Back-Garden HQ: Why Your Shed Deserves a Real Circuit, Not a Long Orange Lead

There's a sentence that gets said on driveways up and down the country, usually while someone points at a shed: 'It's only a shed.' It's said cheerfully, as a way of closing a conversation. It has probably cost British homeowners more money than any other six words in the language.

Because in 2026 the shed isn't a shed. It's a tax return, a pilates studio, a recording booth, a treatment room, a place where a small business actually happens. It has a heater, a monitor, a router, a kettle and a client sitting in it. And a surprising number of them are still being powered by a 25-metre extension lead trailed through a kitchen window and out across the patio.

How we got here

The garden room boom was supposed to be temporary. It wasn't. What started as a stopgap for hybrid working has hardened into permanent infrastructure — and the electrical demands followed. A desk with two monitors and a laptop dock is one thing. Add a 2kW panel heater, a dehumidifier for the damp months, a 3kW kettle, LED lighting, a small server rack and an EV charger on the outside wall, and you're designing a small commercial installation whether you meant to or not.

Insurers have noticed too. When a claim lands on a garden office that was fed by a plug-in lead, the questions about installation adequacy get awkward quickly.

What the Extension Lead Actually Does Wrong

It's worth being specific, because 'it's dangerous' is easy to dismiss.

• Voltage drop. Twenty-five metres of 1.5mm² flex carrying 10A loses meaningful volts. Motors run hot, electronics misbehave, heaters underperform — and the current creeps up to compensate.
• Mechanical damage. Cable across a lawn gets walked on, mown over and kinked against a step. A damaged flex with no upstream RCD is a genuine shock risk.
• Water. Plugs sitting in a plastic box under a workbench are not IP-rated, whatever the box says.
• No isolation. You can't safely kill the supply to a building you can't isolate at source.
• No discrimination. One fault in the shed can knock out half the house.

Armoured Cable, Sized Properly

Steel wire armoured cable is the right answer for an underground supply, and it's not a place to guess. The cable size depends on two things that must be calculated together: current-carrying capacity and voltage drop over the run. A 20-metre run to a shed with a 2kW heater might be fine in 4mm²; a 40-metre run with an electric heater and a hot tub almost certainly won't be. Voltage drop limits for a final circuit are tight, and long garden runs eat into them fast.

Practical points that get skipped:

• Bury deep enough and protect with warning tape or duct; if that isn't achievable, run it overhead or clipped on a catenary with proper support.
• Terminate with correct SWA glands, and use gel-filled or IP68 joints if any connection must be buried. Joints in a puddle are how you find out about earthing the hard way.
• Never leave a spare core floating without noting it on the certificate.

Protection: One RCD Is Not a Design

Every socket in the outbuilding should be protected by 30mA RCD or RCBO. That isn't a nice-to-have; it's the baseline. In practice that usually means a dedicated RCBO at the origin (so a shed fault doesn't take the kitchen down) plus a small consumer unit in the garden room with its own main switch and individual RCBOs for lighting, sockets and any fixed heating.

Earthing deserves a proper conversation with your electrician. If the house has a PME supply, exporting that earth to a metal-clad outbuilding is a decision, not a default — many designs use a local earth electrode and a TT arrangement in the outbuilding instead. This is exactly the kind of thing that separates a proper job from a hopeful one.

And if the garden room houses several thousand pounds of equipment, ask about surge protection at the outbuilding board. It's cheap insurance against the kind of spike that arrives via a long overhead-ish run.

The Part P Conversation Nobody Wants

New circuits to an outbuilding are notifiable work in England and Wales. That means either using a registered competent person who can self-certify, or going through building control before you start. Garden rooms fall squarely inside this — the idea that a structure without a permanent bed in it is exempt is a persistent myth.

Scotland, Wales and Northern Ireland each run their own variations, so the notification route differs, but the principle holds: the work needs certifying. That certification is also what your insurer and any future buyer will ask for. An Electrical Installation Certificate with schedules of test results is the document that turns 'it's only a shed' into 'here's the proof'.

Commissioning Before the First Kettle

A supply that's installed but never tested is a supply nobody can vouch for. Before energising properly, expect to see insulation resistance tests, polarity checks, earth loop impedance or RCD trip-time verification, and a full schedule of results signed off. Then the functional bits: does the isolator at the house end actually kill everything in the outbuilding? Is the board labelled? Can it be locked off for maintenance?

That last point matters for businesses. If your garden room is a workplace, you need a way to isolate safely before anyone pokes at a socket.

A note on loads people forget

Kettles and heaters are the obvious ones. Less obvious: infrared panels, treadmills, air conditioning, dehumidifiers, kilns, welders, hot tubs, EV chargers and any future electric car the household buys next year. Build in headroom, because the garden room always gets fuller.

The verdict

Spending proper money on a garden room's electrics feels like spending money on nothing visible. There's no new sofa at the end of it. But a correctly sized armoured feed, proper RCD protection, real certification and a lockable isolator is the difference between a building that adds value to your property and one that quietly undermines it.

The extension lead is fine for a hedge trimmer. It was never fine for an office.