How much does house extension electrical work cost?

For a small single-storey rear extension (up to 15m²), the electrical package typically costs £1,200 to £2,000. This covers first-fix cable installation, a socket circuit, lighting circuit, and second-fix installation, testing, and Part P notification. A medium extension (15 to 30m²) costs £2,000 to £3,200, and a larger extension (30m²+) or one that includes a kitchen or utility room costs £3,000 to £4,000 or more. A consumer unit upgrade (if required) adds £400 to £800 to the package. The main cost drivers are floor area, number of circuits, whether a kitchen is included, and the distance from the consumer unit.

What is the difference between first fix and second fix electrical work?

First fix is all the electrical work carried out before plastering: installing back boxes (flush or surface), running cables through the walls, ceiling, and floor, and installing conduit where cables cannot be concealed in the structure. First-fix cables are left with tails for connection at second fix. Second fix is carried out after the plastering and decoration is complete: fitting faceplates (sockets, switches, light fittings), connecting back boxes, and carrying out the full test and inspection of all circuits. The split allows the builder to manage the programme — electricians go in twice rather than interrupting the build.

Should I use new circuits or extend existing circuits for a house extension?

New dedicated circuits from the consumer unit are almost always the correct choice for a house extension. Extending an existing ring final to supply a new room is technically permissible if the extended circuit still complies with BS 7671 (maximum ring floor area of 100m², maximum circuit current within the MCB rating after the extension). However, the practical arguments for new circuits are strong: the existing circuit load is not increased, fault discrimination is better, the EIC is simpler and cleaner, and the homeowner has dedicated circuit protection for the new rooms. Extending the existing lighting circuit to supply a single light in a small extension is acceptable and cost-effective — but new socket circuits are strongly recommended for any extension of meaningful size.

Does a house extension require Part P notification?

Yes. New circuits in a house extension are notifiable under Part P of the Building Regulations. This covers all new wiring from the consumer unit to the extension, whether the work is done in the original house fabric or in the new extension structure. The simplest route is to use an electrician registered with a competent person scheme (NICEIC, NAPIT, ELECSA) who can self-certify and issue the Part P certificate. If the electrician is not registered, a building notice must be submitted to the local authority before the electrical work begins.

Does a house extension need a consumer unit upgrade?

Not always. If the existing consumer unit has sufficient spare ways and is an 18th edition-compliant unit with RCD protection, the new extension circuits can be added without upgrading the board. If the board is full, is a pre-18th edition unit without RCD protection, or is a plastic consumer unit (which does not comply with the current non-combustible enclosure requirement), an upgrade is required. A new 18th edition metal consumer unit costs £400 to £800 to supply and install and is usually worth including in the extension quote as it upgrades the whole house protection.

What circuits are typically required in a house extension?

The circuits required depend on how the extension is used. A rear living room or dining room extension typically requires: a lighting circuit (or extension of the existing house lighting circuit), a ring final or radial socket circuit, and possibly a spur for a wall-mounted TV or electric fireplace. An open-plan kitchen-diner extension requires: dedicated circuits for a cooker or hob (6mm or 10mm cable on a 32A or 45A circuit), dishwasher, washing machine, a refrigeration spur, a ring final socket circuit for countertop appliances and sockets, an extractor fan spur, and lighting. An orangery or garden room extension with underfloor heating adds a dedicated heating circuit to the list.

How is voltage drop affected by long cable runs to a rear extension?

Long cable runs from a consumer unit at the front of a house to a rear extension can exceed the 3% voltage drop limit for final circuits if undersized cable is used. For a 20m run of 2.5mm twin and earth on a 32A socket circuit, the voltage drop is approximately 2.9V (1.3%) — within limit. However, if the same cable runs 30m or more, the voltage drop may become marginal, and uprating to 4.0mm is recommended. The voltage drop calculator should always be used on extension surveys where the cable run from the consumer unit to the furthest outlet in the extension exceeds 15m.

