Does a house extension need its own consumer unit?

Not necessarily. If the existing consumer unit has spare ways and the additional load is within the capacity of the main fuse, new circuits can be added to the existing board. Where there are no spare ways, a second small consumer unit can be installed and fed from the existing board as a sub-board. If the existing installation is old, has inadequate RCD protection or shows deterioration on inspection, replacing the whole consumer unit at the same time as the extension wiring is the neater and safer option.

What size cable should I use for sockets in a house extension?

Socket circuits in a house extension are typically 2.5 mm squared twin and earth on a 20 A MCB or RCBO, wired as radials. A single 20 A radial can serve up to around 50 square metres of floor area. Ring final circuits (2.5 mm on a 32 A device) can also be used. Lighting circuits use 1.0 mm or 1.5 mm twin and earth on a 6 A device. Cable sizes must be chosen to account for the method of installation and thermal insulation, following the current-carrying capacity tables in BS 7671 Appendix 4.

Is house extension wiring notifiable under Part P?

Yes. Installing new circuits, including any wiring for a house extension, is notifiable work under Building Regulations Part P in England. The person carrying out the work must either be registered with a government-approved competent person scheme such as NICEIC, NAPIT or ELECSA, or notify the local building control authority before starting and arrange for the work to be inspected on completion. An Electrical Installation Certificate must be issued at the end of the job regardless.

Can a homeowner do the first fix wiring themselves?

A homeowner may carry out electrical work themselves, but house extension wiring is notifiable under Part P. If you do the work yourself, you must notify building control before starting, and a qualified electrician must inspect and test the work before it can be used. In practice, most people find it simpler and more cost-effective to have an electrician carry out first fix and second fix and self-certify the whole job, rather than splitting the work and still paying for inspection.

What tests must be done before the new circuits are energised?

Before any new circuit is connected to the supply, the electrician must carry out dead tests: continuity of every circuit protective conductor, insulation resistance between live conductors and earth on every circuit (minimum 1 Megohm at 500 V DC), and polarity at every point. These results are recorded on the Schedule of Test Results. Once the supply is restored, live tests include voltage measurement, RCD operating times and earth fault loop impedance at each point.

What documentation do I get at the end of the job?

You should receive an Electrical Installation Certificate signed by both the designer and the person who carried out inspection and testing (often the same qualified electrician), a Schedule of Inspections and a Schedule of Test Results listing the measured values for every circuit. If the electrician is registered with a competent person scheme, they will also notify Building Control and you will receive a completion certificate from the scheme. Keep all these documents; you will need them when selling the property or arranging buildings insurance.

How to Wire the Electrics in a House Extension (UK)

Helpful video reference. This guide uses a video by Artisan Electrics — Jordan Farley's Cambridge-based electrical contracting channel with over 350,000 subscribers — showing real house extension electrical work carried out to UK standards. The video covers first fix cable runs, consumer unit connections and second fix in a domestic extension setting. Watch it on YouTube.

Before you start. Extension wiring is notifiable work under Building Regulations Part P. Either use a registered competent person scheme electrician who can self-certify, or notify your local building control office before any work begins. Do not energise any new circuit until dead testing is complete and satisfactory. Never work on live circuits.

1. Plan the circuit layout and confirm the existing supply can cope

Before any cable goes in, produce a circuit schedule. List every socket outlet, lighting point, and fixed appliance the extension will contain. Add the design currents and apply diversity using the guidance in BS 7671 Appendix 1 and IET Guidance Note 1. If the total demand, including the existing installation, is close to the rating of the main fuse, speak to the local Distribution Network Operator (DNO) about an upgrade before the work starts.

Count the spare ways in the existing consumer unit. Each new circuit needs one. If there are not enough spare ways, you have two options: replace the consumer unit with a larger one, or add a separate sub-board fed from the existing board. A sub-board is practical when the extension is a single-storey structure some distance from the house, but a replacement consumer unit is often cleaner when the existing board is dated.

2. First fix: run cables before the walls are boarded

First fix is the wiring stage that happens while the frame is visible and the walls are open. Drill through studs and joists using flat wood bits, keeping the centre of the hole at least 50 mm from the face of the timber where possible. This reduces the risk of the cable being pierced by nails or screws during boarding and decoration.

Use 2.5 mm twin and earth for socket radials, 1.0 mm or 1.5 mm twin and earth for lighting circuits. Staple cables to the sides of studs at regular intervals. Leave a tail of at least 150 mm protruding from each back box position. Label each cable at the consumer unit end before the access is closed up.

