximum permitted for the protective device in the house consumer unit. Where this is the case, the protective device rating or cable size must be adjusted, or a local RCD relied upon for additional protection.

03 · Installation Guide

EV Charging Consideration for Garden Rooms

Many garden room installations are adjacent to a driveway, and homeowners increasingly want to combine the garden room electrical installation with an electric vehicle charging point. Planning for EV charging at the design stage is far more cost-effective than adding it retrospectively.

Load impact — a 7kW single-phase EV charger draws up to 32A continuously. Adding this to a garden room installation that also has lighting, sockets, and heating significantly increases the total load. The SWA cable from the house must be sized for the combined maximum demand — typically 16mm² or 25mm² rather than 6mm².
Dedicated circuit — the EV charger requires a dedicated 32A circuit from the garden room sub-board (or from the house consumer unit if the charger is closer to the house). Under BS 7671 Section 722, the circuit must include appropriate overcurrent protection, and the charger must comply with The Electric Vehicles (Smart Charge Points) Regulations 2021, which requires smart charging capability (load management and remote control).
EV charger earthing — under BS 7671 Regulation 722.411.4, EV charger installations require a protective earth connection and may require a Protective Earth-Neutral (PEN) fault protection device. For TT-earthed garden room installations, a separate RCD on the EV charger circuit provides the required protection.

If EV charging is a future possibility, ask your electrician to install a 25mm² SWA feed cable and a sub-board with a spare 32A way at the initial installation. The additional cost is modest compared to the cost of replacing a correctly sized cable later.

04 · Installation Guide

Internet and Data Infrastructure for Garden Room Home Offices

For a garden room used as a home office, reliable internet connectivity is as important as the electrical supply. Planning data infrastructure at the same time as the electrical installation saves significant money and disruption.

CAT6 ethernet — the reliable option — a CAT6 ethernet cable run in conduit alongside or within the SWA cable trench provides a permanent, gigabit-speed internet connection. At the house end, the cable connects to the router or a network switch. At the garden room end, a flush-mounted ethernet socket provides a clean installation. This is the recommended solution for anyone working from home who relies on video conferencing or large file transfers.
Conduit for future cabling — even if a CAT6 cable is not installed immediately, run an empty 32mm conduit in the trench. This allows future data cables (or additional ethernet cables) to be pulled through without excavating the garden again. The cost of the conduit is negligible; the cost of re-excavating the garden is not.
Wi-Fi mesh systems — for shorter cable runs or where a CAT6 cable is not practical, a dedicated outdoor Wi-Fi access point mounted on the house or a pole can provide reliable garden room coverage. However, this requires a power supply to the access point and does not match the reliability or speed of a wired connection.

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05 · Installation Guide

Insulation Requirements and Consumer Unit Options

The thermal performance of the garden room and the specification of the consumer unit are closely linked — poor insulation means higher heating loads, which affects the electrical circuit requirements.

Insulation and heating load — a well-insulated garden room (75mm wall insulation, 100mm roof insulation, double-glazed windows) may need only 1kW of electric heating to maintain a comfortable temperature. A poorly insulated room may need 3kW or more. Higher heating loads require larger dedicated circuits and a higher-rated SWA feed cable. Investing in good insulation reduces the electrical specification and running costs.
RCBO consumer unit — the best choice — a metal consumer unit with individual RCBOs for each circuit is the optimal solution for a garden room. Each RCBO provides both overcurrent protection and 30mA RCD protection for its circuit. A fault on one circuit (for example, the heater) does not affect the other circuits (lighting, sockets, computer). Under Regulation 421.1.201 of BS 7671, the enclosure must be non-combustible — metal is the standard.
IP rating for the consumer unit position — if the consumer unit is positioned where it could be exposed to moisture (near a door, in an unheated area, or where condensation is possible), the enclosure must have an appropriate IP rating. For most garden room interiors, IP2X is acceptable. Where the unit is in a porch or unheated lobby, IP44 or higher is preferred.

06 · Installation Guide

Part P Notification and Building Regulations

Garden room electrical installation work — including the cable from the house, the sub-board, and all circuits within the garden room — is notifiable under Part P of the Building Regulations 2010. There are no exemptions for garden rooms or outbuildings.

Competent person scheme — use a NICEIC, NAPIT, or ELECSA registered electrician to self-certify the work. They notify the scheme on completion, and you receive a notification certificate. No separate building control inspection of the electrical work is required.
EIC on completion — an Electrical Installation Certificate (EIC) covering the entire installation — SWA cable, sub-board, all circuits, earthing system, and test results — must be issued on completion. Use the Elec-Mate EIC app to complete and issue the EIC on site, including the earth electrode resistance measurement.
Property sale implications — when the property is sold, the buyer's solicitor will ask for evidence that the garden room electrical installation is compliant. Without an EIC and Part P notification, the sale may be delayed or an indemnity insurance policy required. A properly certified installation adds value; an uncertified one creates doubt.

07 · Installation Guide

For Electricians: Garden Room Electrical Installations

Garden room and garden office installations are among the most enjoyable domestic electrical jobs — a complete new installation from scratch, with interesting design decisions, testing, and full certification. With the growth in home working, demand for quality garden room electrical work continues to increase.

Complete the EIC With Earth Test Results

Use the Elec-Mate EIC certificate app to record all test results on site, including the TT earth electrode resistance. Generate a professional PDF and send it to the client before you leave. Many electricians omit the earth electrode test result — don't be one of them.

Upsell EV and Data Infrastructure

When quoting a garden room installation, always discuss EV charging and data infrastructure. A client who is not planning an EV now may be planning one in two years — upsizing the cable now costs little but saves them thousands later. Use the quoting app to produce clear alternative quotes for standard, EV-ready, and full-spec options.

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Frequently Asked Questions About Garden Room Electrical Installation

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Garden Room Electrical Installation: Home Office Wiring Guide UK

Key Takeaways

Dedicated Circuit from the House or Sub-Board in the Garden Room?

Get the BS 7671 A4:2026 Cheat Sheet — free

Cable Sizing for Distance: Getting the Voltage Drop Right

EV Charging Consideration for Garden Rooms

Internet and Data Infrastructure for Garden Room Home Offices

Try Elec-Mate free for 7 days

Insulation Requirements and Consumer Unit Options

Part P Notification and Building Regulations

For Electricians: Garden Room Electrical Installations

Complete the EIC With Earth Test Results

Upsell EV and Data Infrastructure

Manage garden room electrical jobs with Elec-Mate

Frequently Asked Questions About Garden Room Electrical Installation

Do I need a sub-board in my garden room?

How far can electricity travel to a garden room without voltage drop issues?

Can I add an EV charger to my garden room electrical installation?

What internet connection options work for a garden room home office?

Do I need planning permission to wire a garden room?

Can I use a normal extension lead to power my garden room temporarily?

What is the best consumer unit for a garden room?

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