Loft Electrical Calculator — Circuits, Cable Routes & Consumer Unit

Yes. Electrical work in a loft conversion is notifiable under Part P because it involves new circuits in a new habitable space. The work must be done by a registered competent person (e.g. NICEIC or NAPIT) who can self-certify, or be notified to building control and inspected. An Electrical Installation Certificate plus a Building Regulations compliance certificate is issued on completion.

Typically: 1 lighting circuit (6A MCB — about 12 points at the ~100W/point demand convention), 1 ring final circuit for sockets (32A MCB, limited by ≤100m² floor area not socket count per BS 7671), and optionally a dedicated shower circuit. The shower MCB follows the design current Ib = kW×1000÷230V, choosing the smallest standard MCB with In ≥ Ib: an 8.5kW shower draws ~37A (40A MCB), 9.5–10kW draws 41–43.5A (45A MCB), and 10.5–12kW draws 45.7–52.2A (50A MCB, because 10.5kW = 45.7A exceeds a 45A breaker). A smoke detector circuit is also needed if not already interconnected with the existing system.

Yes. Under the 18th Edition Wiring Regulations (BS 7671:2018+A4:2026, the current Orange Book) every new domestic socket-outlet circuit must have additional protection by a 30mA RCD under Reg 411.3.3, and every lighting circuit under Reg 411.3.4 — so the new loft circuits need a dual-RCD or all-RCBO consumer unit. An electric shower in a bathroom requires a Type A RCD/RCBO. If your existing board has no spare ways or is an old rewireable fuse board, it will likely need upgrading.

Possibly. You need spare ways in the consumer unit for the new circuits (typically 2-4 ways). If your existing CU has no spare ways or is an old-style rewireable fuse board, it will need upgrading to a board that gives 30mA RCD protection to all the new final circuits (dual-RCD or all-RCBO) to comply with the 18th Edition Wiring Regulations (BS 7671:2018+A4:2026).

A loft conversion is designed to BS 5839-6 Grade D1, Category LD2 (referenced by Approved Document B): interlinked, mains-wired alarms with battery backup, fitted on every storey along the escape route plus the principal habitable room, with a HEAT alarm (not smoke) in any kitchen. For a typical two-storey house becoming three storeys that means at least an alarm in the new loft room and on each landing below, all interlinked so they sound together.

Cables typically run from the consumer unit (usually ground floor) up through the existing ceiling void, through the floor structure to the loft. Cables in walls must be in safe zones (vertical or horizontal from accessories) or protected in conduit. Cable routes through fire barriers need fire-stopping.