Wiring a house extension involves designing new circuits, connecting to the existing distribution board (or installing a sub-main), coordinating with the building project timeline, and ensuring compliance with Part P and BS 7671. This guide covers every aspect of extension electrics — from initial planning through to testing and certification.
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Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.
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The electrical design for a house extension should be planned at the same time as the architectural design — not as an afterthought once building work has started. The position of the consumer unit, cable routes through the existing house to the extension, socket and light positions, and external circuit requirements all affect the building work and must be agreed before the builder starts.
Discuss what the extension will be used for. A kitchen extension has very different electrical requirements to a bedroom extension or a home office. Key questions: How many sockets are needed and where? What lighting style is preferred (recessed downlights, pendants, track lighting)? Are there any dedicated circuits required (cooker, dishwasher, boiling water tap, electric underfloor heating)? Does the homeowner want external lighting, outdoor sockets, or EV charger preparation?
Before designing the extension circuits, assess the existing consumer unit. Count spare ways available. Check the condition of existing wiring — if the existing installation has significant deficiencies, these should be discussed with the homeowner before adding new circuits to it. Consider whether the incoming supply and main fuse are adequate for the additional demand. An EICR on the existing installation before starting extension work is strongly recommended.
The builder needs to know where cables will run so that suitable chase routes, floor voids, and wall cavities are maintained. First fix electrical work must happen after the structure is weather-tight but before insulation, boarding, and plastering. Agree the programme with the builder and confirm your attendance dates for first fix and second fix. Late arrival for first fix delays the entire project.
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The number of new circuits depends on the size and use of the extension. As a guide, the following circuits are typically required:
Smoke and heat detectors in the extension must be mains-powered and interconnected with the existing detection system in the main house — this is a Building Regulations requirement.
The new circuits for the extension must connect to the property's electrical distribution. There are two main approaches, depending on the capacity of the existing consumer unit.
If the existing consumer unit has sufficient spare ways (at least as many as the number of new circuits), the new circuits can be connected directly. Each new circuit gets its own RCBO. This is the simplest and most cost-effective approach. However, adding circuits to an existing consumer unit still requires a full Electrical Installation Certificate (EIC) for the new work, not just a Minor Works Certificate, because new circuits are being added.
If the existing consumer unit is full, or if the extension is large enough to warrant its own distribution, a sub-distribution board can be installed in the extension. This is fed by a sub-main cable from a spare way in the main consumer unit (or a dedicated way added to the main board). The sub-main is sized to carry the maximum demand of all the extension circuits. A typical sub-main for a domestic extension is 10mm² or 16mm² twin-and-earth, protected by a suitably rated device at the main board.
Never double up circuits on existing ways — connecting two circuits to one MCB or RCBO is not compliant with BS 7671 and creates an overload risk. Never spur new circuits from existing socket outlets as a way of avoiding adding circuits to the consumer unit — a spur is an addition to an existing circuit, not a new circuit. Each new circuit in the extension must have its own protective device at the distribution board.
The lighting design in a modern extension typically goes beyond simple pendant fittings. Homeowners expect a professional lighting scheme that creates the right ambience and provides task lighting where needed.
All lighting switch positions should be agreed with the homeowner during the design phase. Consider wiring colour conventions for switched live and two-way switching installations.
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Start free trialThe number and position of socket outlets in the extension should be planned based on the room's intended use. BS 7671 does not specify a minimum number of sockets, but the IET Guidance Note 1 and the NHBC standards provide recommendations.
Allow for 6-8 double socket outlets above the worktop, plus dedicated connections for fixed appliances (oven, hob, dishwasher, washing machine, boiling water tap, fridge/freezer). Position worktop sockets 150mm-200mm above worktop level. Consider USB charging sockets for at least one or two positions.
Allow for double sockets at each seating position, TV/media wall, and any likely desk or table locations. A minimum of 4-6 doubles for a typical single-room extension. Consider floor outlets for central positions away from walls (e.g., kitchen island power, central dining table).
Multiple doubles at desk level, at least one data point (CAT6 alongside mains), and consider a dedicated circuit for computing equipment if the homeowner has significant IT infrastructure. A separate lighting circuit with task lighting above the desk area is beneficial.
Standard socket mounting height is 450mm from finished floor level (centre of plate). Mounting sockets higher — at 600mm or even desk height (1050mm) — can be more convenient in some situations and improves accessibility. Discuss preferences with the homeowner during the design phase.
House extensions often include new external circuits for garden lighting, patio sockets, and security lighting. These have specific requirements under BS 7671:
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Try it free for 7 daysAll new electrical work in a house extension is notifiable under Part P of the Building Regulations. This includes every new circuit, the connection to the existing distribution, and any outdoor circuits. The work requires a full Electrical Installation Certificate (EIC), not a Minor Works Certificate.
There are two routes to compliance:
If the electrician is registered with a competent person scheme (NICEIC, NAPIT, Elecsa, etc.), they can self-certify the work. The scheme provider issues a Building Regulations Compliance Certificate to the homeowner. This is the most common route for professional electricians — it avoids the need for a separate Building Control inspection and is faster and cheaper for the homeowner.
If the electrician is not registered with a competent person scheme, the homeowner must notify Building Control before the work starts. Building Control will inspect the work and issue a completion certificate. This adds cost (Building Control fees) and time (waiting for inspection appointments). For extension projects, Building Control is usually already involved for the structural work, so this route can be coordinated with the overall building project.
The EIC for extension work must cover the design, construction, and testing of all new circuits. Use Elec-Mate's digital EIC form to complete the certificate on site with all test results validated automatically.
Planning and installing electrics for a house extension from design to certification.
Walk through the extension plans and agree every socket, light, and switch position. Identify all dedicated circuits (cooker, underfloor heating, external circuits). Design the circuit schedule and cable routes. Check the existing consumer unit for spare capacity. Use Elec-Mate's AI Circuit Designer for a compliant specification.
Confirm dates for first fix (after weather-tight, before insulation and boarding) and second fix (after plastering). Agree cable routes through the existing house if the consumer unit is not adjacent to the extension. Ensure the builder leaves suitable access for cable runs through floors and walls.
Run all cables from the consumer unit or sub-board position to every accessory point. Install back boxes in agreed positions. Clip cables at correct intervals per BS 7671. Install fire-rated downlight cans and any conduit runs. Label all cables at both ends. Install smoke and heat detector cables.
After plastering is complete, fit all accessories (sockets, switches, light fittings, cooker connection). Connect all circuits at the consumer unit or sub-board. Carry out the full testing sequence — continuity, insulation resistance, polarity, earth fault loop impedance, prospective fault current, and RCD tests.
Complete the Electrical Installation Certificate with all design, construction, and test data. Submit Part P notification through the competent person scheme or coordinate with Building Control. Issue copies of the EIC to the homeowner. Provide the circuit chart for the consumer unit.
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