Wingham and the Little Stour villages are full of converted outbuildings - old kilns turned into utility rooms, oast roundels turned into snugs, garden studios that started as a shed and grew. Almost none of them are supplied to modern standards. This is how we do it properly.
Sub-main sizing
You need to know two things: how much load the outbuilding will draw at peak, and how far the run is. A garden office with a heater, computer, and kettle draws maybe 3kW at peak. A workshop with a table saw and welder draws 8kW. Run length matters because voltage drop compounds - the longer the cable, the thicker it needs to be to deliver the same voltage at the far end.
Typical spec
- Garden office, 15m from house, 3kW peak: 4mm SWA, RCBO on the house board, small 4-way consumer unit at the office.
- Workshop, 30m from house, 8kW peak: 10mm SWA, dedicated 40A supply, 6-way board at the workshop with RCBOs.
- Stables or barn, 60m from house, mixed load: 16mm SWA minimum, main switch and small distribution board at destination.
Trench and route
Direct-buried SWA in a 450mm-deep trench with a 150mm layer of soft sand below and above the cable, marker tape 150mm above that, then backfill. Yellow warning tape at 300mm below finished surface. Under a driveway or path we ducktube it. Bends are gentle, no sharp corners.
Earthing
On outbuildings we always TT the sub-board with a dedicated earth rod, unless the main installation is very obviously TN-S with a decent main earth. The reason is that TN-C-S (PME) earthing has some edge cases with detached outbuildings and metal cladding, and a dedicated rod removes the ambiguity. RCD protection on every outgoing circuit at the sub-board.
Part P notification
A new circuit to an outbuilding is notifiable work under Part P. We notify Dover District Council Building Control through the Competent Person Scheme, and you get the Electrical Installation Certificate for your records. That is the paperwork you want when you sell the house.
Solar and battery to an outbuilding
If the outbuilding has a good south-facing roof, a small standalone solar-plus-battery array on the outbuilding itself often makes more sense than running heavier SWA. Ask us if the numbers work for your specific case.