The cable is the job. Everything else is detail.
Garden offices, studios and converted outbuildings have become one of the most common things we are asked to power, and the request almost always arrives the same way: the building is already ordered, or already up, and power is the thing nobody costed.
It is worth knowing what is actually involved, because the gap between doing it properly and running an extension lead across the lawn is larger than it looks, and only one of those is still working in three winters.
Key Takeaways
- A garden building needs its own supply, not a spur off an existing circuit
- The cable must be armoured, correctly sized and buried at the right depth
- The building needs its own small consumer unit and means of isolation
- Size it for what the building will be used for in five years
- Plan the trench before the landscaping, not after
Why An Extension Lead Is Not The Answer
It needs saying because it is extremely common, and because it usually works for a while, which is the problem.
A domestic extension lead is designed for temporary indoor use. Run permanently across a garden it is exposed to water, frost, UV and whatever a mower or a spade does to it. It has no protection designed for the run, no isolation at the far end, and nothing to tell you when the insulation has started to fail. It is also carrying a load it was never rated for, continuously, which is how a lead that felt fine in June becomes warm in January.
A permanent building needs a permanent supply. That means a cable specified for the job, installed so that it is protected from everything above it, and terminated properly at both ends.
What A Proper Installation Involves
A dedicated supply from the house. Not a spur off a nearby socket circuit. The garden building gets its own circuit from the consumer unit, correctly protected, sized for the load and the distance it has to travel.
Armoured cable, correctly sized. Two things determine the size: how much power the building will draw, and how far it has to travel to get there. Distance causes voltage drop, so a cable that would be fine over ten metres may not be over forty. This is the calculation that gets skipped in cheap quotes.
Buried at the right depth and protected. Deep enough to survive whatever happens above it, which is different under a lawn, a flower bed and a drive with vehicles on it. It should also be marked, so that the next person to dig knows it is there before the spade finds it.
Its own consumer unit in the building. A small board at the far end gives the building its own protection and its own means of isolation, so it can be worked on, or switched off, without killing the power to the house. It also means a fault in the garden office does not take out the kitchen.
Testing, inspection and certification on completion, with the paperwork handed over.
Size It For Five Years, Not For Today
This is the single most useful decision you will make, and it costs almost nothing at the right moment.
Most people describe a garden office as needing a laptop, a couple of lamps and a router. That is a trivial load. But the building rarely stays that. It gains a heater, because it is cold in January and nobody enjoys working in a coat. It gains a small kitchen area. The studio gains a kiln, or a compressor, or a welder. The gym gains a treadmill.
Each of those is a step up in demand, and the cable is the one part you cannot upgrade without digging the trench again. Sizing up at installation is a modest cost on a job you are doing anyway. Retrofitting it means lifting the lawn you just relaid.
So the question we will ask is not what the building needs now. It is what you might plausibly want it to do, and whether heating is going in at some point. Answer that generously.
Lighting, Heating And Data
Lighting inside is straightforward; outside is where it is worth some thought. A path between the house and the building that is unlit in December is the reason the building stops getting used. External fittings also need to be rated for the position they are in, because an indoor fitting in a sheltered spot will survive one winter and fail in the second.
Heating should be decided before the cable is sized, for the reasons above. An electric heater is the most common choice and the most common thing added later.
Data is worth doing at the same time as the power, in the same trench, while it is open. Wi-fi from the house to a building thirty metres away through two walls is unreliable at the best of times, and running a data cable afterwards means either a second trench or giving up. If the building is going to be used for work, this is not optional.
Planning Permission And Building Regulations
Worth a word, though it depends on your specific building and site.
The garden building itself may fall under permitted development or may not, depending on size, height, position and your property. That is a question for your local planning authority or the supplier of the building, and it is separate from the electrics.
The electrical work is notifiable and must be designed, installed, tested and certified to current standards. That is our side of it, and you should receive the certification regardless of who supplies the building. If a garden building company tells you their installer will “sort the power out”, it is reasonable to ask what certification you will be given.
Get The Trench In Before The Landscaping
A practical point that saves real money.
The order most projects happen in is: building goes up, ground gets tidied, landscaping goes in, then somebody thinks about power. By then the route between the house and the building has a new patio, a resown lawn or a gravel path across it, all of which have to come up.
If you are having a garden building, get the supply run while the ground is already disturbed. Even if the building is not there yet, the cable can go in and be left ready. It is the cheapest sequencing decision available on the whole project.
How We Approach It
We come and walk the route. We look at where the consumer unit is, where the building is going, what is in between and what the building is realistically going to be used for. Then we size the cable for that rather than for the minimum that would pass.
You get a free, itemised quote covering the full scope including the groundworks and reinstatement, so the trench is not a surprise line item halfway through. As NICEIC Approved Contractors with over twenty years of experience, everything is tested, certified and left tidy.
We carry out outdoor lighting and power and electrical installations across Salisbury, Downton and the surrounding villages, including the more spread out properties around Whiteparish and the SP5 villages where the house and the outbuildings are rarely close together.