3. Fit back boxes and surface-mounted boxes

Mark each back box position on the stud or masonry before the boarding goes on. For dry-lining on a timber frame, use metal dry-lining boxes rather than surface-mounted pattress boxes. Knock out the entry holes, feed the cable tail through, and secure the box so its face finishes flush with the eventual plasterboard surface.

On a masonry extension, chop out the recess, fit the back box with suitable fixings and make good the plaster around the edge. Ensure the box is straight and at the correct height: 450 mm from finished floor level for socket outlets is the standard, though the homeowner may have preferences.

4. Route cables in safe zones and protect them correctly

BS 7671 Appendix 10 defines horizontal and vertical safe zones around accessories. Cables in these zones may be run without mechanical protection beyond the cable sheath. Outside the zones, cables concealed in a wall must be in earthed metal conduit, in armoured cable, or protected by a 30 mA RCD and run in a zone. Under floors, run cables through holes drilled in joists rather than across the tops where they could be damaged by flooring fixings. In a roof space, keep cables close to the ceiling joists and away from any areas where people might walk or store items.

5. Connect new circuits at the consumer unit

Each new radial or lighting circuit terminates at a MCB or RCBO in the consumer unit. All circuits in a domestic installation must have 30 mA RCD protection under BS 7671 Regulation 411.3.4. An RCBO provides both overload protection and RCD protection in a single device and is the tidiest solution for an extension. If the board uses split RCDs, assign the new circuits to an RCD that is already protecting circuits of a similar type.

Dress the new cables neatly down the sides of the enclosure. Strip the line, neutral and CPC to the correct length, sleeve the bare CPC in green-and-yellow, and terminate each in the correct bar. Label each MCB or RCBO clearly before the lid goes back on.

6. Second fix: connect sockets, switches and light fittings

Once plaster is fully dry — usually two to four weeks for traditional sand and cement, shorter for bonding coat — strip back the cable tails at each back box. At every socket outlet, connect brown to L, blue to N and green-and-yellow to earth, tighten the terminal screws, fold the conductors neatly into the back box and secure the faceplate. At each switch, mark a switched live cable in brown sleeving if the cable is grey. At ceiling roses and pendant fittings, follow the loop-at-ceiling-rose or junction-box layout used for the rest of the lighting circuit.

Pull-test each conductor after making the connection to confirm the terminal has gripped it properly. A conductor that pulls free is a loose joint waiting to become a fault.

7. Carry out dead testing before energising

With the new circuits disconnected from the supply at the consumer unit, carry out the dead tests required by BS 7671 Part 6. Test the continuity of every circuit protective conductor using the wander-lead method. Test insulation resistance between live conductors and earth on each circuit at 500 V DC — the minimum acceptable result is 1 Megohm, but a healthy new circuit should read much higher. Test polarity at every socket and switch using a continuity tester or a multifunction installation tester set to resistance.

Record every result on the Schedule of Test Results before moving on. Any reading that falls outside the acceptable range stops the job until the cause is found and the fault is cleared.

8. Live test, issue the EIC and notify Building Control

Restore the supply to the new circuits and carry out the live tests. Measure the voltage at each point. Test the operating time of every RCD or RCBO at half and full rated tripping current using a multifunction tester — a 30 mA RCD must trip within 300 ms at full rated current (and within 40 ms if the push button is rated at 5x In). Test earth fault loop impedance (Zs) at each socket outlet and at the farthest point of each lighting circuit. Compare every Zs result against the maximum permitted in BS 7671 Table 41.4 for the type and rating of the protective device.

Once all results are satisfactory, complete the Electrical Installation Certificate, the Schedule of Inspections and the Schedule of Test Results. Sign and date them. Give the original to the homeowner and keep a copy. If you are registered with NICEIC, NAPIT or ELECSA, notify Building Control through the scheme's online portal — this removes the need for a local authority inspector's visit and the homeowner receives a completion certificate by post within a few days.

Stop and call an electrician if: the consumer unit has no spare ways and no safe way to add circuits, the insulation resistance test reveals a fault that cannot be traced, the Zs test results exceed the maximum permitted for the installed protective device, or there is any sign that the existing installation is in poor condition and may need upgrading before new circuits can safely be added to it.

When to call us

House extension electrical work is notifiable under Part P and must be accompanied by an Electrical Installation Certificate. Richard is registered with a competent person scheme and covers Sandwich and east Kent. He can take a house extension from circuit design and first fix through to the completed EIC and Building Control notification, leaving you with a fully documented installation that will stand up to any future EICR.

Adding an extension in east Kent?

Richard handles the electrical side from first fix to the EIC. Complete documentation, no shortcuts.

Contact Richard

How to wire the electrics in a house extension