Garden Room and Outbuilding Electrical Installation in Kent: A Complete Guide

The garden room boom that began during the pandemic has shown no signs of slowing down across Kent. From Sittingbourne to Maidstone, homeowners are converting outbuildings, installing purpose-built garden offices, and creating workshops, gyms, and hobby rooms at the bottom of the garden. Every one of these structures needs a reliable, safe electrical supply — and getting it right from the outset saves significant cost and disruption later.

An outbuilding electrical installation is not a job for a competent DIYer. It involves work on the main consumer unit, underground cable installation, and a new distribution board — all of which fall under Part P of the Building Regulations and must be carried out or certified by a registered electrician. This guide covers what is involved, what it costs, and what to look for when choosing an electrician for the job.

What Does an Outbuilding Electrical Installation Involve?

A typical garden room or outbuilding electrical installation has four main components: a new circuit from the main consumer unit in the house, an underground cable run to the outbuilding, a small consumer unit (distribution board) inside the outbuilding, and the final circuits for sockets, lighting, and any other equipment. Each component has specific regulatory requirements that determine how it must be installed.

Underground Cable: Depth and Protection Requirements

The cable run between the house and the outbuilding is the part of the installation that most homeowners underestimate. BS 7671 (the IET Wiring Regulations) requires underground cables to be buried at a minimum depth of 500mm under areas where excavation is unlikely (such as lawns and borders) and 600mm under areas subject to mechanical disturbance (such as driveways and vegetable plots). The cable must be either armoured (SWA — steel wire armoured) or protected by a conduit, and the route must be marked with cable warning tape above it. A cable route plan must be provided to the homeowner on completion.

The Outbuilding Consumer Unit

Every outbuilding with an electrical supply requires its own consumer unit (distribution board) inside the structure. This provides local isolation — the ability to switch off all power to the outbuilding without going back to the house — and houses the RCD and MCB protection for each circuit within the outbuilding. The outbuilding consumer unit must be a 17th or 18th Edition compliant unit with RCD protection, and must be installed in a dry, accessible location inside the structure.

Earthing and Equipotential Bonding

Outbuildings require additional earthing and bonding that is not required inside the main house. Any metalwork within the outbuilding — metal window frames, metal sinks, metal pipework — must be connected to the earthing system by supplementary bonding conductors. If the outbuilding contains a shower or other water installation, the bonding requirements are more extensive. This is a detail that is frequently omitted in DIY or informal installations and is a common cause of electric shock risk.

What Can You Power in a Garden Room?

A properly installed outbuilding supply can power virtually anything a domestic property can — sockets, lighting, heating, air conditioning, EV chargers, and data points. The limiting factor is the capacity of the supply cable and the spare capacity in the main consumer unit. For a typical garden office with lighting, sockets, and a small electric heater, a 6mm² SWA cable on a 32A MCB is usually sufficient. For a workshop with power tools, or a garden room with air conditioning and EV charging, a larger supply may be required.

Building Regulations and Part P Notification

An outbuilding electrical installation is notifiable work under Part P of the Building Regulations. This means it must be carried out by a registered electrician (such as a NICEIC-registered contractor) who can self-certify the work, or alternatively notified to the local authority building control before work begins. A registered electrician will issue an Electrical Installation Certificate (EIC) on completion, which is an important document for your property records and will be requested by solicitors when you sell the property.