Garden Room Electrics: Why an Outbuilding Gets Its Own Board

A garden room, workshop or cabin needs more than a cable run down the garden. On a recent job we fitted a new consumer unit in the house, with surge protection and RCBO protection, and a separate board in a timber outbuilding with its own sockets and lighting circuits. Tidy, labelled and fully tested.

That separate board is the part people are sometimes surprised by. Here is why it is there, and what else decides whether an outbuilding supply is done properly.

The job: a new board in the house and one in the outbuilding

This installation had two halves. In the house, a new consumer unit went in with surge protection and RCBO protection. In the outbuilding, a second board takes the supply from the house.

In the outbuilding, a small board takes that supply and splits it into a sockets circuit and a lighting circuit, with a spare way left for whatever gets added later. The main switch is right there on the wall, so the building can be isolated from inside it.

Why an outbuilding gets its own board

You could, in theory, run a socket circuit down the garden and plug everything into it. It is a bad idea for several reasons.

• Isolation where you are working. If something needs switching off in the garden room, you should not have to walk back to the house to do it. A board in the building gives a main switch at the door.
• Separate circuits. Lights on their own circuit means a tripped socket does not leave you in the dark. That matters in a workshop with power tools.
• Room to grow. Heaters, a second monitor, a kettle, and sometimes an EV charger all end up in outbuildings. A board with a spare way is ready for that. A single socket circuit is not.
• Protection sized properly. The supply cable is protected at the house for its size and length, and each final circuit in the outbuilding is protected for what it feeds.

The cable is most of the job

The board is the quick part. Getting the supply to it is where the planning goes. A buried run needs cable designed for it, normally steel wire armoured, at a sensible depth with warning tape above it so the next person with a spade finds the tape before the cable.

Distance matters too. Voltage drops along any cable, and a supply that is fine at ten metres may not be at forty. The cable is sized for both the load you expect and the length of the run. Undersizing it to save a little money is the most expensive mistake in outbuilding electrics, because the fix is digging the trench again.

Our outdoor electrics page covers the other outdoor essentials – enclosure ratings, UV-stable materials and why indoor accessories fail outside.

Earthing: the question to ask any installer

How the house is supplied decides how the outbuilding is earthed. On some supplies, the earth from the incoming service can be taken out to the outbuilding. On others – particularly where the outbuilding has exposed metalwork or feeds something like a charge point – a separate earth rod at the building may be the right answer.

It is a genuine cost difference between quotes, and a reasonable question to ask. An installer who has not thought about it has probably not tested for it either.

Why the house board mattered on this job

The new outbuilding circuit starts at the house board, so the house board has to be fit to take it. On this job that meant a new consumer unit with surge protection and RCBO protection.

The point of an RCBO is that a fault trips only its own circuit, rather than a whole bank of the house. If you want the detail on what a board change gets you, our guide to RCBO and split-load consumer units goes through it. Board replacements are covered on our consumer units page.

Air con and heat pumps need a proper supply as well

The same thinking applies to anything big that lives outside. On another job we ran a dedicated supply and fitted an outdoor rotary isolator for a new air conditioning unit, all ready for the installer to connect.

The isolator next to the unit means whoever services it can switch it off and see that it is off, right where they are working. Air con and heat pump installers often expect the electrical supply to be in place when they arrive, so it pays to get it booked in first rather than holding up their day.

What to tell your electrician before the quote

An outbuilding supply is sized for what it will do, so the more we know up front, the more accurate the quote and the less chance of digging twice:

• What the building is for. A home office, a gym, a workshop with power tools and a summer house with a kettle all need different things.
• Heating. Electric heating is usually the biggest load in a garden room and has the most effect on cable size.
• The distance and the route. How far from the house, and what the cable has to cross – lawn, patio, a path or a driveway.
• Future plans. An EV charger, a hot tub or a second building later on. It costs little to allow for them now.
• Data. If you will work from the building, a network cable laid in the same trench is far more reliable than Wi-Fi from the house.

Is garden room electrical work notifiable?

Yes. Running a supply to an outbuilding means installing a new circuit, which the Building Regulations list as notifiable work. It has to be certified, and as a registered installer we notify it through our scheme so you do not have to deal with building control yourself.

You should get an Electrical Installation Certificate with the test results for every circuit, and a compliance certificate follows. If you ever sell, or the garden room is used for business, you will be asked for both.