COST GUIDE

House Extension Electrical Cost: First Fix, Second Fix and Part P

A house extension electrical package typically costs £1,200 to £4,000. This guide covers first and second fix stages, when to use new circuits versus extending existing rings, consumer unit considerations, and Part P notification.

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12 min readUpdated 2026-06-10Andrew Moore, Founder of Elec-Mate

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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Key Takeaways

1A house extension electrical package typically costs £1,200 to £4,000, depending on the floor area, number of circuits, and whether a consumer unit upgrade is required.
2Electrical work is split into two phases: first fix (conduit, back boxes, and cables installed before plastering) and second fix (faceplates, testing, and certification after decoration).
3New circuits from the consumer unit are almost always preferable to extending existing ring finals — they provide better fault discrimination, cleaner documentation, and avoid overloading existing circuits.
4All new circuits in a house extension must be notified under Part P of the Building Regulations, either through a competent person scheme or via a building notice to local authority Building Control.
5Extensions that include a kitchen or utility room add significant cost: dedicated circuits for cooker, dishwasher, washing machine, and fridge/freezer are all required.

01 · Cost Guide

House Extension Electrical Work: What Is Involved

A house extension — whether a single-storey rear addition, a side return, an orangery, or a two-storey extension — requires new electrical work that must be integrated with the existing installation and comply with BS 7671:2018+A4:2026 and Part P of the Building Regulations.

The electrical package is divided into first fix (pre-plaster) and second fix (post-plaster), with testing and certification carried out at second fix. The scope varies significantly based on the use of the extension: a simple living room extension has much simpler electrical requirements than an open-plan kitchen-diner extension.

This guide covers typical costs, the first and second fix stages, when to use new circuits versus extending existing circuits, Building Regulations requirements, and how to quote extension electrical work professionally.

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02 · Cost Guide

Typical Costs for House Extension Electrical Work

Costs vary with floor area, room type, and number of circuits. Typical ranges:

Small extension — living room or utility (up to 15m²) — £1,200 to £2,000. Lighting circuit, socket circuit, first-fix and second-fix labour, testing, and Part P notification.
Medium extension — open-plan living (15–30m²) — £2,000 to £3,000. Multiple circuits, potentially underfloor heating or bi-fold door lighting, more complex cable routing through existing structure.
Large extension — kitchen-diner or two-storey (30m²+) — £2,800 to £4,000+. Dedicated kitchen circuits (cooker, dishwasher, washing machine, fridge), lighting, sockets, extractor fan, potentially consumer unit upgrade.

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03 · Cost Guide

First Fix: Cables, Back Boxes, and Conduit

First fix is all electrical work carried out before the plasterer. The objective is to install all cables, conduit, and back boxes in their final positions so that nothing needs to be disturbed after plastering. Key first-fix tasks for an extension:

Cable routes from consumer unit — run new circuit cables from the consumer unit to the extension. Route through the original house structure (typically beneath floorboards on the ground floor, or through the loft on upper floors) before entering the extension. Protect cables where they pass through the wall at the boundary between original house and extension with a fire-stopping sleeve.
Back boxes — install flush metal back boxes in blockwork or timber stud walls before plasterboarding. Mark out socket, switch, and light switch positions with the architect/homeowner before fixing. Standard mounting heights: sockets at 450mm from finished floor level, switches at 1050mm.
Ceiling cables — for downlight installations, run cables in the ceiling void above the plasterboard, dropping down at each downlight position. Mark out downlight positions before boarding. Pre-install downlight fire hoods in the ceiling void if the extension has insulation above.

First fix is ideally carried out in one visit, co-ordinated with the builder. Returning multiple times for small first-fix elements is inefficient — agree the programme with the builder before pricing.

