Electrical First Fix in London Renovation: How to Specify It Correctly and Avoid the Mistakes That Last Forever

The electrical first fix is invisible in the finished building. Once walls are plastered, ceilings are boarded, and decoration is complete, the cable routes behind them cannot be seen, cannot be changed without destruction, and cannot be added to without drilling. The electrical installation is permanent infrastructure — as fundamental to the building as its structure or drainage — and it must be designed with the same rigour and foresight.

In a prime London renovation, the electrical installation is complex. A modern home has a high density of electrical demand: multiple appliances with dedicated circuits, extensive lighting circuits on individual dimmer channels, home automation infrastructure, EV charging, high-capacity kitchen appliances, home office data and power, AV distribution, security systems, and increasingly electric vehicle charging. Each of these has cable route requirements, back box requirements, consumer unit capacity requirements, and interaction with the building's other systems that must be resolved in design — not improvised on site.

This guide covers what a client commissioning a prime London renovation should specify, understand, and verify at the electrical design and first fix stage.

The Consumer Unit (Fuse Board)

The consumer unit is the heart of the electrical installation — the point where the incoming supply is distributed to all circuits in the house, protected by circuit breakers (MCBs) and residual current devices (RCDs).

Specification for a prime renovation:

A house of 250–400m² with a comprehensive electrical specification typically requires a consumer unit with 24–40 way capacity. For a premium installation, a dual-consumer-unit arrangement (one unit for power circuits; one for lighting circuits) or a split-load consumer unit with multiple RCD banks provides better discrimination — a fault on a single circuit trips only that circuit's protection rather than an entire bank of unrelated circuits.

All new consumer units in domestic installations must use RCBO (Residual Current Breaker with Overcurrent protection) protection for each individual circuit, per the 18th Edition IET Wiring Regulations. An RCBO combines the function of an MCB and an RCD in a single device, providing individual earth leakage protection for each circuit — a fault on the oven circuit trips only the oven circuit, not all kitchen circuits.

Incoming supply capacity: A modern prime London home has a high electrical demand — particularly if it has an induction hob (7–14kW), electric oven (2–4kW), EV charger (7–22kW), heat pump (4–12kW), and electric underfloor heating in secondary zones. The total diversified demand may exceed 60–100A. The incoming single-phase supply to most London houses is 100A — adequate for typical loads but potentially limiting if an EV charger and heat pump are running simultaneously. For a large renovation with high electrical demand, a three-phase supply upgrade (enquiry to UK Power Networks, typically 6–12 month lead time) provides 200A+ capacity and distributes load across phases.

Location: The consumer unit must be accessible to adults but not to children (a lockable enclosure satisfies Part P of Building Regulations). In a prime renovation, the consumer unit should be in a dedicated electrical cupboard or plant room — not in the kitchen, not behind a decorative panel, but in a purposefully designed service space with adequate clearance for safe working.

Circuit Design: How Many Is Enough?

The number of circuits in a prime London renovation is the specification decision that most directly determines the long-term flexibility and usability of the electrical installation. Too few circuits create shared loads, increased fault disruption, and inability to manage lighting zones independently. Too many creates an oversized consumer unit; in practice, it is almost always better to err on the side of more circuits.

Lighting circuits: Each floor should have at minimum two independent lighting circuits (allowing the floor to be half-lit without turning off all lighting on a fault). In a lighting-control installation (Lutron, KNX), each dimmer channel has its own dedicated circuit from a distribution board — 40–80 circuits for a large house.

Power circuits: Kitchen ring main (or radial circuits for high-demand appliances on dedicated circuits): induction hob (32A radial), double oven (20A radial), fridge/freezer (unswitched, dedicated), dishwasher (13A), washing machine (13A), tumble dryer (13A). General ring mains for sockets: one per floor minimum; two per floor for a large floor plate.

Dedicated circuits: EV charger (32A or 40A radial from a dedicated RCBO — required by regulation for all new EV charge points); heat pump (typically 16–32A depending on model); MVHR unit (6A or 10A); security alarm panel (6A, unswitched); CCTV NVR (6A); home automation server/rack (10–16A); pool or spa equipment.

Outdoor circuits: Garden lighting (weatherproof circuit with RCD protection); garden power sockets (IP44 rated, on a dedicated circuit); external security lighting; gate motor; outbuilding supply (typically via an armoured cable and a sub-distribution board in the outbuilding).

