Electrics for a Home Extension London 2025: First Fix, Second Fix, Consumer Unit, Part P, and Costs

**Why first-fix electrical planning must happen before walls are closed**:

First-fix electrics is the stage of electrical installation that takes place before walls are plastered and before ceilings are boarded — cables are run through wall cavities; across joists; through the structural frame of the extension; and into back-boxes set into the wall. Once the plasterboard goes up or the screed goes down, first-fix is complete and any change requires re-opening walls or floors — at significant cost.

For a London house extension, good first-fix electrical planning must address:

*1. Load assessment and consumer unit capacity*:

Before any first-fix work can begin, the electrician must assess whether the existing consumer unit (distribution board — DB) has sufficient spare capacity to serve the new extension circuits.

• •**Modern consumer unit (2016 or later)**: most consumer units installed in London since 2016 comply with BS 7671:2018 (the IET Wiring Regulations, 18th Edition) and are fitted with Residual Current Circuit Breakers with Overcurrent protection (RCBOs) or split-load consumer units with Residual Current Devices (RCDs) and MCBs. If spare ways (slots) are available, extension circuits can be added relatively straightforwardly
• •**Older consumer unit (pre-2016; fuse box)**: if the existing consumer unit is a fuse board (rewirable fuses or older MCB boards without RCD protection), it is very likely that the electrician will recommend or the Building Control Inspector will require a consumer unit upgrade as part of the extension works. A compliant consumer unit upgrade for a London house: £500–£900 (consumer unit supply + fitting; testing; Part P notification; EICR if required)
• •**Three-phase supply (rare in residential)**: most London residential properties have a single-phase 100A supply. Where the extension is very large or involves three-phase equipment (unusual in residential), the supply upgrade must be discussed with UK Power Networks before design is finalised — a supply upgrade can take 3–9 months to arrange and involves a connection quotation from the DNO

*2. Circuit types for a London extension*:

**Ring final circuit (RFC) vs. radial circuit** — the choice affects cable sizes, breaker ratings, and maximum floor area served:

| Circuit type | Cable | Breaker | Max floor area | Typical use | |---|---|---|---|---| | Ring final circuit (RFC) | 2.5mm² twin-and-earth | 32A RCBO/MCB | 100m² | General purpose; power sockets in large open-plan spaces | | Radial circuit (20A) | 2.5mm² twin-and-earth | 20A RCBO/MCB | 50m² | Power sockets in smaller rooms | | Radial circuit (32A) | 4mm² twin-and-earth | 32A RCBO/MCB | 75m² | Power sockets in larger rooms; dedicated appliance circuits | | Lighting circuit | 1mm² or 1.5mm² twin-and-earth | 6A or 10A RCBO/MCB | No limit by floor area | Lighting throughout extension | | Kitchen dedicated circuits | 6mm² twin-and-earth | 32A or 45A | Single appliance | Double oven; induction hob; electric shower | | Electric vehicle (EV) charger | 6mm² (7kW) or 10mm² (10–22kW) | 32A (7kW) or higher | Single dedicated circuit | EV charger on external wall or in garage |

• **Standard circuits for a typical London rear extension including kitchen**:
• •1× RFC or radial for general power sockets (open-plan kitchen-dining-living)
• •1× lighting circuit (extension)
• •1× 45A circuit for induction hob (hob must have its own dedicated circuit)
• •1× 32A circuit for double oven
• •1× 20A or 32A circuit for dishwasher
• •1× 30mA RCD-protected circuit for fridge-freezer (cold storage)
• •1× spur or circuit for extractor fan (if power rather than transformer-fed)
• •1× circuit for electric towel rail or bathroom heating if extension includes a bathroom
• •1× EV charger circuit if planned (see Section 3)

*3. Cable routing through a London extension*:

Cable routing in a new London extension must comply with BS 7671:18th Edition 'safe zones' to avoid future accidental damage from fixings:

