An extension usually needs new circuits rather than extended ones, and the first question is whether the existing consumer unit has spare ways and adequate capacity. The work is notifiable under Part P and needs Building Control sign-off. Every socket, switch and light position must be fixed before first fix, because changing them afterwards means chasing finished walls.
- Check the consumer unit for spare ways before the design is finalised. It is often the constraint.
- Extending an existing ring into a new room is usually the wrong answer.
- Electrical work in an extension is notifiable — it must be certified and signed off.
- Decide every outlet position before first fix. Afterwards is chasing plaster.
Start at the consumer unit
This is the question to ask before the drawings are finished, because the answer can change the budget and occasionally the design.
An extension adds circuits. Those circuits need somewhere to land, and the existing board may not have room.
Spare ways. Count the empty positions. A kitchen extension can easily need three or four new circuits — sockets, lighting, cooker, and often one for an outdoor supply or underfloor heating. If the board is full, it needs replacing or a secondary board adding.
The main fuse and supply capacity. Most domestic supplies are adequate, but adding a large extension with electric underfloor heating, an induction hob, a shower and an EV charger can push the demand calculation. An electrician will do a maximum demand assessment; where it exceeds the supply, the distributor has to be involved and that takes time.
The age and type of the board. Adding circuits to a board with no RCD protection is not acceptable, so an old board turns into a replacement as part of the works. That is better discovered now than mid-build.
The earthing arrangement. The type of supply earthing affects what is permitted, particularly for an outdoor supply or an EV charger, and it occasionally requires an earth rod.
The practical point: get an electrician to look at the board before you commit to a layout. A consumer unit replacement is a known, modest cost when planned, and a disruptive surprise when it emerges after the plasterboard is up.
New circuits, not extended ones
The cheap approach is to spur off the nearest existing socket and carry the ring into the new room. It is usually the wrong answer, for three reasons.
Loading. A ring final circuit serves a defined floor area and a defined set of demands. Adding a kitchen-diner to a circuit already serving half the house puts the load somewhere it was not designed for.
Diagnosis. When something trips, you want the fault isolated to a part of the house. A circuit spanning old and new means the whole lot goes off together and tracing takes longer.
Inheriting faults. An extended circuit carries forward whatever is wrong with the original — no RCD, degraded insulation, a borrowed neutral. New work on a compromised circuit is still compromised.
What an extension typically needs:
- A dedicated socket circuit for the new area.
- A separate lighting circuit, or at least a separate way on the board.
- A cooker circuit where there is a hob or oven, sized to the appliance.
- A supply for underfloor heating, if fitted, with its own control.
- An outdoor socket and garden supply, far cheaper to run while the ground is open.
- Data cabling to the new room, which costs very little at first fix and is awkward later.
- Smoke and heat alarms interlinked with the existing system, which Building Control will expect.
Part P and sign-off
Electrical work in an extension is notifiable. It falls under Part P of the Building Regulations and it has to be certified.
There are two routes, and which one applies depends on how the extension itself is being controlled.
Through the main building control application. Where the extension is already under a full plans or building notice application, the electrical work is inspected as part of it. The electrician issues an Electrical Installation Certificate, and Building Control takes that as evidence.
Self-certified by a registered electrician. An electrician registered with a competent person scheme can certify their own work and notify it, which is the usual route for the electrical element.
Either way you should end up holding an Electrical Installation Certificate with the test results on it, and the Building Regulations completion certificate for the extension. Both matter years later: a buyer's solicitor asks for them, and without them the options are a regularisation application, an indemnity policy or a price reduction.
A point worth being firm about: an electrician who cannot or will not provide a certificate is not an option on notifiable work, whatever the price difference. The certificate is not paperwork for its own sake — it records the test results that prove the installation is safe.
Decide the layout before first fix
First fix is when the cables go in, before the walls are closed up. Every position has to be known by then, and this is where most of the regret in an extension's electrics comes from.
What needs deciding:
- The kitchen layout, in full. Where the sink, hob, oven, fridge, dishwasher and washing machine go, because each needs a supply and some need isolation above worktop level.