Back Box and Cable Route Specification

The back box — the metal or plastic enclosure recessed into the wall behind a socket, switch, or data outlet — must be correctly positioned and of sufficient depth before plastering. This requires the electrical design to be produced from a dimensioned interior design drawing, not from a site walk-through.

Socket heights: In a prime renovation, sockets are typically positioned at a consistent height throughout — 450mm from finished floor level (FFL) is a common standard that reads as intentional rather than opportunistic. In a kitchen, sockets above the worktop are typically at 150mm above worktop level. USB sockets (incorporating USB-A and USB-C charging ports) should be specified in bedside locations, home office locations, and kitchen charging zones.

Switch heights: Light switches at 1350mm from FFL is the standard. In a lighting-control installation, the switch plate locations are fixed in the design by the lighting designer; the electrician installs back boxes and data cables to those positions.

Data outlets: Every room that may ever be used as a home office, media room, or child's bedroom should have a hardwired data outlet (Cat 6A minimum; Cat 7 for future-proofing). Wireless connectivity (WiFi) does not replace hardwired data for high-bandwidth applications (4K video streaming, large file transfers, latency-sensitive video calls). The data outlets connect back to a central network rack (in the plant room or a dedicated data cupboard) via individual cable home runs — no daisy-chaining.

TV/AV points: In a distributed AV system, coaxial or HDMI matrix cabling runs from the AV rack to TV points in each room. The back boxes for TV points should accommodate the wall plate and the cable loop required by the AV installer. Specifying these positions before first fix — and running the correct cable type (typically two coax, one HDMI, one Cat 6A per TV point) — determines what AV distribution is possible for the life of the building.

Special Considerations for London Renovation

Rewiring vs. extending existing installation: A comprehensive London renovation almost always requires a full rewire — the existing installation in a Victorian or Edwardian property is likely to be a combination of original rubber-insulated cable (now brittle and potentially dangerous), partial upgrades, and additions by previous occupants over decades. A full rewire on a stripped-out renovation (walls open, floors lifted) is far cheaper than extending or repairing a piecemeal existing installation in a completed interior. The cost of a full rewire for a 250m² London house: approximately £18,000–£35,000 for the electrical installation, depending on circuit count and complexity.

Part P compliance: All electrical work in dwellings in England and Wales must comply with Part P of Building Regulations. Most electrical work must be carried out by a registered competent person (NICEIC or NAPIT registered) or notified to the local authority building control. For a full rewire in a major renovation, the work will be covered under the building regulations application for the renovation project; the electrician must issue an Electrical Installation Certificate at completion.

Underfloor heating (electric mat) circuits: Each electric UFH mat zone requires a dedicated circuit from the consumer unit — typically 16A or 20A, depending on mat wattage — with a dedicated RCBO. The thermostat controlling the mat must be accessible after installation (typically a surface-mounted or flush-mounted unit within the room); the floor sensor probe must be installed in the correct position within the screed before the floor finish is laid.

EV charger circuit: The EV charger circuit should be installed as part of the renovation even if EV charging is not immediately required. Running a 40A armoured cable from the consumer unit to the intended charger location (garage or front of house) costs approximately £400–£800 during a renovation; retrofitting the same cable after completion costs £1,500–£4,000. Government grant funding (OZEV) for EV charger installation requires the charger to be installed by an approved installer — the grant applies to the charger and its installation, not the circuit preparation.

Testing and Certification

On completion of the electrical installation, the following tests must be carried out and documented by the electrician:

• —Continuity tests: Verify all protective conductors (earth cables) are correctly connected
• —Insulation resistance tests: Verify cable insulation has not been damaged during installation
• —Polarity tests: Verify live and neutral are not reversed at any outlet
• —Earth fault loop impedance tests: Verify the earth fault path impedance is within limits for the installed protective devices
• —RCD tests: Verify each RCD/RCBO operates within the specified time (typically 40ms for 30mA RCDs)
• —Prospective fault current measurement: Verify the consumer unit's breaking capacity is adequate for the fault level at the supply point

The test results, along with the installation schedule, are recorded in the Electrical Installation Certificate (EIC) — a legal document that must be retained by the building owner and provided to any future purchaser or their solicitor on request. The EIC is part of the building's compliance documentation alongside planning permissions, building regulations completion certificates, and gas and oil safety certificates.