• •**In walls**: cables run vertically from socket back-boxes to ceiling level (in prescribed safe zones — horizontal at the top of the wall; vertical drops from horizontal runs; not diagonal). In block-work or brick-and-block walls, cables run in conduit or are surface-run in trunking
• •**In timber-frame extensions**: cables run through drilled holes in studs (not notched in stress zones); holes must be at least 50mm from the edge of the stud face (to prevent accidental piercing by fixings); or cables run in oval conduit clipped to the face of studs in non-insulated cavities
• •**Through insulated external walls**: where the extension has insulated walls (cavity, solid, or SIP panel), cable routes must be planned before insulation is installed — cables running in thermally insulated zones may be derated (must carry less current than their standard rating due to reduced heat dissipation) and cable sizing must account for this derating factor (Correction Factor Ci)
• •**Below screed/slab**: in-slab cables (underfloor heating wiring; below-screed power for an island or kitchen pod) must be in armoured cable or in conduit before the screed is poured — no second chances once the screed is down

*4. Pre-planning Building Control requirement*:

For a rear extension with a new structural opening into the existing house, the builder will typically remove the existing rear wall in one phase and install the structural support (steel beam; padstones; temporary support — see `structural-steel-beams-guide`). The electrician must ensure that any existing circuits passing through the rear wall are rerouted before demolition — if an existing ring circuit or lighting circuit feeds rear rooms through the rear wall, breaking that wall without first rerouting the circuit will cut power to part of the house during works.

**Part P of the Building Regulations (Electrical Safety in Dwellings)**:

Part P of the Building Regulations (England) — introduced in 2005 and revised in 2013 — governs electrical installation work in dwellings. For a London home extension, Part P means:

*Which electrical work requires notification or certification?*:

• Under the Part P framework, all electrical installation work in a dwelling must be either:
• •**Notified to Building Control** (so a Building Control Officer can inspect and certify the work): this is the route if the electrician does not belong to a Part P self-certification scheme; or
• •**Certified by a registered competent-person electrician** under a Part P self-certification scheme: the electrician certifies their own work and notifies the work directly to Building Control and to the homeowner — no Building Control inspection required for the electrical works specifically

*Part P self-certification schemes — NICEIC, NAPIT, and ELECSA*:

The main Part P competent-person registration bodies for domestic electricians in London:

| Scheme | Acronym | Notes | |---|---|---| | National Inspection Council for Electrical Installation Contracting | NICEIC | Most widely recognised; Gold Card = highest competency level; Domestic Installer scheme for Part P | | National Association of Professional Inspectors and Testers | NAPIT | Full-scope registered electricians; also covers gas and other services | | ECA Certification (Electrical Contractors' Association) | ELECSA | Certification scheme from the ECA; widely used in South East |

For a London homeowner: **always use a NICEIC-registered, NAPIT-registered, or ELECSA-registered electrician for your extension electrics**. The registered electrician's self-certification covers the Part P requirement — no separate Building Control notification needed for the electrical element of the extension (the structural extension will still have its own Building Control Full Plans application covering the building fabric).

• *What the registered electrician provides on completion*:
• •**Electrical Installation Certificate (EIC)** — the primary document certifying that the new installation has been designed, constructed, inspected, and tested in accordance with BS 7671
• •**Schedule of Inspections** — a formal record of each inspection item checked
• •**Schedule of Test Results** — the test values measured for each circuit (insulation resistance; polarity; earth continuity; RCD operating time; loop impedance)
• •**Building Regulations Compliance Certificate (BS7671 Compliance Certificate)** — the Part P certificate, which is sent to the Local Authority by the scheme provider. You will receive a copy — keep it with your property documents (solicitors will request this on sale)

*What if the electrician is NOT registered with a Part P scheme?*:

If the electrician carrying out the extension work is not registered with NICEIC/NAPIT/ELECSA, the homeowner must **notify the Local Building Control before work begins** (not after). The Building Control Inspector will inspect the electrical installation at appropriate stages (typically after first-fix and after second-fix) and issue the Part P compliance certificate. Notification cost: typically included in the Building Regulations fee for the extension works overall.

**The Electrical Installation Condition Report (EICR) — when it is required for an extension**:

• An EICR (formerly called a Periodic Inspection Report) is a condition assessment of the existing electrical installation in the house — it identifies any defects, deterioration, or non-compliance with current regulations in the existing wiring. An EICR is NOT automatically required as part of a home extension — but a Building Control Inspector or a registered electrician may recommend or require one where:
• •The existing consumer unit is very old (pre-2000; fuse board)
• •The existing wiring appears deteriorated or the installation appears to have been modified informally over time
• •The extension will connect to the existing installation and the electrician is concerned about the safety of the existing circuits
• EICR cost for a London house: £150–£350 depending on size and number of circuits.