- Any island. Services to an island go in the floor before the slab, and a decision made later means breaking it up.
- Every socket. Walk the room with the furniture drawn on the plan. Where does the table go, which side is the sofa, where will someone plug a lamp or a laptop in.
- Switch positions and which way doors open, because a switch behind an open door is a daily irritation.
- Lighting: the circuits, the zones, whether anything is dimmed, and whether downlights clash with joists or rooflights.
- Outdoor: sockets, lighting, and ducting to the garden while the ground is open.
- Television and data points, plus a noggin behind any wall-mounted screen.
- Extraction for the hob, and its route out.
There is a simple test for whether the layout is finished: name, for every corner of the new space, what will plug in there. The extension with no socket where the lamp needs to be is remarkably common, and it was a two-minute decision taken a fortnight too late.
Where old meets new
The junction between the existing house and the extension is where the electrical work gets least attention and causes most trouble.
The wall that disappears. If the rear wall is being removed or opened up, whatever was wired into it has to go somewhere — sockets, switches, an outside light, the alarm bell box, the burglar alarm sensor. Those circuits need tracing and rerouting rather than cutting and capping.
Cables buried in the old wall. Chasing into an existing wall to reroute means finding what is already in it. Cables should run in permitted zones but in older houses frequently do not, and a cable run diagonally across a wall is a real hazard when someone drills.
The alarm, doorbell and external lighting all tend to be forgotten until the scaffold comes down.
Interlinking the smoke alarms. New alarms in the extension have to interlink with the existing ones, and if the existing system is old or battery-only this usually means upgrading the lot.
The boiler and heating controls, where the extension affects the pipework or the thermostat position.
Flagging these at survey stage costs nothing. Discovering them when the wall is already open costs a day and a variation.
What it costs
These are the electrical figures published across this site:
| Job | Typical Coventry guide price | Notes |
|---|---|---|
| Electrician, hourly | £45–£90/hr | Certified, Part P |
| Typical job | £233–£473 | Sockets, lighting, faults |
| Consumer unit upgrade | £450–£700 | RCD protection |
| Full rewire (3-bed) | £3,000–£6,000 | Whole-house |
Extension electrics are usually priced per job rather than hourly, because the circuit count and the cable runs drive it. A consumer unit replacement, where the existing board is full or has no RCD, is the item most likely to be missing from an early estimate.
We look at the consumer unit and the earthing before the layout is fixed, because a full board is the constraint people find out about late.
Frequently asked questions
Does an extension need its own electrical circuits?
Usually yes. Extending an existing ring into a new room puts load where the circuit was not designed for it, means a fault takes out old and new together, and carries forward any problem in the original circuit. A dedicated socket circuit and a separate lighting way is the normal answer.
Is electrical work in an extension notifiable?
Yes, it falls under Part P. It is either inspected as part of the extension's building control application or self-certified by an electrician registered with a competent person scheme. You should end up with an Electrical Installation Certificate carrying the test results.
Will I need a new consumer unit for an extension?
Often. A kitchen extension can need three or four new circuits, and if the board has no spare ways it needs replacing or supplementing. A board with no RCD protection also has to be dealt with, since new circuits cannot be added to it. Check before the design is finalised.
When do I need to decide where the sockets go?
Before first fix, when the cables go in and before the walls are closed. Changing a position afterwards means chasing finished plaster. Walk the room with the furniture drawn on the plan and name what will plug in at each corner — that is the test for whether the layout is done.
What happens to the wiring in the wall being knocked through?
It has to be traced and rerouted, not cut and capped. Sockets, switches, an outside light, the alarm bell box and its sensors all tend to live on that wall. Cables in older walls also often run outside the permitted zones, which matters when anyone drills later.
Should I run cable to the garden during the build?
Yes, or at least ducting with a draw cord. The ground is already open, so a run for an outdoor socket, garden lighting or a shed supply costs very little then. Retrofitting means lifting paving or cutting through new planting, which is disproportionately expensive.
Check the board before the drawings
We assess spare ways, maximum demand and the earthing arrangement before the layout is fixed, so the electrical cost is known up front.