Electrical Works in a London Extension or Renovation: What's Involved

Building electrical works are typically divided into two stages — 'first fix' and 'second fix' — which align with different points in the construction programme.

**First fix electrics:** First fix takes place before plastering and finishing — when the structure is in place and the walls are open. First fix involves:

• •Installing the main cables (wiring) from the consumer unit to each circuit: socket circuits, lighting circuits, dedicated appliance circuits (oven, hob, shower, fridge, dishwasher, washing machine, EV charger)
• •Running cables in conduit or clipped directly to the structure as appropriate
• •Installing back boxes for sockets and switches
• •Installing ceiling rose boxes and lighting points
• •First-fix connections for underfloor heating if electric mat UFH is specified
• •Any changes to the consumer unit (new breakers, RCDs, AFDDs as required by BS 7671:2018 18th Edition)

The first fix electrician works before the plasterer — after plastering, all the cable runs and back boxes are enclosed within the finished walls and ceilings.

**Second fix electrics:** Second fix takes place after all plastering and decoration are complete — when the finished surfaces are in place. Second fix involves:

• •Fitting socket faceplates, switch plates, and lighting rose covers
• •Connecting and fitting light fittings
• •Connecting kitchen appliances (oven, extractor fan connections)
• •Connecting underfloor heating thermostats
• •Installing smoke and CO alarms (second-fix position)
• •Testing and certifying the installation
• •Producing the Minor Works Certificate or Electrical Installation Certificate

**The importance of coordination:** Electrical layout decisions — where sockets, switches, and pendant lights are positioned — have a major impact on the finished room's usability and appearance. These decisions should be made during the design phase (before first fix) and confirmed with the client and the electrician before cables are run. Moving a socket position after plastering requires opening up the wall and replastering — an avoidable disruption.

• **Consumer unit (fuse board):**
• A London extension or significant renovation frequently triggers a consumer unit review. The existing consumer unit may:
• •Not have sufficient spare capacity (MCB positions) for the new circuits
• •Not meet the current BS 7671:2018 18th Edition requirements for RCD protection and — in kitchens, living rooms, and bedrooms — AFDD (Arc Fault Detection Device) protection
• •Be an older-style fuse box (rewirable fuse wire, rather than modern MCBs) that should be replaced regardless

For any significant extension, the electrical contractor should confirm whether the existing consumer unit can accommodate the new circuits or whether a new consumer unit (or an additional consumer unit) is required.

**Incoming supply capacity:** A large extension with multiple new circuits, an EV charger, and electric underfloor heating may push the total electrical demand close to or beyond the capacity of the existing incoming supply (typically 60A or 100A for a residential property). If the total demand exceeds the incoming supply capacity, an upgrade to the incoming service (fuse/cutout, meter, and DNO incoming cable) is required. This is arranged with the Distribution Network Operator (UK Power Networks in much of London) and is not within the contractor's scope to carry out — it requires a separate DNO application.

• **Part P — Electrical Safety:**
• Part P of the Building Regulations requires that notifiable electrical work in dwellings is either:
• •Carried out by a competent person registered with an approved scheme (NICEIC, NAPIT, or similar) — the electrician self-certifies
• •Or notified to Building Control and inspected by a Building Control officer

• Notifiable electrical work includes:
• •Installation of a new circuit
• •Consumer unit replacement
• •Any work in a special location (bathroom, shower room, kitchen zones)

All electrical work in an extension is notifiable. The electrician must be scheme-registered or the work must be notified to Building Control.

• **18th Edition BS 7671 requirements:**
• The current edition of the Wiring Regulations (BS 7671:2018, Amendment 2:2022) includes requirements for:
• •RCD protection on all circuits (most circuits in new or extended installations must be protected by a 30mA RCD)
• •AFDD protection on socket circuits in kitchens, living rooms, and bedrooms (this is a significant cost addition — AFDD devices are more expensive than standard breakers)
• •SPD (Surge Protection Device) in new consumer unit installations

These requirements apply to new installations and substantial modifications — confirm with the electrician that the new installation will be compliant with the current edition.

• **EV charging:**
• Electric vehicle chargers are now commonly requested as part of London extension and renovation projects. A dedicated EV charger (typically 7kW single-phase or 22kW three-phase) requires:
• •A dedicated circuit from the consumer unit (typically 32A or 40A single-phase)
• •An outdoor-rated consumer unit or protection device if the charger is external
• •Planning permission if the charger is on the front elevation or in certain conservation area positions (though most home EV chargers fall within PD)
• •OZEV (Office for Zero Emission Vehicles) approval to qualify for any available grant schemes

Including the cable run for an EV charger in the first fix of an extension is cost-effective (adding the cable while walls are open costs very little extra); retrospectively running cables for an EV charger after plastering and decorating is significantly more disruptive and expensive.

• **Solar PV:**
• Solar photovoltaic panels are increasingly requested alongside extensions. Key considerations:
• •Planning: most roof-mounted solar panels fall within PD (subject to PD criteria — not projecting more than 200mm from the roof slope, not on a conservation area front elevation or flat roof visible from a highway)
• •Building Regulations: notifiable under Part P and may be relevant to Part L SAP calculation
• •DNO export: where panels are connected for export to the grid (Smart Export Guarantee scheme), a G98 or G99 application to the DNO may be required
• •Battery storage: increasingly paired with solar for self-consumption — requires additional space (typically a wall in the garage or utility) and a dedicated inverter and battery management system

**Cat6 data cabling:** Wi-Fi is the default home networking solution, but structured data cabling (Cat6 or Cat6a to key points — TV locations, home office, home cinema) installed during first fix is cheap and avoids the performance limitations of Wi-Fi for high-bandwidth applications. Installing data cabling during the first fix of an extension or renovation adds approximately £800–£2,000 to the electrical costs and provides a permanent, high-performance network infrastructure.

**CCTV and security:** Camera positions and intercom wiring are best roughed-in during first fix — the structural access makes it simple to run cables to camera positions without surface-mounted conduit.