You need a separate consumer unit for a shed, garage, or garden office if you want independent control over its circuits the ability to isolate power to just that building without affecting the house, and to protect several separate circuits (lighting, sockets, tools) rather than sharing one with whatever’s already on the house ring. If you only want a single socket or light running off an existing house circuit, a full sub-board is usually more than you need.
That distinction matters more than most product pages let on, because the honest answer to “which one should I buy” depends entirely on what the project actually needs first.
Do You Actually Need a Separate Consumer Unit?
The clearest way to think about it: a separate consumer unit gives you independent control and protection over the circuits in that building. If a fault develops in the garage, only the garage loses power, not the kitchen. If you want several distinct circuits out there lighting, sockets, and something higher-demand like a compressor or an EV charger a dedicated board lets each one be sized and protected individually.
If you genuinely just want one extra socket or a single light, extending an existing house circuit can sometimes be enough, and a full sub-board would be unnecessary cost. The moment you’re talking about multiple circuits, anything power-hungry, or a building any real distance from the house, a proper sub-main is the right call, not just the safer one.
What’s a Sub-Main, and How Does It Work?
A sub-main is simply a dedicated circuit run from your main consumer unit to a second, smaller consumer unit in the outbuilding. That second board then splits into its own individual circuits, exactly the same way your main board does, just on a smaller scale.
Rather than every garage or shed circuit tracing all the way back to the main board’s own individual breakers, one properly sized and protected sub-main cable feeds a local board that does the actual distribution on-site. It’s the same principle behind the small consumer units covered in our sizing guide, applied specifically to a building that isn’t physically part of the house.
The Cable Matters More Than the Board
Whichever consumer unit ends up on the wall, the cable feeding it is doing most of the safety work, particularly if it’s running underground. Steel wire armoured (SWA) cable is the standard for this kind of supply, because the metal armour gives real mechanical protection against the cable being damaged by digging, tree roots, or ground movement something a standard twin-and-earth cable was never designed to survive underground.
Current guidance calls for a minimum burial depth of around 500mm in open ground, deeper or with additional protection under anything that gets walked or driven over. Getting this wrong isn’t a minor detail. It’s the difference between a supply that lasts decades and one that fails, or becomes genuinely dangerous, the first time someone puts a spade through it.
RCD Protection Isn’t Optional Here
Under BS 7671 (Regulation 411.3.3), socket outlets and most circuits supplying equipment outdoors need 30mA RCD protection. This isn’t a nice-to-have for an outbuilding supply it’s a standard requirement, precisely because outdoor and underground circuits carry a higher risk of an earth fault than something safely inside a dry house.
In practice, this means the RCD protection on your shed, garage, or garden office board isn’t an upgrade you can skip to save money. It’s built into doing the job properly in the first place.
Sheds, Garages & Garden Offices Different Projects, Different Needs
The right setup depends heavily on what the building’s actually for.
A shed used mainly for storage, with perhaps one light and a socket for a charger or a radio, is often the simplest job a short cable run, a small 2-way board, done in a day. Add power tools or anything more demanding, and it’s worth planning for more capacity than the bare minimum.
A garage is usually a bigger project. Lighting, multiple socket circuits, and increasingly an EV charger or a workshop’s worth of power tools mean a garage board is rarely just 2-way, and it’s exactly the kind of space where under-planning today means a second, more disruptive job in a year or two.
A garden office tends to be the most demanding of the three. Heating, internet, a desk full of electronics, sometimes a kettle or small fridge, all running for hours at a time rather than the occasional burst of a shed or garage. This is also where we’re seeing the most growth across Richmond, Chiswick, and the wider West London area, as more properties convert existing outbuildings or add new garden rooms. It’s worth wiring it properly the first time rather than treating it as an afterthought to a bigger renovation.
What Size Do You Need?
Sizing a sub-board follows the same logic as any small consumer unit: count what’s actually being wired, then leave a little spare capacity for whatever comes next. Our small consumer units guide covers exactly how to work that out, including a quick sizing calculator, if you want a sense of the size before booking a survey.
Planning an EV Charger in the Garage?
Garages are, by some distance, the most common place for a home EV charger to end up, and it changes the sizing conversation. An EV charger needs its own dedicated circuit, with its own protection, which means factoring that in from the start rather than discovering there’s no spare capacity once the charger’s already been bought. If this is on the horizon at all, it’s worth mentioning when you book a survey, even if you’re not ready to install the charger itself yet.
Every outbuilding supply we install is carried out by ECS/JIB-certified, City & Guilds-qualified electricians, fully insured and DBS-checked, with the cable route, board, and testing covered as one job rather than a box off a shelf.
Frequently Asked Questions
Do I need a separate consumer unit for my shed or garage?
If you want independent control over multiple circuits in that building, or if it’s any real distance from the house, yes. For a single extra socket or light, extending an existing house circuit can sometimes be enough.
Do I need a separate consumer unit for sheds, garages, and outbuildings, or can I run an extension lead?
A permanent extension lead isn’t a safe long-term solution for anything beyond very light, temporary use. A proper underground supply with its own protection is the correct way to power a shed, garage, or outbuilding permanently.
What size consumer unit does a garage need?
It depends on what’s being wired, but most garages fall somewhere between 4-way and 8-way once you account for lighting, sockets, and any power tools or an EV charger. Our small consumer units guide covers the full breakdown.
How much does a consumer unit for a shed or garage cost?
It varies with distance from the house, cable routing, and board size. Our consumer unit replacement cost guide covers the general cost factors, though an outbuilding supply typically also includes the cable run itself, not just the board.
What cable do I need to run power to a shed or garage?
Steel wire armoured (SWA) cable is the standard for any underground run, buried to a minimum depth of around 500mm, with additional protection wherever the route passes under a path or driveway.
Can I install a garage consumer unit myself?
Running a new circuit to an outbuilding is notifiable work under Part P of the Building Regulations, so it needs to be carried out or signed off by a registered electrician, the same as any other consumer unit installation.
Does a garden office need its own consumer unit?
In most cases, yes, particularly if it has heating, IT equipment, or anything else running for extended periods. A dedicated board gives you proper circuit separation and the right level of protection for a space that’s effectively being used as a room.
Is RCD protection required for outdoor circuits?
Yes. Under BS 7671, socket outlets and most circuits supplying equipment outdoors require 30mA RCD protection as standard, not as an optional extra.
Planning a Shed, Garage, or Garden Office Project?
Get a free electrical assessment for the whole project cable route, board sizing, and a fixed quote, not just a box off a shelf.