Garden Office & Outbuilding Electrics: Cost, Cable and Regs

By Steven Dixon · NAPIT-registered electrician, Royal Artillery veteran (26 years) · Published 8 September 2026 · Bradford, West Yorkshire

Running a proper electrical supply to a garage, workshop or garden office costs £700-£2,000 in 2026. The building itself barely matters. What decides the price is how far it is from the house, what you intend to run in it, and what the earthing arrangement at the property allows.

That last one catches people out constantly, so it gets a section of its own below. It is also the reason two quotes for what looks like the same job can differ by £400 — one electrician has accounted for it and the other has not.

The 30-second answer

Prices include armoured cable, ducting, the circuit at the house end, the sub-board where one is needed, testing, certification and Part P notification. They exclude reinstating a lawn or block paving, and any building work on the outbuilding itself.

The earthing rule nobody mentions in the quote

Most homes in Bradford, Leeds and the surrounding towns are on a PME supply — protective multiple earthing, also called TN-C-S. The earth comes from the incoming supply cable rather than from a rod in your garden. It is the standard arrangement and it works well indoors.

Outdoors is different. Under a PME fault on the network, the earthing system in your house can briefly rise to a dangerous voltage relative to the true earth outside. Indoors that does not matter because everything metallic is bonded together and rises with it. Standing on wet grass holding a metal garage door handle, it matters a great deal.

BS 7671 therefore restricts exporting a PME earth to outbuildings where there are extraneous conductive parts — metal structures, buried metal pipework, a steel-framed building, anything in contact with the general mass of earth. The practical outcomes are:

• A timber garden room with no metal in contact with the ground — usually fine to export the PME earth. Straightforward.
• A metal-framed or metal-clad garage, or one with a water supply — usually needs a TT island: an earth electrode driven at the outbuilding, that building's supply disconnected from the house earth, and RCD protection sized to the electrode reading. Add £250-£450.
• Anything with an EV charge point — separate rules again, and most modern chargers include open-PEN detection which resolves it. Worth flagging early if a charger is planned, because it changes the design. See our EV charger page.

Cable, trenching and ducting

Armoured cable is the only sensible answer

Steel wire armoured (SWA) cable is designed to be buried. The steel armour resists a spade, and the armour itself can serve as the protective conductor. Two-core or three-core depending on the earthing design, sized for the load and the run length.

Voltage drop is the reason long runs need thicker cable. BS 7671 allows 5% drop for power circuits, and over 30 metres a 2.5mm² cable will exceed that on any meaningful load. Typical sizing:

• Up to 15m, light use (sockets and lighting): 2.5mm² or 4mm² SWA on a 20A or 32A circuit
• 15-30m, garden office with heating: 6mm² SWA, 32A
• 30m+, or a workshop with machinery: 10mm² SWA, 40A, sometimes larger

The cost difference between 4mm² and 10mm² over 30 metres is real money, which is why "how far is it and what are you running in there" is the first question we ask.

How deep, and what goes in the trench

BS 7671 does not give a number. It requires the cable to be at a depth where reasonably foreseeable disturbance will not damage it. Established practice:

• 600mm under lawn or garden — below normal digging and planting depth
• Deeper, and in duct, under a driveway — vehicle loading and future resurfacing
• Bed of sand under and over the cable so stones cannot work into the sheath
• Yellow warning tape above the sand, so whoever digs there in fifteen years gets a warning before the spade finds the cable
• Ducting wherever practical — adds perhaps £60-£120 in materials and means a future upgrade or a damaged cable can be pulled through without opening the garden again. Worth every penny.

On Pennine plots we hit rock more often than not. A trench through shale or a stone-flagged yard takes considerably longer than one through a lawn, and any honest quote reflects that after actually looking at the ground.

Does it need its own consumer unit?

A shed with one socket and one light can be fed as a single circuit from the house. Anything beyond that wants a small consumer unit in the outbuilding, and it is not a big cost — typically £120-£200 fitted as part of the job.

Why it is worth having:

• Circuits can be isolated locally, so a fault in the workshop does not mean walking back to the house in the dark
• Separate RCBOs mean a tripped socket circuit does not take the lighting with it
• Adding a circuit later — a heater, an EV point, a hot tub — is a small job rather than another trench
• It is what an inspector expects to see, so it does not generate observations on a future EICR

If the house board has no spare way for the new circuit, that becomes part of the conversation too — sometimes it is more sensible to change the consumer unit at the same time than to fit a separate enclosure alongside it.

Garden offices specifically

Garden rooms have gone from a niche to a standard request since 2020, and they need more thought than a garage:

• Heating dominates the load. A 2kW panel heater or a small air-conditioning unit is most of your design current. Size the cable for it even if it is not going in on day one.
• Data, not just power. Run a Cat6 alongside the SWA in the same trench while it is open. Wi-Fi across a garden through two insulated walls is not a plan you want to depend on for work calls.
• Sockets where the desk will be. Sounds obvious. Decide the layout before first fix, not after the insulation and boarding are in.
• Smoke detection. Not required for a detached outbuilding, but if anyone will realistically fall asleep in there it is a sensible addition.
• Outside lighting and a socket on the exterior. IP-rated, RCD protected, and far cheaper to do while the electrician is already there.

Certification and Part P

Supplying an outbuilding is a new circuit, which makes it notifiable under Approved Document P of the Building Regulations in England and Wales. Registered installers self-certify and notify Building Control automatically; unregistered work means applying to the council yourself, typically £200-£500 and an inspection you have to arrange.

You should receive an Electrical Installation Certificate with a full schedule of test results, including the earth electrode resistance if a rod was installed. Keep it — it is the document a buyer's solicitor asks about, and the baseline for the next inspection. gov.uk reference.

The bottom line

Budget £700-£1,100 for a straightforward detached garage supply and £1,200-£2,000 for a garden office done properly, with a sub-board, heating capacity and data. Add for trenching through hard surfaces and for an earth electrode if the earthing arrangement calls for one. Put ducting in while the trench is open.

And do not run an extension lead across the lawn in the meantime. It is the single most common cause of outbuilding fires we get called to, and it is coded on every EICR.

Book a free survey and we will measure the run, look at the ground and the earthing, and give you a fixed price in writing within 24 hours. Or try the cost calculator for a ballpark first.