How power gets to a garden office or garage

Why not just run an extension lead?

Because it is the single most common cause of the outdoor electrical incidents that get reported. An extension lead is designed for temporary indoor use. Left outside it takes water at the connections, gets driven over by a mower, gets nicked by a spade, and has no protection sized for a permanent load.

It also means the outbuilding has no isolation of its own. If something goes wrong out there, you are dealing with it from the house.

A proper supply is not much of a job in the scheme of a garden office that has already cost real money, and it is the part that keeps the rest of it safe.

What does the installation actually involve?

A dedicated circuit leaves the consumer unit in the house, protected at the right size for the load and with RCD protection. It runs in steel wire armoured cable, which is the type with a metal braid under the outer sheath that protects it from being cut.

Under a garden that cable is buried at a minimum of 600mm, deeper under a drive, with warning tape above it so the next person with a spade gets a chance. Along a wall or fence it can be clipped instead, which is cheaper, though it has to be out of the way of damage.

At the far end it lands in a small consumer unit in the outbuilding, which then feeds the sockets and lighting out there. That local board is what lets you isolate the building without switching off the house.

How much power can I have out there?

This is decided at the house end, not the shed end. The question is how much headroom the incoming supply has once the house is accounted for, exactly the same calculation as for an EV charge point.

For a garden office with a laptop, a couple of monitors, lighting and a heater, the load is modest and it is rarely a problem. A workshop with a welder, a compressor or a kiln is a different conversation and may run into the main fuse.

Tell the electrician what is actually going in there, including the things you are not sure about yet. Sizing the cable for the workshop you might build is cheap now. Digging the trench twice is not.

Is there anything special about earthing outside?

Yes, and it is the part a general builder will not know about. Most homes use a PME earthing arrangement, and BS 7671 restricts extending that earth to certain outdoor installations, for the same reason it matters with EV chargers: under a rare supply fault, exposed metalwork outdoors could become live.

Depending on what the outbuilding is made of and what is in it, the answer is either exporting the earth is fine here, or the outbuilding needs its own earth electrode and separated arrangement.

This is a judgement a qualified electrician makes on site. It is not a corner to cut, and it is a good question to ask whoever is quoting, because the answer tells you quickly whether they know the regulations.

Does it need signing off?

Yes. A new circuit is notifiable under Part P, and a supply to an outbuilding is a new circuit. You should receive an Electrical Installation Certificate and a building regulations compliance certificate.

That matters more than it sounds. When you sell, an unregistered garden office supply is exactly the sort of thing a buyer’s solicitor asks about, and producing the certificate is a lot easier than explaining its absence.

Questions we get asked about this