04 · Cost Guide

Second Fix: Faceplates, Testing, and Certification

Second fix is carried out after plastering and decoration. The cables that were left tailed at first fix are now connected and all accessories fitted. The second-fix sequence is:

Fit socket faceplates, switch plates, and connect wiring to terminals
Install light fittings or downlights; connect to ceiling roses or junction boxes
Connect circuit cables at the consumer unit and fit MCBs or RCBOs
Test all new circuits: continuity of CPCs, insulation resistance (500V DC, minimum 1 megohm), polarity, earth fault loop impedance, and RCD operation
Complete and issue the Electrical Installation Certificate and submit to the competent person scheme for Part P certification

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05 · Cost Guide

New Circuits vs Extending Existing Circuits

One of the most common decisions in extension electrical work is whether to run new circuits from the consumer unit or to extend existing circuits. The answer is nearly always: new circuits.

Extending Existing Circuits

Technically permissible if the extended ring still complies (area under 100m², load within MCB rating, voltage drop within limits). However: the existing circuit is made more complex, fault-finding is harder, the EIC must reference the existing installation condition, and overloading risk increases if the extension adds significant load. Not recommended for kitchen or heating circuits.

New Circuits (Recommended)

New circuits from the consumer unit provide clean fault discrimination, correct voltage drop design, and a simple EIC covering only the new work. The homeowner gets properly protected dedicated circuits, the documentation is straightforward, and the installation is future-proof. The extra cable cost is modest compared to the benefits.

The exception where extending is acceptable: a single light point added to the existing house lighting circuit to illuminate a small lean-to or porch. In this case, extending the lighting circuit with a spur is cost-effective and appropriate.

06 · Cost Guide

Building Regulations: Part P Notification

All new circuits in a house extension are notifiable under Part P of the Building Regulations. The requirements:

Competent person scheme (preferred route) — NICEIC, NAPIT, or ELECSA-registered electricians can self-certify the work. The scheme issues the Part P certificate directly. No building notice required; the scheme notifies the local authority on behalf of the electrician.
Building notice (alternative route) — the homeowner or contractor submits a building notice to local authority Building Control before work begins. A Building Control officer inspects the completed work. Slower and more expensive than using a competent person scheme.
Documentation for sale — the Part P certificate must be provided to the homeowner and is required for the property sale conveyancing process. Without it, the extension electrical work is flagged as non-compliant by the buyer's solicitor. Electricians who do not issue Part P certificates create problems for their customers.

07 · Cost Guide

For Electricians: Winning Extension Electrical Work

House extensions are a reliable source of electrical work. Builders need a reliable electrical sub-contractor who understands the build programme and can hit first-fix and second-fix dates. Winning repeat business from builders is more valuable than winning individual homeowner quotes.

Survey Before Quoting

Always visit the site before submitting a quote. Assess the consumer unit, the cable route from the board to the extension, the floor construction, and the builder's programme. A quote based on a visit wins more often than a quote based on a phone conversation.

Check Voltage Drop on Survey

Use the cable sizing calculator on the survey to check that the cable size is appropriate for the cable run length. Specifying 4mm cable on a long socket circuit run during the quote avoids a materials change once work has started.

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Frequently Asked Questions About House Extension Electrical Work

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House Extension Electrical Cost: First Fix, Second Fix and Part P

Key Takeaways

House Extension Electrical Work: What Is Involved

Get the BS 7671 A4:2026 Cheat Sheet — free

Typical Costs for House Extension Electrical Work

Price house extension electrical packages accurately

First Fix: Cables, Back Boxes, and Conduit

Second Fix: Faceplates, Testing, and Certification

Price the job in minutes, not evenings

New Circuits vs Extending Existing Circuits

Extending Existing Circuits

New Circuits (Recommended)

Building Regulations: Part P Notification

For Electricians: Winning Extension Electrical Work

Survey Before Quoting

Check Voltage Drop on Survey

Quote and certify house extension electrical work on your

Frequently Asked Questions About House Extension Electrical Work

How much does house extension electrical work cost?

What is the difference between first fix and second fix electrical work?

Should I use new circuits or extend existing circuits for a house extension?

Does a house extension require Part P notification?

Does a house extension need a consumer unit upgrade?

What circuits are typically required in a house extension?

How is voltage drop affected by long cable runs to a rear extension?

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