outdoor
Garage, shed and outbuilding supplies
A submain to an outbuilding with a small board at the far end costs £400 to £1,200. The variable is the trench. Twenty metres across a lawn is an afternoon; the same twenty metres under a block drive is a different job with a different number on it.
What this covers
What it costs. A submain and a small board is £400 to £1,200 on a straightforward run. Full cost breakdown.
What the job covers
- Running a submain in armoured cable to an outbuilding
- Fitting a small consumer unit at the far end
- Lighting and sockets for a workshop
- Supplies sized for a welder, compressor or car lift
- Certifying the submain as a new circuit
What is involved
The right answer for anything beyond a single socket is a submain: armoured cable from its own way in the house consumer unit, buried at depth with warning tape, terminating in a small consumer unit inside the outbuilding. That gives the building its own protective devices, so a fault in the garage does not put the house in the dark, and it gives room to add circuits later without going back to the house. An extension lead through a window is the thing this replaces and it is worth saying plainly that it is not a substitute.
Sizing follows what goes in the building rather than what is there today. Lighting and a couple of sockets is modest. A workshop with a table saw, a compressor and a welder is not, and neither is a garage that will take an EV charger, which is worth deciding now because the cable is the expensive part and upgrading it means digging the trench twice. Ask for the submain to be sized for the charger even if the charger is two years away.
Earthing is the detail that catches people out. An outbuilding is often treated differently from the house, and on a PME supply exporting the house earth to a detached metal building is not always acceptable, which can mean a rod at the outbuilding and its own arrangement. That is a decision for the person doing the design rather than a preference, and it is one of the reasons this job is not a long extension lead with a plug on it.
Choosing someone for the job
Decide now what the building will hold in five years, including an EV charger, and have the submain sized for that. Ask whether the trench is in the price and what it crosses. Ask what the earthing arrangement at the outbuilding will be. Ask for a small consumer unit at the far end rather than a single switched fused connection.
Common questions
Can I just run an extension lead?
For a lawnmower on a dry afternoon, people do. For anything permanent it is unprotected, untested, a trip hazard and uninsurable. A submain is the job this replaces.
How deep does the cable go?
Deep enough to be clear of normal digging, with warning tape above it, and deeper again under a drive. The depth is part of what makes the trench the largest line on the quote.