Short answer
An extension usually gets one or two circuits of its own, connected to the existing meter cupboard, provided there is room. Run cables straight away for outdoor lighting, an outdoor socket, network and possibly a future air conditioner or sun shading. The extra cost of that is small now and considerable later.
Who this article is for Anyone building an extension. The best moment to read this is before the walls are closed.
What belongs in it as standard
- lighting, usually several points with separate switches or a dimmer
- sockets along the walls, and more than you think you need
- an outdoor socket and outdoor lighting by the back door
- network, because wifi through an insulated wall is disappointing
- a separate circuit, or two if heavy equipment is coming
An extension often becomes a kitchen or living room, and then the same principles apply as in electrics in the kitchen.
Connecting to the existing installation
The extension gets its supply from the existing meter cupboard. Two questions:
For a large extension or a garden room, a sub-distribution board is usually the wisest set-up.
Is there room in the consumer unit? If not, this is the moment to replace or extend it, see consumer unit replacement.
How does the cable get there? From the meter cupboard to the extension there is often a route through the crawl space or along a ceiling. Decide that route before the floor is closed.
For a large extension or a garden room a sub-distribution board can be sensible: one heavier cable to a small distribution board on site, and the circuits from there. That saves metres and makes extending simple later.
What to include now that you will be grateful for later
| Lay now | Why |
|---|---|
| Empty conduit to the roof | air conditioning, solar panels, sun shading |
| Network cable to the extension | wifi rarely gets through insulation |
| Cable to the garden or garden room | lighting, camera, pond pump |
| Extra conduit to the meter cupboard | whatever comes in future |
| Supply for electric sun shading | blinds are often bought afterwards |
The extra cost of this during the build is a fraction of what it costs later.
Insulation and penetrations
Where cables pass through the insulation or the airtight layer, it must be sealed neatly. A sloppy penetration causes draughts and condensation, exactly where you cannot see it. I finish them with the sleeves made for the purpose and coordinate it with the builder.
Include outdoor lighting straight away
An extension with a patio behind it calls for outdoor light. A cable and a switch point are quickly laid during the build. Also think about a switch inside by the door and possibly a dusk sensor or motion sensor.
From my own work
For an extension in Geleen, besides the normal circuits I also laid an empty conduit to the roof and a network cable to the garden room. Two years later an air conditioner and a camera by the back door were added. Both were simply a matter of pulling cable through. Without that conduit it would have meant chasing a freshly plastered wall.
When to call an electrician
- An extension, conservatory or garden room is being added
- You do not know whether the meter cupboard can take more circuits
- A cable to the garden or garden room needs to be included
I get involved early in the build, while the walls are still open. Request a quote.
Frequently asked questions
What should I include in an extension that becomes expensive later?
An empty conduit to the roof, a network cable, a cable to the garden and an extra conduit to the meter cupboard. Almost free now, another renovation later.
Do I need a sub-distribution board?
For a large extension or a garden room usually yes: a heavier cable to it and the circuits on site. That saves metres and makes extending simple.
Does my meter cupboard need upgrading for an extension?
If there is no more room on the rail or not everything has RCD protection, yes. During the build that is considerably cheaper than afterwards.
Further reading
What does an electrician cost in 2026? Hourly rate and call-out charge explained
Hourly rate, call-out charge, material margin and VAT, without beating about the bush. Including what I charge myself and why a fixed call-out charge is fairer.
Electrics in the bathroom: what is allowed where
The bathroom is divided into zones. For each zone the standard sets what may be installed and how well it must be protected.
Electrics in a new kitchen: which circuits and where
A modern kitchen easily has four to six circuits. Induction almost always needs its own power circuit.
Questions and comments
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