Does loft electrical work need Part P notification?

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, ELECSA) or inspected by building control. A certificate is required on completion.

How many circuits does a loft conversion need?

Typically: 1 lighting circuit (6A MCB, max 12 points), 1 ring final circuit for sockets (32A MCB, max 100m² floor area), and optionally a dedicated shower circuit (40A MCB for 8.5kW). A smoke detector circuit is also needed if not already interconnected with the existing system.

Do I need to upgrade my consumer unit?

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 may need upgrading to a modern dual RCD or RCBO board to comply with current BS 7671 requirements.

What about smoke detectors in a loft conversion?

A loft conversion requires interlinked smoke detectors on every floor of the house (Part B requirement). At minimum, you need a smoke detector in the new loft room(s) and on the landing below, all interconnected so that if one triggers, they all sound. Mains-wired with battery backup.

What cable routes are used in a loft conversion?

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.

TradeCalculator.co.uk

Loft Electrical Calculator — Circuits, Cable Routes & Consumer Unit

Calculate electrical requirements for a loft conversion: lighting circuits, ring finals, shower circuits, consumer unit spare ways and cable routes.

Light Points

Ceiling lights, wall lights, switches

Double Sockets

13A double socket outlets

Shower Circuit

Dedicated circuit for electric shower

USB Sockets

USB-A/C integrated sockets

Smoke Detectors

Interlinked mains-wired (Part B)

Price Per Point (optional)

Average installed cost per point

How We Calculate This

This calculator estimates the electrical circuits and components needed for a loft conversion.

The formula

Lighting Circuits = ceil(Light Points / 12)

Ring Finals = ceil(Sockets / 10)

CU Ways = Total Circuits + Spare Ways

Typical circuits

Lighting: 6A MCB, 1.0mm² T&E cable
Ring final: 32A MCB, 2.5mm² T&E cable
Shower: 40A MCB, 10mm² T&E cable
Smoke detectors: Interconnected, mains + battery

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.