Outbuildings, Out of Sight: A Safe Power Plan for Garden Offices and Summerhouses

Garden rooms have a way of growing. A summerhouse becomes a home office; a shed gains a heater, a kettle and a second monitor; a studio acquires a treadmill. The first cable is often an extension lead trailed across the lawn. It works for a weekend, then becomes permanent, and the outbuilding quietly acquires a supply that was never designed for the load.

A garden office can be a safe and comfortable workspace, but it needs a proper electrical plan. The aim is to treat the outbuilding as part of the installation, not as an accessory to a kitchen socket.

Begin with the load, not the cable

Before choosing cable or digging a trench, list what the building will actually power. A 2 kW heater, a 3 kW kettle, a large monitor, lighting and a laptop charger can add up quickly. If the plan includes a hot tub, air conditioning, power tools or an electric vehicle charger, the demand is higher still.

A single 13 A socket circuit is not a free pass to run everything at once. The electrician should calculate the maximum demand, apply diversity where appropriate, and check the capacity of the existing supply. Sometimes the right answer is a larger supply cable; sometimes it is a change to how the building is used, such as a lower-power heating system. Deciding this early is cheaper than discovering it after the trench is backfilled.

Cable route and choice

For an underground supply, steel wire armoured cable is the usual choice because it provides mechanical protection. The route should be agreed before digging, with adequate depth, a suitable sand bed or duct, warning tape and protection at the building entry. The cable should be kept away from other buried services, and the trench should be planned so that future maintenance does not require guesswork.

Overhead supplies are possible, but they need proper support, strain relief and UV-rated cable. A cable that swings in the wind or can be reached from a fence is not a sound arrangement.

Cable size is not a matter of guesswork. It depends on current-carrying capacity, installation method, ambient temperature, grouping and voltage drop. A long run to the bottom of a garden can leave a heater performing poorly or lights flickering if the cable is too small. The electrician should calculate the size and record the result.

Protection, isolation and earthing

The outbuilding supply should have RCD or RCBO protection selected for the installation. A local consumer unit in the garden room gives useful separation, but it must be correctly rated and labelled. A double-pole isolator, ideally lockable, allows the supply to be switched off for maintenance or in an emergency.

Earthing needs thought. The electrician will confirm the earthing arrangement at the main property and decide whether the earth can be exported safely or whether a local earth electrode is required. Metallic services entering the outbuilding may need bonding. Outdoor sockets and equipment need suitable IP ratings, and any heater or fixed appliance should be installed according to the manufacturer's instructions.

Part P and the paperwork

In England and Wales, adding a new circuit to an outbuilding is generally notifiable under Part P of the Building Regulations. The work should be carried out by a registered competent person, or notified to building control before it starts. Scotland and Northern Ireland have their own procedures, so the route to approval depends on where the property is.

At the end, the homeowner should receive the correct certificate, test results and schedules. This is not paperwork for its own sake. It records what was installed, proves the work was tested, and matters when the property is sold, let or insured.

A checklist before you dig

• Decide what the building will power and how often.
• Ask an electrician to check the existing supply capacity.
• Agree the cable route and depth before digging.
• Check for buried services and utility plans.
• Confirm cable type, size and voltage drop.
• Specify RCD/RCBO protection and local isolation.
• Confirm the earthing arrangement and any bonding.
• Choose weatherproof equipment with suitable IP ratings.
• Confirm Part P notification and who will certify the work.
• Leave room for future demand, but do not oversize blindly.

A garden room should feel like part of the home, not an electrical outpost. With a proper supply, protective devices and certification, it can be used safely through winter as well as summer. S&DG Electric can plan and install outbuilding supplies, consumer units and outdoor points, and advise on the checks that should happen before any spade goes into the ground.