Start with the supply, not the sockets

Most garden cabin projects begin with a picture in your head: a desk under a window, a kettle in the corner, a warm lamp on a shelf. Power is usually the last thing anyone thinks about, and that is exactly why it ends up expensive or disappointing. A garden room is a small building, so it deserves its own dedicated supply rather than a long extension lead trailed across the lawn.

Before you buy a single socket, decide three things: what the cabin will be used for, how far it sits from the house, and whether you want the electrics done once, properly. A home office needs far less than a workshop with a welder, and a 30-metre run of cable needs a very different size from a 10-metre one. Get those answers clear and everything else falls into place.

Work out the load before you choose the cable

Add up what you genuinely expect to run at the same time, not everything you might one day own. A practical home office might be a laptop, two monitors, a printer, LED lighting, a small heater and a kettle — perhaps 2.5 to 3kW in total. A 16A supply at 230V gives you roughly 3.7kW of headroom, which is comfortable for that. Add a 2kW fan heater and a 1kW oil radiator and you are already close to the limit.

• Lighting: modern LED fittings draw very little — a whole cabin might use 60W.
• Sockets: allow one double socket per wall as a minimum, plus a dedicated one for a heater.
• Big loads: heaters, air conditioning, welders, kilns and power tools should be listed individually.
• Future-proofing: if a hot tub, EV charger or garden gym might follow, size the supply once and thank yourself later.

A 32A supply fed by a 6mm² or 10mm² armoured cable covers most domestic garden buildings generously, but the cable size must be calculated — not guessed — against the run length.

Route the cable properly and bury it deep enough

Steel wire armoured (SWA) cable is the standard choice for underground garden runs. It is tough, UV-resistant and mechanically protected, and it must be terminated into metal enclosures using proper glands so the armour is earthed. Flexible cable in a hosepipe is not an acceptable substitute, however well it seems to work.

Dig a trench around 450mm deep, line it with sand, lay the cable with a gentle sweep rather than a tight bend, then cover it with sand, a layer of warning tape and the backfill. Where the cable crosses a patio or path, run it through a duct so it can be replaced without lifting the slabs. Take photographs of the route before you fill it in — you will forget exactly where it runs within a year.

Volt drop matters more than most people expect. Over a long garden run, a cable that looks adequate on paper can drop enough voltage to make lights flicker and tools run slow. This is the single most common reason a garden supply underperforms, and the fix is a larger cable or a shorter route.

Consumer units, RCDs and earthing

The cabin should have its own small consumer unit rather than being fed from the house's spare way alone. That unit gives you local isolation, individual circuits for lighting and sockets, and a clearly labelled point for future maintenance. Fit RCBOs where you can — each circuit then has its own 30mA RCD protection, so a tripped socket circuit does not plunge the cabin into darkness.

Earthing needs a proper decision, not a default. Your electrician will establish whether the house supply's earth can be exported safely to the cabin, or whether the outbuilding should be set up as a TT system with its own earth electrode and RCD protection. Metal-clad cabins, metal framed buildings and longer runs all influence that call. Add surge protection at the cabin board too if you are running computers, a network switch or a TV in there.

In the UK, most new circuits of this kind are notifiable electrical work. Use a registered electrician who can certify and notify the job — the paperwork will matter when you sell the house.

Outdoor-rated sockets, switches and lighting

Every fitting outside the insulated envelope should carry a suitable IP rating. An IP65 socket with a spring-loaded lid is the right choice for an exposed wall; IP44 is acceptable under a generous canopy where rain cannot drive in. Standard indoor sockets in a shed will fail within a couple of seasons.

• External sockets: fit one on each side of the cabin for mowers, strimmers and pressure washers.
• Lighting: use warm white LEDs around 3000K — crisp enough to work by, soft enough to sit under in the evening.
• Security: a PIR floodlight over the door and a low-level light along the path make the building feel, and look, safer.
• Switching: two-way switching between house and cabin is a small luxury that stops a lot of torch-lit dashes.

Inside, choose dimmable LED downlights or a simple pendant on a dimmer, and put sockets at 300mm above floor level so they stay clear of clutter. Add a weatherproof isolator outside the cabin door so power can be cut without going back into the house.

Heating, data and the finishing touches

Electric panel heaters are cheap to install and easy to control, and a 1kW panel is often enough for a well-insulated 12m² cabin. Underfloor heating mats suit tiled floors but draw serious current, so include them in your load calculation. A small wall-mounted convector with a timer keeps the damp and condensation at bay through winter.

Run a Cat6 cable in a separate duct alongside the power — never in the same conduit — or plan for a mesh access point mounted on the outside wall. Wiring for data, and for any future security camera, costs almost nothing while the trench is open and a great deal afterwards.

Finally, label the consumer unit clearly, keep a spare way or two free, and ask your electrician to test the RCDs annually. A garden cabin with a well-planned supply is a building you can use all year round; one with a rushed supply is a shed with expensive furniture.