**EV charger wiring during a London extension — best practice**:

If the extension includes a garage; a rear outbuilding; an external parking space adjacent to the extension; or a side return where a vehicle will eventually park, the time to wire for an EV charger is during the extension first-fix — not as a retrofit after the extension is complete.

*Why wire during first-fix?*:

• •Running a dedicated 6mm² or 10mm² armoured cable from the consumer unit to the EV charger location while walls are open costs a fraction of what it costs to surface-run conduit or open up finished surfaces after completion
• •EV charger wiring must run in a dedicated circuit from the consumer unit — it cannot share a circuit with any other load
• •If the route from consumer unit to charger location passes through the extension structure (internal wall; floor; roof space), the first-fix stage is the only practical time to install it

• *OZEV grant for EV charger (Electric Vehicle Infrastructure Grant — EVIG)*:
• •As of 2025, OZEV grants for domestic EV chargers are available to homeowners with off-street parking (£350 toward the cost of an EV charger and installation). Grant requires the installation to be by an OZEV-authorised installer. The grant does not cover the wiring done during extension first-fix separately — it covers the full EV charger supply and installation when the vehicle is purchased

• *Specification and cost*:
• •7kW smart EV charger (single phase; the standard for London residential): 6mm² T+E or SWA cable; 32A MCB or RCBO; dedicated circuit from consumer unit; outdoor IP-rated enclosure
• •Supply and installation of 7kW OZEV-grant smart EV charger in London 2025: £500–£900 (after OZEV £350 grant deduction; gross cost £850–£1,250)

**Kitchen extract duct wiring — a common first-fix coordination issue in London rear extensions**:

A kitchen extension requires a cooker hood or ceiling extract with a duct to the outside (recirculation-only hoods are not compliant with Part F ventilation requirements for new extensions). The duct route affects both the structural design of the extension roof and the electrical provision:

• •**For a flat-roof extension**: the extract duct typically runs through the flat roof structure (insulation and waterproofing layers) to an upturned cowl on the roof surface. This penetration must be built into the roofing specification — a retrofit penetration through a warm flat roof risks waterproofing failure
• •**For a pitched-roof extension**: the duct can exit through the gable wall (if applicable) or through the slope of the roof, or into the roof space and then through the soffit
• •The cooker hood or extract fan requires a fused connection unit or socket outlet within 2m of the unit — first-fix cable routing must reach this location before ceilings are boarded

**Second-fix electrics — what is included and what is client-supplied**:

Second-fix electrics is the fitting stage — sockets; switches; light fittings; consumer unit connections; final testing. Under the standard RCB terms, **second-fix materials are client-supplied** (the client selects and purchases their own sockets, switches, and light fittings). This is particularly relevant for a kitchen extension where the client has often chosen a specific socket, switch, and light fitting specification to match their kitchen design.

• *Standard second-fix items for a London rear extension with open-plan kitchen (client-supplied)*:
• •Double sockets (brushed steel; white; or chrome — client choice)
• •USB-integrated sockets (for worktop level)
• •Rocker or flat-plate switches (same finish as sockets)
• •Under-cabinet LED strip lighting driver and strip (typically 12V LED)
• •Pendant or recessed ceiling light fittings (LED downlights for extension ceiling — client to specify cut-out size; typically 70mm or 90mm)
• •Extractor hood (client supplies; electrician connects)

**Realistic electrical costs for a London rear extension with open-plan kitchen 2025**:

| Element | Cost range | |---|---| | First-fix labour (cable routing; back-boxing; consumer unit work; EV cable if planned) | £1,500–£3,000 | | Consumer unit upgrade (if required; supply and fit) | £500–£900 | | Second-fix labour (fitting client-supplied sockets; switches; lights; testing) | £700–£1,500 | | Part P certification (NICEIC/NAPIT self-certification) | Included in above | | EV charger installation (if planned; after OZEV grant) | £500–£900 | | **Total electrics for London rear extension with new kitchen (excl. client-supplied second-fix materials)** | **£2,700–£5,400** |