Getting proper power to a shed, garden office or outbuilding is a fixed electrical installation, not a job for an extension lead trailed across the lawn. It usually means running buried steel-wired armoured cable from your house to the building, fitting the right protection at both ends, and often a small consumer unit in the outbuilding. The work is notifiable, so it has to be tested and certified. The right design depends on how far away it is and what you are running out there.
Can’t I just run an extension lead?
For a quick one-off, like a strimmer on a dry afternoon, a properly rated outdoor extension lead is fine. As a permanent supply it is not. A trailing lead across a garden is a trip hazard, it is not rated to be left out in the weather, it can be damaged by mowers, frost and water, and it cannot safely carry the load of a heated office, tools or a fridge for any length of time. A garden office you work in every day, or a shed with a freezer, lighting and power tools, needs a proper fixed supply that is buried, protected and certified.
What is actually involved in a proper installation?
A correctly done outbuilding supply has a few non-negotiable parts:
- Steel-wired armoured cable (SWA). This is the tough, armoured cable designed to be buried underground. The size of the cable is chosen for the load and the distance, since a long run loses voltage and needs a bigger cable to compensate.
- A buried route at the right depth. The cable is run in a trench deep enough to protect it from spades and forks, typically with warning tape laid above it so anyone digging later gets a warning before the cable.
- Protection at the house end. The new circuit is added at your fuse board with its own correctly rated protective device.
- A small consumer unit in the outbuilding. Most installations of any size get their own little sub-board in the shed or office, with RCD protection, so the lights, sockets and any heating out there are properly protected and separately controlled.
- Earthing arrangements that are correct for an outbuilding, which is a specific design point an electrician handles, not something to improvise.
Done this way, the outbuilding behaves like a safe, certified extension of your home’s electrics.
How deep does the cable need to be buried?
Deep enough that normal garden activity will not reach it. As a rule of thumb that means a trench around 450mm deep under a lawn or border, and deeper where it runs under a driveway or anywhere vehicles go, with cable warning tape laid above the cable so future digging is warned off. If the route has to cross a patio, decking or a path, that is part of what the survey works out. SWA can also be clipped along a wall or fence at high level in some situations, which can save digging, and that is a judgement call the electrician makes on site.
Does the outbuilding need its own consumer unit?
Usually, yes, if you want more than a single light or socket out there. A small sub-board in the outbuilding gives you:
- Local RCD protection for the sockets and circuits in the building.
- Separate switching, so you can isolate the outbuilding without killing power to the house.
- Room to add circuits, such as lighting, sockets, heating or outdoor and garden lighting, without overloading a single feed.
A single outdoor socket or one light might be wired more simply, but anything you actually spend time in, an office, a workshop, a gym, is far better served by its own little board.
Will my house supply and fuse board cope?
This is the question that decides a lot of jobs, and it is why the work starts with a look at your existing setup. Adding an outbuilding load on top of everything already in the house draws on your main service fuse and your consumer unit. If your board has a spare way and is in good condition, the new circuit drops in. If the board is old or full, it may need attention first, the same check that applies before an EV charger or heat pump, explained in will my fuse board cope. If the board is tired, you may want to read signs your fuse board needs replacing. Either way, you are told what your property actually needs before any price is set.
Does it need to be notified or certified?
Yes. A new circuit feeding an outbuilding is notifiable work under the Building Regulations, so it must be designed, installed, tested and certified to the current wiring standard, then notified through the proper channel. Jack tests and certifies the work to BS 7671 and issues the paperwork as part of the job, so you end up with paperwork that proves the installation is safe and signed off, which matters if you ever sell the house.
What affects the cost?
There is no flat price, because every garden is different, but the main factors are straightforward:
- Distance from the house. A shed ten metres away is a short run. An office at the bottom of a long garden needs more cable and a bigger trench.
- The route and ground. Soft lawn is quick to dig. Crossing a patio, driveway, decking or mature borders is more work and more making-good.
- The load you need out there. A couple of lights and a socket is modest. A heated office, a workshop full of tools or a hot tub draws far more and needs heavier cable and more protection.
- Your existing fuse board and supply, which may take the new circuit as-is or may need upgrading first.
Because of all that, the honest way to price it is to look at your garden and your board in person. Snelling Electrical gives you a written quote within 24 hours so you know the exact figure before anything starts, and you can see the full scope on the outbuilding power page.